Toxoplasmosis

Introduction
Diagnosis
Treatment and Control
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Introduction

Toxoplasmosis is a parasitic disease caused by the protozoan, Toxoplasma gondii. The parasite infects warm-blooded animal, including birds. The disease and protozoan are found worldwide. The definitive host is any of the wild or domestic felids (cats). A definitive host is one in which the organism reaches maturity and reproduces. There are 3 infectious stages of the organism, tachyzoites (rapidly multiplying form), bradyzoites (tissue cyst form), and sporozoites (in oocysts). Animals, including humans, may be infected by contact with cat feces containing the infectious oocysts, consuption of raw infected meat (bradyzoites), or trasmission from mother to fetus (tachyzoites).

Following consumption of uncooked meat containing tissue cysts (carnivores) or feed or drink contaminated with cat feces containing oocysts (all warm-blooded animals), Tgondii initiates extraintestinal replication. Bradyzoites and sporozoites, respectively, are released and infect intestinal epithelium. After several rounds of epithelial replication, tachyzoites emerge and disseminate via the bloodstream and lymph. Tachyzoites infect tissues throughout the body and replicate intracellularly until the cells burst, causing tissue necrosis. Young and immunocompromised animals may succumb to generalized toxoplasmosis at this stage. Older animals mount a powerful cell-mediated immune response to the tachyzoites (mediated by cytokines) and control infection, driving the tachyzoites into the tissue cyst or bradyzoite stage. Tissue cysts in the host remain viable for many years, and possibly for the life of the host.

In young animals, particularly puppies, kittens, and piglets, tachyzoites spread systemically and cause interstitial pneumonia, myocarditis, hepatic necrosis, meningoencephalomyelitis, chorioretinitis, lymphadenopathy, and myositis. After infection of a pregnant ewe, tachyzoites spread via the bloodstream to placental cotyledons, causing necrosis. Tachyzoites may also spread to the fetus, causing necrosis in multiple organs. Finally, immunocompromised adult animals (eg, cats infected with feline immunodeficiency virus) are extremely susceptible to developing acute generalized toxoplasmosis.

Between 22 - 33% of the US population has been exposured to the protozoan and have developed antibodies to the organism. This rate can be even higher with up to 60% of the world's population haven been exposed to the organism.

Diagnosis

During the first phase of infection, the individual may exhibit mild flu-like symptoms. Unless immune comprimised, after this intial phase there are no symptoms. Diagnosis is made by biologic, serologic, or histologic methods, or by some combination of the above. Clinical signs of toxoplasmosis are nonspecific and are not sufficiently characteristic for a definite diagnosis.

Treatment and Control

For animals other than humans, treatment is seldom warranted. Sulfadiazine and pyrimethamine act synergistically and are widely used for treatment of toxoplasmosis. While these drugs are beneficial if given in the acute stage of the disease when there is active multiplication of the parasite, they will not usually eradicate infection. These drugs are believed to have little effect on the bradyzoite stage. Certain other drugs, including diaminodiphenylsulfone, atovaquone, and spiramycin are also used to treat toxoplasmosis in difficult cases. Clindamycin is the treatment of choice for dogs and cats and is given for 14-21 days.

Tgondii is an important zoonotic agent (can be transmitted between animals and humans). Toxoplasmosis is a major concern for people with immune system dysfunction (eg, people infected with human immunodeficiency virus). In these individuals, toxoplasmosis usually presents as meningoencephalitis and results from the emergence of Tgondii from tissue cysts located in the brain as immunity wanes rather than from primary Tgondii infection. Toxoplasmosis is also a major concern for pregnant women because tachyzoites can migrate transplacentally and cause birth defects in human fetuses. Infection of women with Tgondii may occur after ingestion of undercooked meat or accidental ingestion of oocysts from cat feces. To prevent infection, the hands of people handling meat should be washed thoroughly with soap and water after contact, as should all cutting boards, sink tops, knives, and other materials. The stages of Tgondii in meat are killed by contact with soap and water. Tgondii organisms in meat can also be killed by exposure to extreme cold or heat. Tissue cysts in meat are killed by heating the meat throughout to 67°C or by cooling to -13°C. Toxoplasma in tissue cysts are also killed by exposure to 0.5 kilorads of gamma irradiation. Meat of any animal should be cooked to 67°C before consumption, and tasting meat while cooking or while seasoning should be avoided. Pregnant women should avoid contact with cat litter, soil, and raw meat. Pet cats should be fed only dry, canned, or cooked food. The cat litter box should be emptied daily, preferably not by a pregnant woman. Gloves should be worn while gardening. Vegetables should be washed thoroughly before eating because they may have been contaminated with cat feces.

In about 40 percent of the cases in which a pregnant woman has toxoplasmosis, the baby is also infected. Infants who become infected during pregnancy are said to have 'congenital toxoplasmosis' infection. In the United Sates, 1 to 2 per 1000 babies that are born each year have toxoplasmosis. About one in 10 infected babies has a severe Toxoplasma infection evident at birth. These newborns often have eye infections, an enlarged liver and spleen, jaundice (yellowing of the skin and eyes), and pneumonia. Some die within a few days of birth. Those who survive can have mental retardation, severely impaired eyesight, cerebral palsy, seizures and other problems. Although up to 90 percent of infected babies appear normal at birth, between 55 and 85 percent of them develop problems months to years later, including eye infections that may affect sight, hearing loss and learning disabilities. Toxoplasmosis during pregnancy also can result in miscarriage or stillbirth.

Only active infections result in problems during pregnancy. Previous infection does not appear to cause problems during human pregnancies. Even in cats, they appear to only be infected once during their life, developing immunity to subsequent infection.

At present there is no vaccine to prevent toxoplasmosis in humans.

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