Infection: Toxoplasmosis

Definition
Description
Demographics
Causes and Symptoms
Diagnosis
Treatment
Prognosis
Prevention
The Future
For more information

Definition

Toxoplasmosis is an infectious disease caused by a one-celled parasitic protozoon. It gets its name from the scientific name of the parasite, Toxoplasma gondii. It is classified as a zoonosis because it can be transmitted from animals to people.

Description

Toxoplasmosis is an infectious disease caused by a one-celled parasite called Toxoplasma gondii. Although the parasite reproduces sexually only inside cats, it is widespread in many other animals, including rats, mice, birds, deer, and pigs. Most humans who become infected get toxoplasmosis from eating raw or undercooked meat or drinking water containing the parasite rather than from pet cats.

About 90 percent of people who have been infected by the parasite have no symptoms at all; some people develop a flu-like illness that lasts about a month; and a few people with weakened immune systems develop severe disease that can particularly affect the brain. It is also possible for a pregnant woman who is infected by the parasite for the first time during pregnancy to pass the infection to her unborn baby.

Demographics

Toxoplasmosis is very common in the developed world, particularly in countries where people like to eat raw or undercooked meat. It is

estimated that about 20 percent of the population of the United States, or 60 million people, have been infected by the parasite. Around the world, the average rate of infection is about 30 percent, but in some countries like France, the rate hovers around 75 percent because so many people prefer undercooked meat.

Toxoplasmosis is the third most common cause of fatalities from food-related illnesses in the United States; it causes about 250,000 cases each year, resulting in 5,000 hospitalizations and about 750 deaths. About 3,500 babies are born each year in the United States with toxoplasmosis.

The rate of illness is high among patients with AIDS because of their weakened immune systems. Florida has a particularly high rate of toxoplasmosis in AIDS patients; as many as 40 percent are infected. Between 1 and 5 percent of AIDS patients in the United States eventually develop encephalitis (inflammation of the brain) from toxoplasmosis.

Toxoplasmosis affects both sexes and all races equally, as far as is known.

Causes and Symptoms

The protozoon that causes toxoplasmosis has a complex life cycle but reproduces sexually only inside cats. When a cat eats a dead mouse, rat,

bird, or raw meat that contains cysts (a capsule containing the parasite in its resting stage), the cysts open up within the cat's digestive tract. There they reproduce and form new cysts, which are expelled in the cat's feces. Cats shed these cysts for only one or two weeks, after which they cannot spread the infection further.

It takes the cysts in the cat's feces between one and five days to mature. This delay is the reason why cleaning a cat's litter box on a daily basis helps to prevent toxoplasmosis in humans. The parasite's cysts are hardy; they can live for a year in garden soil. They can survive freezing temperatures as low as 10°F and cannot be killed by household bleach or detergents. The cysts can, however, be killed by freezing at temperatures below 0°F (− 18°C) or by heating food containing them to temperatures above 150°F (66°C).

People can get toxoplasmosis in one of several ways: from eating uncooked meat containing cysts; by eating unwashed vegetables that have been contaminated with cysts from garden soil; by drinking water contaminated by the parasite; by failing to wash hands after handling cat feces and getting cysts into the mouth accidentally; by direct transmission from mother to child; and by receiving an infected organ by transplantation or infected blood through transfusion (very rare). People cannot get the disease from touching or being close to an infected child or adult or from petting an infected cat.

Keeping Kitty Healthy

Although cats are carriers of the toxoplasmosis parasite, people do not get the disease from petting or playing with a pet cat, since cats do not carry the parasite on their fur. They also do not pass the parasite to humans by scratching them. People who enjoy having cats as pets do not have to give them up even if they have AIDS, are pregnant, or are being treated for cancer.

The American Veterinary Medical Association (AVMA) recommends the following steps to lower the risk of getting toxoplasmosis from a cat:

• Keep the cat indoors. In addition to lowering the cat's risk of getting the parasite by eating dead animals, keeping kitty indoors protects him or her from automobiles, coyotes and other predators, and other parasitic infections.
• Do not feed the cat raw or undercooked meat. Feed only well-cooked meat or processed cat foods.
• Scoop the cat's litter box every day. Disposing of feces as soon as possible prevents toxoplasmosis cysts from maturing.
• Pregnant women should ask another family member to clean the cat's box, or wear disposable gloves and wash the hands well in hot soapy water afterward.
• Have a sick cat checked by a veterinarian. Most cats with toxoplasmosis, like most humans, have no signs of infection at all and recover completely even without treatment.

Once inside the human body, the cysts release the parasite, which forms new cysts, especially in the muscles and brain. In people with healthy immune systems, the cysts usually remain dormant for the rest of theperson'slife.In personswith immune systems weakened by AIDS or chemotherapy for cancer, however, the parasites may cause a flu-like illness or severe infections of the central nervous system.

