Tropical diseases

Nowhere is the prevalence of certain illnesses more striking than in areas where tropical diseases flourish. In many parts of Africa , South America, and Asia , diseases exist that are rarely seen in the United States. These include malaria , which infects from 300 to 500 million people annually and kills up to 2.7 million people every year, and leishmaniasis, which affects some 12 million people internationally. Other ailments, such as measles and diarrhea, are well-known in developed areas such as the United States but are rarely fatal.

Young people are highly susceptible to death from disease in these areas. Children under five account for 40-60% of all deaths in Africa, Asia, and South America, although they make up only about 15% of the population. Such high death rates reflect the fact that tropical diseases are most prevalent in poor areas where health care is limited. Efforts to improve community health in areas where tropical diseases thrive include the establishment of health clinics and the development of new vaccines.

Battles against malaria

Malaria, a parasitic disease spread by mosquitoes , is the best-known tropical disease and infects the largest number of people internationally. It has been called the "most devastating disease in history" based on the number of people it has attacked or killed. Currently, malaria has been identified in about 100 countries, although 80% of the clinical cases are reported in Africa.

The history of malaria is different than the history of other tropical diseases, due to its immense reach and the concerted effort over time to defeat the disease. But in as much as it has been eliminated or controlled in developed countries and remains a major killer in those that are poor and less developed, its history is also similar to that of other tropical diseases.

The ancient Chinese wrote about malaria as early as 2700 b.c., describing the symptomatic fever and characteristic enlargement of the spleen. Ancient Greeks and Romans also suffered from malaria. Hippocrates (460-375 b.c.), offered an accurate account of malarial symptoms and theorized that the disease was caused by a miasma, or poisonous cloud, rising up from marshy land.

The first effective treatment for malaria was developed in the seventeenth century and utilized a traditional Peruvian treatment for fever, the cinchona tree bark . In 1630, Jesuit missionaries in Peru introduced the bark of the cinchona tree to Europeans. The active substance in the bark, quinine , was isolated in 1820. Though there were common, serious side effects to the drug, such as ringing of the ears and hearing loss, the substance was effective in treating malaria, which was common in Europe at that time.

Development of a treatment for malaria cleared the way for large scale exploration of tropical areas by Europeans. Development of a synthetic quinine, called chloroquine, in the 1940s, offered effective treatment with fewer common side effects. In the 1960s, malarial parasites became resistant to chloroquine, and the substance no longer worked in many areas.

The most recent effort to control malaria has been effective in many areas, including the southern United
States. The pesticide DDT was used, successfully, in the Tennessee River Valley, Greece, Puerto Rico and other locations to kill the mosquitoes carrying malaria and eliminate the disease. A world campaign to eliminate malaria internationally used DDT from 1956-1969. However, mosquitoes became resistant to DDT and the effort failed.

There are four different types of malaria protozoa . One of the four, P. falciparum, is responsible for almost all deaths from the disease. Symptoms of the disease do not occur for one to two weeks. They often include chills, muscle aches, fatigue, and abdominal discomfort. They may include tremors and high fever, which comes and goes at regularly spaced intervals. Complications from severe malaria include renal failure, pulmonary edema , coma , and hypoglycemia.

The delayed appearance of malaria's symptoms reflects the measured course the disease takes in the body. The activity of the malaria parasite in the body is aggressive and thorough. The disease is spread by female anopheline mosquitos that inject malaria parasites into the bloodstream with their bite. The parasites, called sporozoites, travel to the liver, where they enter cells of the liver tissue . Once in the liver, the sporozoite changes to a spore , which replicates itself until there are thousands of spores in a cyst-like structure which has replaced the cell . Malaria manifests no symptoms while this process occurs.

Symptoms develop when the cyst bursts and the spores, called merozoites, are released into the blood stream. At this point, sweating and high fever can occur

as the spores enter the host's red blood cells. The parasite consumes hemoglobin from within the red blood cells.

As the parasite grows from consuming the hemoglobin, its nucleus divides into from six to 32 parts, each of which becomes a spore of its own. The red cell bursts, and the spore moves to another cell.

After the merozoites have launched their process of reproducing in the red blood cells of the host, some of the merozoites are transformed to male or female game-tocytes. These sexual cells form the base for more parasites if the host is bitten again by the female anopheles mosquito. When this occurs, the gametocytes develop into a number of differentiated cells, called gametes, zygotes, ookinetes and oocysts, within the gut of the mosquito. Eventually the parasite reproduces and sporozoites that travel in the mosquito are formed. These sporozoites move up to the mosquito's salivary gland, where they are ready to infect another host.

Treatment of malaria is usually effective. Because many malaria parasites are resistant to chloroquine, physicians have returned to using quinine in many instances. Travelers and others at risk of malaria receive chloroquine or other drugs, including mefloquine, if journeying to areas where the drugs still work. However, there is no totally effective preventive measure against malaria.

The deadly sandfly and leishmaniasis

Another disease spread by insects is leishmaniasis, a sickness caused by several types of protozoa carried by sand flies . In the mid 1990s, the disease was infecting about 12 million people worldwide, killing about 1,000 annually. It is spread when sand flies draw blood from individuals who already are infected. Hosts can be humans, dogs, or other mammals .

Each of the four major clinical syndromes of the disease have a long incubation period that ranges from three months to 24 months. The disease is marked by one or more skin lesions and varies in severity depending on the type. Kala Azar, one of the types, is commonly found in East Africa and the Sahara. Kala Azar causes fever, diarrhea, enlarged liver, and anemia . Another type, Old World leishmaniasis, usually heals on its own and is marked by multiple lesions.

Treatment depends on the type of leishmaniasis and may include the use of drug therapy and transfusions for several weeks. The need for extended medical care makes treatment of leishmaniasis impossible for many poor individuals in developing countries, raising the mortality rate of the disease.

Dangerous worms

Ascariasis and hookworm

A common disease in tropical countries is ascariasis. The annual death rate due to ascariasis is about 20,000, most due to complications within the intestine, where the worms settle as adults. An estimated 700 million individuals are affected at any given time by the disease.

Ascariasis is spread through dirt. The worm which grows in the intestine, the roundworm Ascaris lumbricoides, is the largest such parasite, growing to a length of 19.3 in (49 cm). It can live about a year in the human body.

The disease can cause difficulties, such as bronchial asthma , in the lungs, when the parasite settles in that part of the body. The greatest threat to the human host is the loss of food nutrients which go to the worm instead of the host. Individuals whose diet is already sparse may suffer from malnutrition after infection with ascariasis.

