What Is Toxoplasma Gondii?

Toxoplasma infection, which can lead to toxoplasmosis, can be transmitted by cats to humans, and poses a special danger to pregnant women.

Toxoplasmosis is a disease of cats as well as other mammals and birds caused by a parasite called Toxoplasma gondii. Toxoplasma infection is common, but full-blown disease is rare. T. gondii is important because virtually all warm-blooded animals, including man, can become infected with it. Domestic, wild, and feral cats can transmit Toxoplasma infection to humans.

Cats have a 20% to 60% infection rate with T. gondii. Its prevalence is related to factors including the ingestion of infected animals such as rodents and birds or raw meat. Stray and feral cats have a higher incidence of infection than pet cats, and older cats have more of a chance of acquiring it.

Toxoplasmosis is most common in cats less than two years old, possibly because of their poorly developed immune response. In an older cat, a recurrent infection may be due to the presence of feline leukemia virus or feline immunodeficiency virus, which suppress its immune response.

Symptoms of toxoplasmosis include anorexia, weight loss, lethargy, difficulty breathing (because of pneumonia), eye inflammation, and fever. Other symptoms are vomiting and diarrhea, neurological symptoms, swollen lymph nodes, and jaundice.

The life cycle of the T. gondii parasite has three stages: cyst, oocyst, or tachyzoite. A cat may eat cysts in infected rodents or birds, or other raw meat, whereupon the organisms will begin to multiply in the wall of the small intestine, producing the second stage, oocysts. These are excreted in the feces for two to three weeks. Then they may become spores, and become infectious to other animals, including humans. Most exposed cats shed oocysts during acute Toxoplasma infection, but not after. Oocysts are very hardy and can survive in moist shaded soil or sand for months.

In the intestine, some Toxoplasma organisms multiply as tachyzoites, which spread to other sites in the body. This stage eventually ends, and a "resting" stage produces cysts in the muscles and brain. Most of these cysts probably remain dormant indefinitely.

Most cats show no symptoms of infection with Toxoplasma. Sometimes toxoplasmosis occurs, mostly to kittens and young adult cats, with lethargy, depression, loss of appetite, and fever. Pneumonia is often a major symptom. Liver inflammation may lead to vomiting, diarrhea, and jaundice. The pancreas may be inflamed and the lymph nodes enlarged. Other signs are inflammation of the retina, abnormal-sized pupils, blindness, lack of coordination, personality changes, circling, ear twitching, difficulty in chewing and swallowing food, seizures, and loss of control over urination and defecation.

As much as 50% of the human population in the United States and 20% to 80% of all domestic animals are infected with Toxoplasma, harboring its cyst form. If given the opportunity, the cyst can produce disease in immunocompromised patients, such as people with AIDS.

But the greatest concern for humans is transmission of Toxoplasma from mother to fetus. Up to 45% of American women between the ages of 20 and 39 have already been exposed to Toxoplasma and are therefore immune. But in mothers who first acquire Toxoplasma infection during their pregnancy, about one-third to one-half of their infants born to mothers are also infected. Toxoplasma infection of the fetus is least common, but the disease is most severe, if the mother has been infected during the first trimester of pregnancy. Toxoplasma infection is most common, but the disease is least severe or without symptoms, if the infection occurs during the third trimester. Most women infected during pregnancy have no symptoms of the infection themselves, but there is about a 40% chance that the fetus will acquire the infection, and be born prematurely or even stillborn. In about 10% of these cases, severe neurological or eye disease will result. Toxoplasma infection results in over three thousand human congenital infections annually in the United States, most of them with no symptoms. In symptomatic individuals, symptoms may appear at different times: at birth, or weeks, months, or even years later.

In AIDS patients or those being treated for cancer or organ transplantation, enlargement of the lymph nodes, eye and neurologic disturbances, respiratory problems, and heart disease are common symptoms. For these patients the mortality rate is high.

The good news is that most studies show that contact with a pet cat or a neighbor's cat will not increase the risk of acquiring toxoplasmosis, unless these animals are allowed to roam and hunt or eat raw meat. However, contact with stray and feral cats can be risky.

Blood serology tests may indicate recent infection with T. gondii, but not a definitive diagnosis of toxoplasmosis. Its diagnosis relies on the history of the patient, symptoms, and the results of laboratory tests, including an antibody titer test and microscopic examination of tissues. If a healthy cat has high antibody levels, that means it has been previously infected and is now probably immune and not excreting oocysts to other animals. If there is no antibody, the cat can become infected and will shed oocysts for one to two weeks after exposure.

Pyrimethamine and sulfadiazine are the two drugs used to treat toxoplasmosis. They act together to inhibit Toxoplasma reproduction. However, pyrimethamine may be unpalatable or even toxic to some cats. Another choice is the antibiotic clindamycin, which has fewer side effects. Alternative veterinarians recommend modalities such as herbs, homeopathy, and acupuncture to treat the symptoms of toxoplasmosis.

No vaccine is available as yet.

What can you do to control the spread of Toxoplasma? Restrict your pet cats from access to rodents and birds, and give them only cooked meat, commercially prepared cat food, and pasteurized dairy products. Don't allow them to scavenge in garbage cans.

Transmission of oocysts present in garden soil, children's sandboxes, litter boxes, and anywhere cats may defecate can be avoided by wearing rubber gloves during contact with contaminated soil or litter. Wash your hands thoroughly after contact. Cover sandboxes to prevent cats from defecating in them. Get rid of feces from litter boxes every day, and disinfect them with scalding water or dry heat; chemical disinfection is not effective in destroying oocysts.

If you are pregnant, or plan to become pregnant soon, test your cats for antibodies to Toxoplasma. As mentioned above, do not allow cats access to birds, rodents, uncooked meat, and also unpasteurized dairy products. Don't handle litter boxes; have another person change them daily. Stay away from free-roaming cats because they can be contaminated with oocysts. Keep any outdoor cat from contact with bedclothes and any furnishings that you use. Don't handle a cat that seems ill. Wash uncooked vegetables thoroughly before eating. Wash hands thoroughly with soap and water after contact with soil and cats.

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