Toxoplasmosis (Toxoplasma Infection)

Introduction

Disease History, Characteristics, and Transmission

Scope and Distribution

Treatment and Prevention

Impacts and Issues

BIBLIOGRAPHY

Introduction

Toxoplasmosis (TOX-o-plaz-MO-sis) refers to an infection caused by a type of microorganisms known as a protozoan. The particular protozoan responsible for the infection is Toxoplasma gondii. The infection is part of a parasitic association between T. gondii and a human host—the microbe benefits from the association, but the host does not. In the case of toxoplasmosis, the infection enables the protozoan to complete its life cycle.

Toxoplasmosis (sometimes called “toxo”) is a serious concern in people whose immune systems are not functioning properly, such as those with acquired immunodeficiency syndrome (AIDS, also cited as acquired immune deficiency syndrome). For those with AIDS, toxoplasmosis can be lethal.

Disease History, Characteristics, and Transmission

Toxoplasmosis is an example of a zoonotic disease—a disease that is passed from animals to humans. The animal that is most important in the spread of toxoplasmosis is the cat. The United States Centers for Disease Control and Prevention has estimated that approximately 30% of domestic cats in the United States harbor T. gondii. Cats can acquire the protozoan by eating an infected rodent. Other animals can also carry the protozoan, in particular cattle, sheep, or other livestock, which poses an increased risk to farmers, ranchers, and others who come in contact with farm animals.

T. gondii has a life cycle that involves two forms of the organism. The actively growing and dividing form actually causes the disease. But, typically this is not the form of the organism that first enters the body. Rather, a person ingests the form that is called an oocyst. An oocyst is a smaller and hardier form of T. gondii that is analogous to a bacterial spore—an oocyst is designed to survive environmental conditions that would otherwise kill the growing protozoan. When ingested, the oocyst can convert to the growing form in the less hostile conditions of the intestinal tract.

WORDS TO KNOW

OOCYST: An oocyst is a spore phase of certain infectious organisms that can survive for a long time outside the organism and so continue to cause infection and resist treatment.

PROTOZOA: Single-celled animal-like microscopic organisms that live by taking in food rather than making it by photosynthesis and must live in the presence of water. (Singular: protozoan.) Protozoa are a diverse group of single-celled organisms, with more than 50,000 different types represented. The vast majority are microscopic, many measuring less than 5 one-thousandth of an inch (or 0.005 millimeters), but some, such as the freshwater Spirostomun, may reach 0.17 inches (3 millimeters) in length, large enough to enable it to be seen with the naked eye.

ZOONOSES: Zoonoses are diseases of microbiological origin that can be transmitted from animals to people. The causes of the diseases can be bacteria, viruses, parasites, and fungi.

Oocysts are shed in the feces of cats and the other animals. People can ingest the oocysts after stroking a cat's fur (on which oocysts can stick, although this route is rare), by handling a cat's litter box and not properly washing their hands before hand-to-mouth contact, by eating produce that was irrigated with oocyst-contaminated water, or by eating undercooked meat that contains the protozoan. Eating undercooked meat is the most common route of infection.

IN CONTEXT: PERSONAL RESPONSIBILITY AND PROTECTION

The Centers for Disease Control and Prevention (CDC), Division of Parasitic Diseases recommends the following to reduce chances of becoming infected with Toxoplasma:

• Wear gloves when you garden or do anything outdoors that involves handling soil. Cats, which may pass the parasite in their feces, often use gardens and sandboxes as litter boxes. Wash your hands well with soap and water after outdoor activities, especially before you eat or prepare any food.
• When preparing raw meat, wash any cutting boards, sinks, knives, and other utensils that might have touched the raw meat thoroughly with soap and hot water to avoid cross-contaminating other foods. Wash your hands well with soap and water after handling raw meat.
• Cook all meat thoroughly; that is, to an internal temperature of 160° F (71° C) and until it is no longer pink in the center or until the juices become colorless. Do not taste meat before it is fully cooked.

SOURCE: Division of Parasitic Diseases. The Centers for Disease Control and Prevention (CDC)

Following the regeneration of the T. gondii oocysts into the growing form, the symptoms of toxoplasmosis are produced. These include a fever that comes and goes, swollen lymph nodes, generalized muscle pain, and fatigue. For those who recover fairly rapidly, protection from a future infection is guaranteed for life. For others, toxoplasmosis can persist—this is generally referred to as a chronic infection. Chronic toxoplasmosis can cause retinochoroiditis, which is an inflammation of the eyes. This condition can cause a yellowing of the skin and the whites of the eyes that is called jaundice. More seriously, inflammation of the brain, which is called encephalitis, can produce numbness, severe headaches, impaired vision or even blindness, and convulsions.