The life cycle of the creature is similar to the life cycle of the blood fluke, though there are some differences. The disease is spread when Ascaris eggs leave the human host in feces. The new host ingests the eggs through soil or by soil-contaminated hands or food. The eggs then travel through the liver, lungs, and throat, before ending up in the intestine. Once they are in the intestine, more eggs are laid, clearing the way for further spread of the disease.

Another parasite that is spread through dirt and grows in the human body is hookworm, which affects approximately 800 million individuals in the developing world and kills about 50,000 annually. Hookworm was at one point common in the American South, but is now seldom reported in the area.

The hookworm, which grows up to 0.5 in (13 mm) in size, sucks blood and normally lives in the small intestine. Establishment of the worm in the host generally results in iron-deficiency anemia because of blood loss, and may cause bronchitis , peptic ulcers , or even heart problems.

Hookworms enter the body as larvae through the skin, taking advantage of hair follicles or other openings to help the creature pierce through the skin with its boring movement. The parasite enters the circulatory system , moving first to the lungs then to the pharynx, where it is swallowed. The final larvae stage typically occurs in the small intestine, where adult worms emerge and ultimately lay eggs. Adult worms normally live from one to 5 years if the host is not treated. They cannot multiply in the body.

Successful control of the hookworm depends on improvements in plumbing and the use of footwear, a challenge in countries where shoes are a luxury. Medication is available to destroy hookworm, but providing it in areas where hookworm infestation is severe does little to alter the problem, as those who are medicated can easily be reinfected.

Schistosomiasis

One of the most common tropical diseases is schistosomiasis, a disease caused by a worm called a blood fluke. The worm, which can be about 0.47 in (12 mm) long, can live in the human body from five to 20 years. About 200 million people are infected by the worm in developing countries, with as many as 500,000 deaths due to the disease each year.

Symptoms include abdominal pain , diarrhea, and weight loss. If untreated, the disease can cause enlargement of the liver, bleeding from blood vessels in the esophagus, and problems to the central nervous system .

The worm, which is usually hosted by Biomphalaria snails , depends on fresh water to survive. The disease is common in areas where bathing occurs in freshwater contaminated by human feces and where many individuals have the disease.

The life cycle of the blood fluke entails travel throughout the human body and through the body of its other host, the snail. The worm enters the human body in a larval stage in fresh water through contact with human skin. The larva enter a blood or lymph vessel and move to the heart and lungs, where they grow for several days. Then they move to the liver and the portal circulation, where they grow for several weeks.

Ultimately, the worms move to the intestinal wall, where they settle. The creatures lay eggs, which are released in feces and become new larva after contact with fresh water. These larva enter the host snail, where they eventually produce cercariae, the final larval stage. These are the creatures that enter the human body after contact with skin.

Drug therapy is an effective treatment for the disease. Prevention efforts depend on controlling infection through drug therapy and on convincing individuals to steer clear of fresh water which may be infected.

Microscopic hazard

Amebiasis, a disease caused by a microscopic protozoa spread through dirty water, infects as many as 500 million individuals in the developing world. The disease kills about 70,000 people annually.

The protozoa can live in the large intestine and not cause damage to the host. But it is also capable of causing ulceration of the colonic wall and damage to the liver and other organs, including the brain .

The disease is spread through water contaminated by fecal matter , by contaminated flies, or by other contaminated substances. Hosts swallow cysts, which divide in the small intestine and again in the large intestine to form amoebae. These amoebae divide and form trophozoites, which feed on fecal bacteria . These trophozoites form cysts and are passed out in the feces of the host. This entire cycle takes from 48 hours to four months.

Diarrhea and cholera

Best known in the United States as a benign but unpleasant condition, diarrhea is one of the leading causes of death in developing countries. Indeed, diarrhea causes about 4.3 million deaths each year, more than malaria or many of the other serious tropical illnesses. Though most of the approximately 28 billion individuals who develop diarrhea survive, the health consequences of diarrhea are great in tropical areas.

Infants and small children are most likely to die of diarrhea. This is because loss of even a small amount of water can be life-threatening to small children. Death due to diarrhea stems from loss of fluid, loss of plasma , and the collapse of the cardiovascular system. Infants being weaned from the mother's breast are particularly susceptible to malnutrition and dehydration. Diarrhea can be caused by bacteria or viruses.

Residents of tropical disease areas are less likely than residents of developed areas to have access to emergency treatment for extreme diarrhea. They are also more likely to be exposed to pathogens that cause intense diarrhea in contaminated food or water. Cholera , a life-threatening condition whose major symptom is watery diarrhea, is spread through dirty water.

Oral rehydration therapy, an approach which uses a solution of glucose or sucrose and salt to hydrate the body, is an effective treatment for life-threatening diarrhea. One problem with the approach is that it must be made available immediately after a child gets sick, as diarrhea can cause enough fluid loss within the first 12 to 24 hours to cause death. Many individuals in tropical disease areas live many hours by foot from medical care. Therefore, health experts are working to make oral rehydration therapy, along with education about the therapy, available at home to residents of tropical-disease areas.

The most infamous type of diarrheal disease is cholera, which is also marked by vomiting, intense thirst, and abdominal cramping. Cholera is so common it is endemic in some parts of the world, such as India. In other parts of the world, it occurs as an epidemic or a pandemic.

The disease is caused by the Vibrio cholerae bacteria, which typically spreads through dirty water, food, or seafood from areas with contamination . The bacteria that causes cholera multiplies in the human intestinal tract, but has also been known to multiply in water.

Symptoms of cholera are a direct result of the cholera enterotoxin, the toxic substance elaborated by cholera bacteria that affects the cells of the small intestine. The substance forces the mucosal cells to secrete large quantities of fluid, resulting in diarrhea.

Control of cholera calls for good sanitation measures, specifically clean water and good food hygiene. Boiling water eliminates the bacteria. A vaccine that offers limited effectiveness—30-80% protection for up to six months—is available.

While treatment is effective, involving antibiotics , nutritional support, and treatment for dehydration, the death rate in epidemics may reach 30%.

Malnutrition

Malnutrition is the underlying cause of death for about two million people annually, and the disease most commonly affects children under five and the elderly. As many as 40% of all children under five in developing areas have suffered from malnutrition.

The disease stems for eating inadequate amounts of food or eating foods without enough protein. Different syndromes reflect different types of food deficits. Kwashiorkor reflects a protein deficiency, and marasmus is caused by an energy deficiency linked to failure to consume enough carbohydrates and fats.

Signs of malnutrition are loss of energy, wasting of muscle, and loss of fat . The condition can result in severe anemia, coma, or heart failure. Treatment requires efforts to rehydrate the individual and to replace deficient eating patterns with adequate consumption of healthy food. Education of some families may help improve future eating patterns. Yet, because the condition most often occurs among impoverished families in areas with inadequate food availability, changing eating habits can be difficult or impossible.