People with the mild form of toxoplasmosis typically have the following symptoms:

• Headache
• Fever and sore throat
• Sore aching muscles
• Swollen lymph nodes
• Night sweats

Patients with a severe toxoplasmosis infection may have:

• Lung infections resembling pneumonia
• Headache
• Loss of coordination
• Seizures
• Mental confusion and disorientation
• Blurred vision (in people with normal or weakened immune systems)

The risk of harm to an unborn baby from toxoplasmosis is highest if the mother becomes infected during the last three months of pregnancy. Toxoplasmosis can cause a pregnant woman to lose her baby before birth. The baby may also be born with seizures, disorders of the liver, or a severe eye infection. Some babies who are infected with toxoplasmosis before birth may have normal health for several years and then develop hearing loss, eye disorders, or mental retardation in the early teen years.

Diagnosis

People are not routinely screened for toxoplasmosis because most who are infected do not develop symptoms. Those with a mild case of the disease may be thought to have an illness with similar symptoms like flu or mononucleosis. The diagnosis of toxoplasmosis can be made, however, by taking samples of the patient's blood and sending them to a laboratory for analysis.

A pregnant woman who thinks she may have been infected can be tested for toxoplasmosis by having a small sample of the fluid surrounding the baby in the womb withdrawn through a needle and tested for the parasite. This test increases the risk of losing the baby, however. The Centers for Disease Control and Prevention (CDC) advises women

who are planning to become pregnant or have recently become pregnant to have a blood test for toxoplasmosis.

A patient who has symptoms of a brain disorder caused by the parasite can be evaluated by magnetic resonance imaging or a CT scan, as the infection produces characteristic patterns of damage to brain tissue that can be distinguished from brain tumors or other disorders.

Treatment

No treatment is needed for patients who have no symptoms of infection. Patients who have a mild case of the disease are treated with a combination of an antibiotic and a drug that was originally developed to treat malaria. People with AIDS might need to take this drug combination for an extended period to prevent symptoms from recurring. Pregnant women are given an antibiotic by itself, as the antimalarial drug can have serious side effects for both mother and child.

Prognosis

The prognosis of toxoplasmosis depends on the severity of the infection. Most people never know they have been infected. People with a mild form of illness usually recover completely in a few weeks. Those with severe toxoplasmosis may have lifelong eye disorders, personality changes, or seizures; about 15 percent will die from the infection.

Prevention

The CDC recommends the following steps to lower the risk of getting toxoplasmosis:

• Cook foods thoroughly, especially meats. Cook beef, lamb, and veal to 145°F (63°C), pork and venison (deer meat) to 160°F (71°C), and poultry to 180°F (82°C).
• Do not eat raw or undercooked meat.
• Wash knives and cutting boards in hot soapy water after use.
• Do not eat raw unwashed fruits and vegetables.
• Freeze meat for several days before cooking.
• Avoid drinking unpasteurized milk or untreated drinking water, particularly when traveling abroad.
• Wear gloves when working in the garden.
• Take proper care of pet cats (see sidebar).

The Future

There is no vaccine to prevent toxoplasmosis in either humans or other animals. Although an effective vaccine may be developed in the future, the best protection against toxoplasmosis at present is prevention.

SEE ALSO AIDS; Encephalitis; Food poisoning

WORDS TO KNOW

Cyst: A capsule or sac containing a parasite in its resting stage.

Encephalitis: Inflammation of the brain.

Protozoon (plural, protozoa): A one-celled animal-like organism with a central nucleus enclosed by a membrane. Many protozoa are parasites that can cause disease in humans.

Zoonosis: A disease that animals can transmit to humans.

For more information

BOOKS

Flynn, Jason. Cat Care. Avon, MA: Adams Media, 2005.

Parker, James N., MD. The Official Patient's Sourcebook on Toxoplasmosis. San Diego, CA: ICON Health Publications, 2002.

PERIODICALS

Adam, David. “Can a Parasite Carried by Cats Change Your Personality?”Guardian, September 25, 2003. Available online at http://www.guardian.co.uk/science/2003/sep/25/medicineandhealth.thisweekssciencequestions1 (accessed April 13, 2008).

WEB SITES

American Veterinary Medical Association (AVMA). What You Should Know about Toxoplasmosis. Available online at http://www.avma.org/communications/brochures/toxoplasmosis/toxoplasmosis_brochure.asp (updated April 2008; accessed April 12, 2008).

Centers for Disease Control and Prevention (CDC). Toxoplasmosis Fact Sheet. Available online at http://www.cdc.gov/toxoplasmosis/factsheet.html (updated January 16, 2008; accessed April 12, 2008).

Cornell Feline Health Center. Toxoplasmosis in Cats. Available online at http://www.vet.cornell.edu/fhc/brochures/toxo.html (updated April 8, 2008; accessed April 12, 2008).