Toxoplasmosis is not readily spread from person to person. An exception is the spread from mother to fetus that can occur during pregnancy. Approximately six of every 1,000 pregnant women acquire the infection; about half of these women pass the infection on to the fetus. In the United States, over 3,000 cases of congenital toxoplasmosis occur each year. In newborns, toxoplasmosis can be rapidly lethal. Other newborns will retain the infection and display symptoms months or years later.

Scope and Distribution

Toxoplasmosis is global in distribution and its incidence is common. Up to 60% of the world's population may carry the protozoan. In the United States alone, over 60 million people are thought to be infected with T. gondii.

Treatment and Prevention

As for many other microbial diseases, good personal hygiene including handwashing is an important preventative measure. Pregnant women should not handle cat litter. Common sense food handling precautions including washing cutting boards after use help minimize the risk of transferring meat-borne T. gondii.

Medication can be prescribed for pregnant women and those with AIDS to kill the protozoan, even those residing in the brain. Other medications prevent the protozoan from acquiring vitamin B, which is vital for its survival.

Impacts and Issues

For most people who become infected with T. gondii, there is little concern, as they have the immune system capability to fight off the infection. But, for people with a malfunctioning immune system, toxoplamosis is a serious, even lethal, disease. The millions of people with AIDS, infants, the elderly, and those whose immune system has been deliberately impaired to avoid rejection of a transplant are at risk.

Toxoplasmosis is also becoming an indicator of how human activity can affect other forms of life. Along the coast of California, deaths of sea lions and sea otters due to toxoplasmosis has been increasing from the 1990s to the present. The cause is thought to be the disposal of cat litter in municipal waste; T. gondii oocysts survive the journey to the ocean water, where they can infect the sea lions and sea otters.

IN CONTEXT: EFFECTIVE RULES AND REGULATIONS FOR KEEPING A CAT

Some people living in public housing or special care centers are forced to give up beloved pets due to illness or fear of Toxoplasma. However, the Centers for Disease Control & Prevention, National Center for HIV, STD, and TB Prevention, Divisions of HIV/AIDS Prevention state that even persons at risk for a severe infection (e.g., you have a weakened immune system or are pregnant) may still keep cats as pets (often offering love, comfort, and other emotional benefits) if at risk persons follow safety precautions as shown below to avoid being exposed to Toxoplasma. Persons at risk should consult with their personal health care provider for full details.

• Have someone who is healthy and not pregnant change your cat's litter box daily. If this is not possible, wear gloves and clean the litter box every day, because the parasite found in cat feces needs one or more days after being passed to become infectious. Wash your hands well with soap and water afterward.
• Keep your cat indoors to prevent it from hunting. Feed your cat dry or canned cat food rather than allowing it to have access to wild birds and rodents or to eat food scraps. A cat can become infected by eating infected prey or by eating raw or undercooked meat infected with the parasite. Do not bring a new cat into your house that might have spent time out of doors or might have been fed raw meat.
• Feed your cat only cat food or cook all meat thoroughly before giving it to your cat.
• Do not give your cat raw or undercooked meat.
• If you adopt or buy a cat, get one that is healthy and at least 1 year old.
• Avoid stray cats and kittens. They are more likely than other cats to be infected.
• Your veterinarian can answer any other questions you may have regarding your cat and risk for toxoplasmosis.

SOURCE: Centers for Disease Control & Prevention, National Center for HIV, STD, and TB Prevention, Divisions of HIV/AIDS Prevention

See AlsoParasitic Diseases; Zoonoses.

BIBLIOGRAPHY

Books

Fields, Denise, and Ari Brown. Toddler 411: Clear Answers and Smart Advice for Your Toddler. Boulder: Windsor Peak Press, 2006.

Joynson, David H.M., and Tim G. Wreghitt. Toxoplasmosis. Cambridge: Cambridge University Press, 2005.

Lindsay, David S., and Louis M. Weiss. Opportunistic Infections: Toxoplasma, Sarcocystis, and Microsporidia (World Class Parasites). New York: Springer, 2004.

Web Sites

Centers for Disease Control and Prevention. “Toxoplasmosis: An Important Message for Cat Owners.” <http://www.cdc.gov/ncidod/dpd/parasites/toxoplasmosis/toxoplasmosis_brochure_8.2004.pdf> (accessed on April 2, 2007).

Brian Hoyle