Infectious disease killers

The most common killers in tropical areas are not exotic diseases. They are infectious diseases, many of which are not considered life-threatening in developed countries such as the United States. In the least industrialized countries, 40% or more of all deaths are caused by infectious disease. In the United States, about 1% of all deaths are caused by such illnesses. The huge difference in death rates stems from the fact that most infectious diseases are the easiest diseases to treat using modern medical care.

Approximately 10 million people die every year from respiratory diseases in the developing world. Such diseases are caused by the influenza virus or bacteria such as pneumococcus and Hemophilus influenza B. Measles, which can be prevented by immunization of infants, kills two million people in the developing world annually. The course of the disease in developing countries often includes complications such as diarrhea, ear infection, pneumonia , and weight loss. For some, weight loss leads to malnutrition. Aggressive efforts to curb measles deaths through immunization have reduced the number of potential deaths in recent years.

Whooping cough is another early childhood disease which is seen rarely in the United States but is common in developing countries. About 600,000 people die of whooping cough annually in the developing world, with many of these deaths occurring among small children. About 55 million people develop the disease. Whooping cough is also prevented by immunization.

Another infectious disease which is a common killer in developing countries is tuberculosis , which kills about 900,000 people every year and infects about seven million people annually. Tuberculosis can be treated through immunization and drug therapy, although drug resistant strains are becoming increasingly prevalent. Though deaths have been reduced internationally over the last 30 years, the disease is still very common in tropical areas.

Tropical-disease areas have also been hit hard by AIDS and HIV infection. An estimated 15 million people internationally are infected by the HIV virus, the World Health Organization estimated in 1994. Of those about one-fifth have developed AIDS, a disease which is fatal in most instances.

A key emphasis of prevention programs in Africa has been promoting the use of condoms during sexual intercourse, a practice which reduces the risk of AIDS. In Africa, condom use increased from two million per year in 1986 to about 70 million in 1993. But problems remain, particularly in places where poverty limits the safety of the blood supply , where condoms are in short supply, where access to accurate information about the disease is limited, and where health providers are far away.

Future trends

A hopeful new development in tropical medicine is the vaccine for malaria, which was in field trials in the middle 1990s. The vaccine, developed by Colombian Manuel Patarroyo, is made up of three peptides synthesized chemically and connected using a fourth peptide from the malaria parasite.

The vaccine is designed to protect against falciparum, the most deadly strain of malaria. Results of clinical tests in Colombia, Ecuador, Venezuela, and Tanzania reported in 1994 show some success against the disease. The Tanzania trial of about 600 children reduced the incidence of malaria by 31%. This shows that the vaccine has some promise. Experts note that while the vaccine did not provide 100% protection, it still would be valuable to cut sickness and death from malaria by 31%. The next step is to see if the vaccine can be improved and to conduct larger field trials.

Other experimental vaccines are also being tested. A therapeutic vaccine for tuberculosis is currently being tested which reduces the time it takes to treat the disease, which currently is about six months. Shortening the treatment time would improve the odds that treatment is completed by the patient.

Another vaccine which could reduce the sickness linked to tropical diseases is a new vaccine for leishmaniasis. The vaccine, which is in clinical trials, has fewer side affects and costs about one-tenth less than drugs currently used to treat the disease. However, because the drug is not in production for distribution yet, the ultimate cost of the medicine is not clear.

Other strategies are also essential to stem the tremendous cost in illness and death from tropical diseases. While World Health Organization efforts have boosted the rate of immunization for many diseases, there are still millions of children who do not receive adequate immunization for common early childhood diseases such as measles and whooping cough. Efforts to provide such proven preventive measures to more children will yield great benefits in health and longer life.

Efforts to boost access to health providers and educational information also stand to benefit residents of tropical disease areas. Knowing how to care for children with diarrhea, adults with hookworm, or many other diseases could make a vast difference. Finally, the prevalence of diseases linked to dirty water and dirty soil drive home the importance of better sewage systems and basic public health measures which western countries generally adopted years ago. Diseases carried by insects are tackled through insecticides , clearing brush and standing water where the carriers may proliferate; sexually transmitted diseases are addressed by educating communities about safer sex practices, and providing condoms; immunization programs may cut down on some diseases; efforts to create vaccines for other diseases are underway; donor blood is undergoing increasingly more stringent screening to avoid transmitting disease through this route.

Resources

books

Desowitz, Robert S. The Malaria Capers. New York: W.W. Norton and Company, 1991.

Goldsmith, Robert, and Donald Hayneman, eds. Tropical Medicine and Parasitology. Norwalk, CT, 1989.

Najera, Jose A., Bernhard H. Liese, and Jeffrey Hammer. Malaria: New Patterns and Perspectives. World Bank Technical Paper Number 183, 1992.

Warren, Kenneth S., and Adel A.F. Mahmoud. Tropical andGeographical Medicine. 2nd ed. New York: McGraw-Hill Information Services Co., 1990.

periodicals

Aldhous, Peter. "Vaccine Shows Promise in Tanzania Test." Science 266, (November 4, 1994): 724.

"No Shortcuts on International Commitment to Combat AIDS." AIDS Weekly (May 16, 1994): 7.

Patricia Braus

KEY TERMS

Cyst

—Refers to either a closed cavity or sac or the stage of life of some parasites during which they live inside a walled in area.

Dehydration

—The condition of having lost too much fluid from the body.

Larva

—Immature developmental stage of various species.

Lesion

—An injury or wound.

Parasite

—Species which depends on another species to survive.

Peptide

—A class of chemical compounds that form proteins.

Protozoa

—One-celled creatures and the simplest forms of animal life.

Spore

—A dormant form assumed by some bacteria, such as anthrax, that enable the bacterium to survive high temperatures, dryness, and lack of nourishment for long periods of time. Under proper conditions, the spore may revert to the actively multiplying form of the bacteria.

Tropical Diseases

Battles against malaria

The deadly sandfly and leishmaniasis

Dangerous worms
Ascariasis and hookworm

Schistosomiasis

Microscopic hazard

Diarrhea and cholera

Infectious disease killers

Stemming tropical diseases

Resources

Nowhere is the prevalence of certain illnesses more striking than in areas where tropical diseases flourish. In many parts of Africa, South America, and Asia, diseases exist that are rarely seen in the United States. Young people are highly susceptible to death from disease in tropical and semi-tropical areas. Children under five account for 40-60% of all deaths in Africa, Asia, and South America, although they make up only about 15% of the population. Such high death rates reflect the fact that tropical diseases are most prevalent in poor areas where health care is limited. Efforts to improve community health in areas where tropical diseases thrive include the establishment of health clinics and the development of new vaccines.

Battles against malaria

Malaria infects up to 500 million people annually and kills from one to three million people every year. A parasitic disease spread by mosquitoes, malaria is the best-known tropical disease and infects the largest number of people internationally. It has been called the ‘most devastating disease in history’ based on the number of people it has attacked or killed. Currently, malaria has been identified in about 100 countries, although 80% of the clinical cases are reported in Africa.

The history of malaria is different than the history of other tropical diseases, due to its immense reach and the concerted effort over time to defeat the disease. But in as much as it has been eliminated or controlled in developed countries and remains a major killer in those that are poor and less developed, its history is also similar to that of other tropical diseases.

The ancient Chinese wrote about malaria as early as 2700 BC, describing the symptomatic fever and characteristic enlargement of the spleen. Ancient Greeks and Romans also suffered from malaria. Hippocrates (460-375 BC), offered an accurate account of malarial symptoms and theorized that the disease was caused by a miasma, or poisonous cloud, rising up from marshy land.

The first effective treatment for malaria was developed in the seventeenth century and utilized a traditional Peruvian treatment for fever, the cinchona treebark. In 1630, Jesuit missionaries in Peru introduced the bark of the cinchona tree to Europeans. The active substance in the bark, quinine, was isolated in 1820. Though there were common, serious side effects to the drug, such as ringing of the ears and hearing loss, the substance was effective in treating malaria, which was common in Europe at that time.

Development of a treatment for malaria cleared the way for large scale exploration of tropical areas by Europeans. Development of a synthetic quinine, called chloroquine, in the 1940s, offered effective treatment with fewer common side effects. In the 1960s, malarial parasites became resistant to chloroquine, and the substance no longer worked in many areas.

Efforts to control malaria has been effective in many areas, including the southern United States. The pesticide DDT was used successfully, in the Tennessee River Valley, Greece, Puerto Rico and other locations to kill the mosquitoes carrying malaria and eliminate the disease in the late 1940s. A worldwide campaign to eliminate malaria used DDT from 1956-1969. However, mosquitoes became resistant to DDT and the effort failed in some areas.

There are four different types of malaria protozoa. One of the four, P. falciparum, is responsible for almost all deaths from the disease. Symptoms of the disease do not occur for one to two weeks. They often include chills, muscle aches, fatigue, and abdominal discomfort. They may include tremors and high fever, which comes and goes at regularly spaced intervals. Complications from severe malaria include renal failure, pulmonary edema, coma, and hypoglycemia.

The delayed appearance of malaria’s symptoms reflects the measured course the disease takes in the body. The activity of the malaria parasite in the body is aggressive and thorough. The disease is spread by female Anopheles mosquitoes that inject malaria parasites into the bloodstream with their bite. The parasites, called sporozoites, travel to the liver, where they enter cells of the liver tissue. Once in the liver, the sporozoite changes to a spore, which replicates itself until there are thousands of spores in a cyst like structure which has replaced the cell. Malaria manifests no symptoms while this process occurs.

Symptoms develop when the cyst bursts and the spores, called merozoites, are released into the blood stream. At this point, sweating and high fever can occur as the spores enter the host’s red blood cells. The parasite consumes hemoglobin from within the red blood cells.

As the parasite grows from consuming the hemoglobin, its nucleus divides into from six to 32 parts, each of which becomes a spore of its own. The red cell bursts, and the spore moves to another cell.

After the merozoites have launched their process of reproducing in the red blood cells of the host, some of the merozoites are transformed to male or female gametocytes. These sexual cells form the base for more parasites if the host is bitten again by the female Anopheles mosquito. When this occurs, the gametocytes develop into a number of differentiated cells, called gametes, zygotes, ookinetes and oocysts, within the gut of the mosquito. Eventually the parasite reproduces and sporozoites that travel in the mosquito are formed. These sporozoites move up to the mosquito’s salivary gland, where they are ready to infect another host.

Treatment of malaria is usually effective. Because many malaria parasites are resistant to chloroquine, physicians have returned to using quinine in many instances. Travelers and others at risk of malaria receive chloroquine or other drugs, including mefloquine or doxycycline, if journeying to areas where the drugs still work. However, there is no totally effective preventive measure against malaria. Philanthropic organizations such as the Bill and Melinda Gates Foundation have made conquering malaria through vaccine development and distribution among their top priorities

The deadly sandfly and leishmaniasis

Another disease spread by insects is leishmaniasis, a sickness caused by several types of protozoa carried by sandflies. Leishmaniasis infects about one million people worldwide per year. It is spread when sand flies draw blood from individuals who already are infected. Hosts can be humans, dogs, or other mammals.

Each of the four major clinical syndromes of the disease have a long incubation period that ranges from three months to 24 months. The disease is marked by one or more skin lesions and varies in severity depending on the type. Kala Azar, one of the types, is commonly found in East Africa and the Sahara. Kala Azar causes fever, diarrhea, enlarged liver, and anemia. Another type, Old World leishmaniasis, usually heals on its own and is marked by multiple lesions.

Treatment depends on the type of leishmaniasis and may include the use of drug therapy and transfusions for several weeks. The need for extended medical care makes treatment of leishmaniasis impossible for many poor individuals in developing countries, raising the mortality rate of the disease.

Dangerous worms
Ascariasis and hookworm

A common disease in tropical countries is ascariasis. The annual death rate due to ascariasis is about 20,000, most due to complications within the intestine, where the worms settle as adults. An estimated 700 million individuals are affected at any given time by

the disease, making ascariasis the most common worm infection in humans.

The disease can cause difficulties, such as bronchial asthma, in the lungs, when the parasite settles in that part of the body. The greatest threat to the human host is the loss of food nutrients which go to the worm instead of the host. Individuals whose diet is already sparse may suffer from malnutrition after infection with ascariasis.

The life cycle of the creature is similar to the life cycle of the blood fluke, though there are some differences. The disease is spread when Ascaris eggs leave the human host in feces. The new host ingests the eggs through soil or by soil-contaminated hands or food. The eggs then travel through the liver, lungs, and throat, before ending up in the intestine. The worm, which grows in the intestine, the roundworm Ascaris lumbricoides, is the largest such parasite, growing to a length of 19.3 in (49 cm). It can live about a year in the human body. Once they are in the intestine, more eggs are laid, clearing the way for further spread of the disease.

Another parasite that is spread through dirt and grows in the human body is hookworm, which affects approximately 800 million individuals in the developing world and kills about 50,000 annually. Hookworm was at one point common in the American South, but is now seldom reported in the area.

The hookworm, which grows up to 0.5 in (13 mm) in size, sucks blood and normally lives in the small intestine. Establishment of the worm in the host generally results in iron-deficiency anemia because of blood loss, and may cause bronchitis, peptic ulcers, or even heart problems.

Hookworms enter the body as larvae through the skin, taking advantage of hair follicles or other openings to help the creature pierce through the skin with its boring movement. The parasite enters the circulatory system, moving first to the lungs then to the pharynx, where it is swallowed. The final larvae stage typically occurs in the small intestine, where adult worms emerge and ultimately lay eggs. Adult worms normally live from one to 5 years if the host is not treated. They cannot multiply in the body.

Successful control of the hookworm depends on improvements in plumbing and the use of footwear, a challenge in countries where shoes are a luxury. Medication is available to destroy hookworm, but providing it in areas where hookworm infestation is severe does little to alter the problem, as those who are medicated can easily be reinfected.

Schistosomiasis

one of the most common tropical diseases is schistosomiasis, also known as bilharzias, a disease caused by a worm called a blood fluke. The worm, which can be about 0.47 in (12 mm) long, can live in the human body from five to 20 years. About 200 million people are infected by the worm in developing countries.

Symptoms include abdominal pain, diarrhea, and weight loss. If untreated, the disease can cause enlargement of the liver, bleeding from blood vessels in the esophagus, and problems to the central nervous system.

The worm, which is usually hosted by Biomphalaria snails, depends on fresh water to survive. The disease is common in areas where bathing occurs in freshwater contaminated by human feces and where many individuals have the disease.

The life cycle of the blood fluke entails travel throughout the human body and through the body of its other host, the snail. The worm enters the human body in a larval stage in fresh water through contact with human skin. The larvae enter a blood or lymph vessel and move to the heart and lungs, where they grow for several days. Then they move to the liver and the portal circulation, where they grow for several weeks.

Ultimately, the worms move to the intestinal wall, where they settle. The creatures lay eggs, which are released in feces and become new larva after contact with fresh water. These larvae enter the host snail, where they eventually produce cercariae, the final larval stage. These are the creatures that enter the human body after contact with skin.

Drug therapy is an effective treatment for the disease. Prevention efforts depend on controlling infection through drug therapy and on convincing individuals to steer clear of fresh water which may be infected.

Microscopic hazard

Amebiasis, a disease caused by a microscopic protozoa spread through dirty water, infects as many as 500 million individuals in the developing world. The disease kills about 70,000 people annually.

The protozoa can live in the large intestine and not cause damage to the host. But it is also capable of causing ulceration of the colonic wall and damage to the liver and other organs, including the brain.

The disease is spread through water contaminated by fecal matter, by contaminated flies, or by other contaminated substances. Hosts swallow cysts, which divide in the small intestine and again in the large intestine to form amoebae. These amoebae divide and form trophozoites, which feed on fecal bacteria. These trophozoites form cysts and are passed out in the feces of the host. This entire cycle takes from 48 hours to four months.

Diarrhea and cholera

Best known in the United States as a benign but unpleasant condition, diarrhea is one of the leading causes of death in developing countries. Indeed, diarrhea causes about 4.3 million deaths each year, more than malaria or many of the other serious tropical illnesses. Though most of the approximately 28 billion individuals who develop diarrhea survive, the health consequences of diarrhea are great in tropical areas.

Infants and small children are most likely to die of diarrhea. This is because loss of even a small amount of water can be life threatening to small children. Death due to diarrhea stems from loss of fluid, loss of plasma, and the collapse of the cardiovascular system. Infants being weaned from the mother’s breast are particularly susceptible to malnutrition and dehydration. Diarrhea can be caused by bacteria or viruses.

Residents of tropical disease areas are less likely than residents of developed areas to have access to emergency treatment for extreme diarrhea. They are also more likely to be exposed to pathogens that cause intense diarrhea in contaminated food or water. Cholera, a life-threatening condition whose major symptom is watery diarrhea, is spread through dirty water.

Oral rehydration therapy, an approach which uses a solution of glucose or sucrose and salt to hydrate the body, is an effective treatment for life-threatening diarrhea. One problem with the approach is that it must be made available immediately after a child gets sick, as diarrhea can cause enough fluid loss within the first 12 to 24 hours to cause death. Many individuals in tropical disease areas live many hours by foot from medical care. Therefore, health experts are working to make oral rehydration therapy, along with education about the therapy, available at home to residents of tropical-disease areas.

The most infamous type of diarrheal disease is cholera, which is also marked by vomiting, intense thirst, and abdominal cramping. Cholera is so common it is endemic in some parts of the world, such as India. In other parts of the world, it occurs as an epidemic or a pandemic.

The disease is caused by the Vibrio cholerae bacteria, which typically spreads through dirty water, food, or seafood from areas with contamination. The bacteria that causes cholera multiplies in the human intestinal tract, but has also been known to multiply in water.

Symptoms of cholera are a direct result of the cholera enterotoxin, the toxic substance elaborated by cholera bacteria that affects the cells of the small intestine. The substance forces the mucosal cells to secrete large quantities of fluid, resulting in diarrhea.

Control of cholera calls for good sanitation measures, specifically clean water and good food hygiene. Boiling water eliminates the bacteria. Several cholera vaccines are being developed, but only one, Dukoral^®, is likely to be effective for long periods. Dukoral is an oral vaccine, which is more easily delivered to populations in the developing world than injectable vaccines. Neither vaccine is effective against a relatively new type of cholera bacteria that emerged in Asia in the early 1990s known as Vibrio cholerae O139.

While treatment is effective, involving antibiotics, nutritional support, and treatment for dehydration, the death rate in epidemics may reach 30%.

Infectious disease killers

The most common killers in tropical areas are not exotic diseases. They are infectious diseases, many of which are not considered life threatening in developed countries such as the United States. In the least industrialized countries, 40% or more of all deaths are caused by infectious disease. In the United States, about 1% of all deaths are caused by such illnesses. The huge difference in death rates stems from the fact that most infectious diseases are among the easiest diseases to treat using modern medical care.

Approximately 10 million people die every year from respiratory diseases in the developing world.

Such diseases are caused by the influenzavirus or bacteria such as pneumococcus and Hemophilus influenza B. Measles, which can be prevented by immunization of infants, kills two million people in the developing world annually. The course of the disease in developing countries often includes complications such as diarrhea, ear infection, pneumonia, and weight loss. For some, weight loss leads to malnutrition. Aggressive efforts to curb measles deaths through immunization have reduced the number of potential deaths in recent years.

Whooping cough or pertussis is another early childhood disease which is seldom seen in the United States but is common in developing countries. About 600,000 people die of whooping cough annually in the developing world, with most of these deaths occurring among small children. Whooping cough is also prevented by immunization.

Another infectious disease which is a common killer in developing countries is tuberculosis, which kills about 900,000 people every year and infects about seven million people annually. Tuberculosis can be treated through immunization and drug therapy, although drug resistant strains are becoming increasingly prevalent. Though deaths have been reduced internationally over the last 30 years, the disease is still very common in tropical areas.

KEY TERMS

Cyst —Refers to either a closed cavity or sac or the stage of life of some parasites during which they live inside a walled in area.

Dehydration —The condition of having lost too much fluid from the body.

Larva —Immature developmental stage of various species.

Lesion —An injury or wound.

Parasite —Species which depends on another species to survive.

Peptide —A class of chemical compounds that form proteins.

Protozoa —One-celled creatures and the simplest forms of animal life.

Spore —A dormant form assumed by some bacteria, such as anthrax, that enable the bacterium to survive high temperatures, dryness, and lack of nourishment for long periods of time. Under proper conditions, the spore may revert to the actively multiplying form of the bacteria.

Tropical-disease areas have also been hit hard by AIDS and HIV infection. An estimated 40 million people internationally are infected by the HIV virus, the World Health Organization estimated in 2006. Over 20 million more people have died of the disease.

A key emphasis of prevention programs in Africa has been promoting the use of condoms during sexual intercourse, a practice which reduces the risk of AIDS. In Africa, condom use increased from two million per year in 1986 to about 70 million in 1993. But problems remain, particularly in places where poverty limits the safety of the blood supply, where condoms are in short supply, where access to accurate information about the disease is limited, and where health providers are far away. Antiretroviral treatment reduces both disability and death from AIDS, but access to these drugs is often not available in developing countries.

Stemming tropical diseases

While World Health Organization efforts have boosted the rate of immunization for many diseases, but there are still millions of children who do not receive adequate immunization for common early childhood diseases such as measles and whooping cough. Efforts to boost access to health providers and educational information also stand to benefit residents of tropical disease areas. Knowing how to care for children with diarrhea, adults with hookworm, or many other diseases could make a vast difference. Finally, the prevalence of diseases linked to dirty water and dirty soil drive home the importance of better sewage systems and basic public health measures which western countries generally adopted years ago. Diseases carried by insects are tackled through insecticides, clearing brush and standing water where the carriers may proliferate; sexually transmitted diseases are addressed by educating communities about safer sex practices, and providing condoms; immunization programs may cut down on some diseases; efforts to create vaccines for other diseases are underway; donor blood is undergoing increasingly more stringent screening to avoid transmitting disease through this route.

Resources

BOOKS

Close, William T. Ebola: Through the Eyes of the People.
Marbletown, WY: Meadowlark Springs, 2001.

Honigsbaum, Mark. The Fever Trail. New York: Farrar
Straus & Giroux, 2002.

Walsh, Nancy. ‘Dengue Fever: A Souvenir of
Tropical Trips’ Skin and Allergy News 35 (July 1, 2004): 55.

PERIODICALS

Meltzer, Eyal, et al. ‘Schistosomiasis among Travelers: New
Aspects of an Old Disease.’ Emerging Infectious Disease. (November 2006): 12 (11).

OTHER

World Health Organization. ‘Control of Neglected Tropical
Diseases.’ <http://www.who.int/neglected_diseases/en/> (accessed on December 3, 3006).

Patricia Braus

Tropical spastic paraparesis

Definition

Tropical spastic paraparesis (TSP) is a slowly progressive spastic paraparesis caused by the human T-cell lymphotropic virus-1 (HTLV-1), with an insidious onset in adulthood. It has been found all around the world (except in the poles), mainly in tropical and subtropical regions.

Description

For several decades the term tropical spastic paraparesis (TSP) was used to describe a chronic and progressive clinical syndrome that affected adults living in equatorial areas of the world. Neurological and modern epidemiological studies found that in some individuals no one cause could explain the progressive weakness, sensory disturbance, and sphincter dysfunction that affected individuals with TSP. During the mid-1980s, an important association was established between the first human HTLV-1 virus and idiopathic TSP. Since then, this condition has been named HTLV-1 associated myelopathy/tropical spastic paraparesis or HAM/TSP and scientists now understand that it is a condition caused by a retrovirus that results in immune dysfunction. The main neurological features of HAM/TSP consist of spasticity and hyperreflexia (increased reflex action) of the lower extremities, urinary bladder disturbance, lower-extremity muscle weakness, sensory disturbances, and loss of coordination. Patients with HAM/TSP may also exhibit arthritis, lung changes, and inflammation of the skin.

Co-factors that may play a role in transmitting the disorder include being a recipient of transfusion blood products, breast-feeding from an infected mother, intravenous drug use, or being the sexual partner of an infected individual for several years.

Demographics

Sporadic cases of TSP have been reported in the United States, mostly in immigrants from countries where this disease is endemic (naturally occurring). In the United States, the lifetime risk of an HTLV-1-infected person developing TSP/HAM has been calculated to be 1.7–7%, similar to that reported for United Kingdom, Africa, and the Caribbean.

The international incidence is difficult to estimate because of the insidious nature of this disease. HAM/TSP is common in regions of endemic HTLV-1, such as the Caribbean, equatorial Africa, Seychelles, southern Japan, and South America. However, it also has been reported from non-endemic areas, such as Europe and the United States. The prevalence in southern Japan is in the range of 8.6–128 per 100,000 inhabitants. An estimated 10–20 million individuals worldwide are carriers of HTLV-1.

HAM/TSP generally affects women more than men, with a female-to-male ratio of 3:1. This disease may occur at any age, with a peak in the third or fourth decade.

Causes and symptoms

The cause of HAM/TSP is still a matter of debate. Whereas only a small proportion of HTLV-1-infected individuals develop HAM/TSP, the mechanisms responsible for the progression of an HTLV-1 carrier state to clinical disease are not clear. However, three hypotheses are considered by scientists as the most likely cause of TSP: direct toxicity, autoimmunity, and bystander damage. The direct toxicity theory of HAM/TSP pathogenesis suggests that HTLV-1-infected cells are directly damaged by certain white blood cells. The autoimmunity theory postulates that the immune system attacks cells that react to HTLV-1 infected cells. In the bystander damage hypothesis, circulating antivirus-specific cells migrating through the central nervous system produce damage to nearby cells that is directed against the infected cells.

Symptoms may begin years after infection. In response to the infection, the body's immune response may injure nerve tissue, causing symptoms including:

• spasms and loss of feeling or unpleasant sensations in the lower extremities, accompanied by weakness
• decreased sense of touch in mid-body areas
• a vibration sensation, especially in the lower extremities, resulting from spinal cord or peripheral nerve involvement
• low lumbar pain with irradiation to the legs
• increased reflexes of the upper extremities
• increased urinary frequency and associated increased incidence of urinary tract infection

Less frequently observed symptoms include tremors in the upper extremities, optical nerve atrophy, deafness, abnormal eye movements, cranial nerve deficits, and absent or diminished ankle jerk reflex.

Diagnosis

During the clinical examination, it is important to exclude other disorders causing progressive spasticity and weakness in the legs. Diagnosis of HAM/TSP criteria typically involve documenting the following:

• absence of a history of difficulty walking or running during school age
• within two years of onset: increased urinary frequency, nocturia, or retention, with or without impotence; leg cramps or low back pain ; symmetric weakness of the lower extremities
• within six months of onset: complaints of numbness or dysesthesias of the legs or feet
• a clinical examination documenting increased reflexes; spasticity of both legs, abnormal gait (manner of walking), and absence of normal sensory level

Laboratory diagnosis using ELISA (enzyme-linked immunosorbent assay) detects the presence of antibodies against HTLV-1, confirmed by the western blot assay. Electrodiagnostic studies and magnetic resonance imaging may also be helpful to show evidence of active denervation, associated with HTLV-1.

Treatment team

Persons with TPS have multiple needs and the team should include a neurologist and a physical therapist. An occupational therapist can prescribe exercises designed to develop fine coordination or compensate for tremor or weakness, or suggest assistive devices. More advanced patients require continual nursing assistance.

Treatment

The US Food and Drug Administration (FDA) has not officially approved any drug for the specific treatment of HAM/TSP in the United States. Many patients benefit from oral prednisolone or equivalent glucocorticoid therapy. A response rate of up to 91% has been reported in less advanced cases. Oral treatment with methylprednisolone may produce excellent to moderate responses in around 70% of patients. Plasmapheresis, interferon, oral azathiaprine, danazol, and vitamin C have been tried and also show transient effects. None of these treatments has been systematically studied in a controlled clinical trial. Antiviral drugs like AZT would be expected to help in reducing viral replication and associated direct cell injury.

Patients with HAM/TSP sometimes report neuropathic pain. Useful drugs include antiepileptics (e.g., carbamazepine , phenytoin, gabapentin , topiramate ), baclofen, and tricyclic antidepressants. The dosages used usually are well below those used in the treatment of epilepsy . Physical therapy is commonly used in combination with medication for nerve pain.

Recovery and rehabilitation

The goal of a rehabilitation program for a person affected with HAM/TSP is to restore functions essential to daily living in individuals who have lost these capacities through injury or illness. Most rehabilitation programs are comprehensive in nature and have several different aspects.

Physical therapy is designed to help restore and maintain useful movements or functions and prevent complications such as frozen joints, contractures, or bedsores. Examples of physical therapy include:

• stretching and range of motion exercises
• exercises to develop trunk control and upper arm muscles
• training in walking and appropriate use of assistive devices, such as ambulatory aids, braces, and wheelchairs
• training in how to get from one spot to another, such as from the bed to a wheelchair or from a wheelchair to the car
• training in how to fall safely in order to cause the least possible damage

Occupational therapy focuses on specific activities of daily living that primarily involve the arms and hands. Examples include grooming, dressing, eating, handwriting, and driving.

Some rehabilitation centers have innovative programs designed to help people compensate for loss of memory or slowed learning ability. Rehabilitation may be carried out in a residential or an outpatient setting.

Clinical trials

In 2004 there were some open clinical trials for the study and treatment of TSP, including:

• "Evaluation of Patients with HAM/TSP," "Phase I/II Study of HTLV-I-Associated Myelopathy/Tropical Spastic Paraparesis (HAM/TSP) Using the Humanized MiK-beta-1 Monoclonal Antibody," and "Assessment of Patients with Multiple Sclerosis," sponsored by National Institute of Neurological Disorders and Stroke (NINDS).
• "Phase I Study of T Cell Large Granular Lymphocytic Leukemia in Humanized MiK-Beta-1 Monoclonal Antibody Directed Toward the IL-2R/IL-15R Subunit (CD122)," sponsored by National Cancer Institute (NCI).

Further updated information on these clinical trials can be found at the National Institutes of Health website for clinical trials at <www.clinicaltrials.gov>.

Prognosis

HAM/TSP is usually a progressive neurological disorder, but it is rarely fatal. Most patients live for several decades after the diagnosis. Their prognosis improves if they take steps to prevent urinary tract infection and skin sore formation, and if they enroll in physical and occupational therapy programs.

Special concerns

An important component in the care of patients with TSP is the prevention of infections with the HTLV-1 virus. Several studies indicate that transmission of the HTLV-1 virus occurs through sexual or other intimate contact, intrauterine exposure, newborn exposure via breast milk, sharing of needles by drug abusers, and blood transfusion from infected persons. Transfusion of HTLV-1 antibody-positive blood causes infection in about 60% of recipients. Breastfeeding is contraindicated for mothers who are carriers of HTLV-1.

Resources

BOOKS

Parker, James N., and Philip M. Parker. The Official Patient's Sourcebook on Tropical Spastic Paraparesis. San Diego: Icon Group International, 2002.

PERIODICALS

Mora, Carlos A., et al. "Human T-lymphotropic Virus Type I-associated Myelopathy/Tropical Spastic Paraparesis: Therapeutic Approach." Current Treatment Options in Infectious Diseases 5 (2003): 443–455.

OTHER

"NINDS Tropical Spastic Paraparesis Information Page." National Institute of Neurological Disorders and Stroke. (April 20, 2004). <http://www.ninds.nih.gov/health_and_medical/disorders/tropical_spastic_paraparesis.htm>.

"Tropical spastic paraparesis." Dr. Joseph F. Smith Medical Library. Thompson Corporation. (April 20, 2004). <http://www.chclibrary.org/micromed/00069230.html>.

ORGANIZATIONS

National Organization for Rare Disorders (NORD). P.O. Box 1968 (55 Kenosia Avenue), Danbury, CT 06813-1968. (203) 744-0100 or (800) 999-NORD (6673); Fax: (203) 798-2291. orphan@rarediseases.org. <http://www.rarediseases.org>.

National Institute of Allergy and Infectious Diseases (NIAID). 31 Center Drive, Rm. 7A50 MSC 2520, Bethesda, MD 20892-2520. (301) 496-5717. <http://www.niaid.nih.gov>.

Francisco de Paula Careta

Iuri Drumond Louro

Disease, Tropical. Tropical disease has plagued American forces from the colonial period to modern times. Malaria, dysentery, dengue, and yellow fever (all endemic to the United States), as well as schistosomiasis, yaws, leishmaniasis, filariasis, and scrub typhus (all from other areas), have decimated garrisons and overwhelmed combat troops. To conquer or contain tropical disease, the U.S. military, sometimes cooperating with civilian scientists, developed preventive techniques, therapies, and cures. Collaborative research controlled and even vanquished most tropical diseases except for malaria, whose parasite became immune to traditional antimalarials, and whose prevention and cure remain elusive.

The military historically follows standard civilian practice regarding contagion, diagnosis, and treatment. Before the Civil War, physicians did not suspect mosquitoes as transmitters, blamed fever on the climate or air, and often could not distinguish one fever from another, treating them with emetics, chinchona bark, bleeding, mercury, wine, aromatics, snakeroot, or arsenic compounds. During the Mexican War, physicians at the Veracruz hospital treated yellow fever with quinine sulfate, first extracted from chinchona bark in 1820, to reduce the fever; mustard plasters and baths to help the circulation; and mercurials to evacuate the bowels. About 28 percent of victims died in the spring and summer of 1847. More prevalent was diarrhea or dysentery, which accounted for one‐third of all hospital admissions. Quinine proved highly beneficial in treating malaria. During the Civil War, the Union army had plenty of quinine, but the disease scourged the Confederate army, which occupied infested areas and had limited supplies. Better sanitation in the post–Civil War period helped curtail communicable disease among troops.

The Spanish‐American War (1898) forced the U.S. Army to lead a fight against tropical disease. American forces could not garrison the islands they had won without controlling yellow fever, typhoid, malaria, and dengue. The work of Maj. William C. Gorgas of the Medical Corps in Havana (1899) and (as colonel) in Panama (1904–06) in preventive medicine halted epidemics of typhoid, dysentery, and yellow fever, and made possible construction of the Panama Canal. Maj. Walter Reed and the Army Medical Board in the autumn and winter of 1900–1901 proved scientifically that the mosquito served as intermediate host for the yellow fever parasite. Reed was indebted to Dr. Carlos Finlay of Cuba, Sir Patrick Manson of Britain, and Maj. Ronald Ross of the British army, each of whom had helped prove that the mosquito was the carrier of disease. Their research pioneered the science of tropical medicine. During the early twentieth century, army medical research boards, such as the Philippine Tropical Disease Board, conducted investigations and began control measures that drastically reduced disease around U.S. bases at home and overseas.

By World War II, the development of vaccines, due largely to support from the Rockefeller Foundation, had eliminated yellow fever and typhus. Improvement of field sanitation controlled the dysenteries. Wartime research under the aegis of the National Research Council improved chemotherapy for the prevention and treatment of malaria and secured development of a powerful insecticide: DDT. Those advances, plus the work of army and navy malaria control units, and the enforcement of malaria discipline by personnel (taking atabrine tablets and wearing protective clothing) reduced incidence to historic lows. In the Southwest Pacific, monthly rates fell from 251 per 1,000 in December 1943 to 62 per 1,000 in November 1944: malaria no longer impeded campaigns.

During the Vietnam War, however, despite research to find better antimalarial drugs or a vaccine, chloroquine‐resistent falciparum malaria from Southeast Asia threatened the U.S. armed forces. Drug‐resistant malaria continues to be the most important military medical problem of the tropics.
[See also Caribbean and Latin America, U.S. Military Involvement in the; Casualties.]

Bibliography

E. C. Andrus et al., eds., Advances in Military Medicine Made by American Investigators Working Under the Sponsorsphip of the Committee on Medical Research, 1948.
John Z. Bowers and Elizabeth F. Purcell, eds., Advances in American Medicine: Essays at the Bicentennial, vols. 1, and 2, 1976.
François Delaporte , The History of Yellow Fever. An Essay on the Birth of Tropical Medicine, 1991.
Mary Ellen Condon‐Rall and and Albert E. Cowdrey , The Medical Department: Medical Service in the War Against Japan, 1998.

Mary Ellen Condon‐Rall

Tropical Spastic Paraparesis

Definition

Tropical spastic paraparesis (TSP) is an incurable viral infection of the spinal cord that causes weakness in the legs. It is caused by the human T-cell lymphotropic virus-1 (HTLV-1) retrovirus.

Description

As the name implies, tropical spastic paraparesis usually occurs in tropical locales. Although isolated cases have been diagnosed in the southeastern United States and other places in the United States, TSP is most frequently found in:

• the Caribbean
• Japan
• the Seychelles Islands
• regions of South America
• western Africa

TSP usually affects adults between the ages of 30 and 40, and is far more common in women than in men.

The disease may remain undetected for years after infection is contracted. When the immune system's response to the virus causes nerve damage, the legs gradually lose strength and flexibility.

Causes and symptoms

TSP is caused by the HTLV-1 virus, which also causes leukemia. The virus can be spread through the placenta, and also through blood transfusions, breast-feeding, contaminated needles, and sexual contact.

Symptoms may begin years after infection. In response to the infection, the body's immune response may injure nerve tissue, causing symptoms that include bladder abnormalities, leg pain, loss of feeling in the feet, tingling sensations, and unpleasant sensations when the skin is touched.

As many as 20% of patients with TSP may also experience:

• deafness
• double vision
• the tendency to incorrectly estimate the amount of motion necessary to accomplish a specific task (dysmetria)
• exaggerated reflexes
• facial paralysis
• tremor.

Diagnosis

Infectious disease specialists use blood tests and magnetic resonance imaging (MRI) of the spinal cord to diagnose this condition.

Treatment

While the disease is incurable, significant improvement has been reported in the condition of TSP patients treated with corticosteroids. These drugs are believed to alleviate symptoms by suppressing the immune system's response to the virus that causes them.

Plasmapheresis, a dialysis-like procedure in which symptom-producing antibodies are removed from the blood, also provides temporary relief.

Prognosis

As noted, TSP cannot be cured.

Prevention

The United States Food and Drug Administration (FDA) has approved screening procedures developed to detect HTLV-1 in donated blood and blood products designated for transfusion. These procedures, which can also be used to diagnose patients with TSP, are designed to prevent the spread of the disease.

Resources

OTHER

"Current Trends Licensure of Screening Tests for Antibody to Human T-Lymphotropic Virus Type I." Centers for Disease Control. May 27, 1998. 〈http://www.cdc.gov/epo/mmwr/preview/mmwrhtml/00001311.htm〉.

KEY TERMS

Retrovirus— A family of RNA viruses containing a reverse transcriptase enzyme which allows the viruses' genetic information to become part of the genetic information of the host cell upon replication.

Virus— A microorganism, smaller than bacteria, which can only replicate within the a cell of a living plant or animal. The virus provides the genetic code and the host cell provides the energy and raw materials for replication.