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[Toxoplasma gondii]

 Toxoplasma gondii tachyzoites, stained with Giemsa, from a smear of peritoneal fluid obtained from a laboratory-inoculated mouse.

Toxoplasma gondii tachyzoites, stained with Giemsa, from a smear of peritoneal fluid obtained from a laboratory-inoculated mouse.

Negative IFA for antibodies to T. gondii, polar stain reaction.

Toxoplasma gondii cyst stained with hematoxylin and eosin.

Causal Agent

Toxoplasma gondii is a protozoan parasite that infects most species of warm blooded animals, including humans, and can cause the disease toxoplasmosis.

Life Cycle

Life cycle of Toxoplasmosis

The only known definitive hosts for Toxoplasma gondii are members of family Felidae (domestic cats and their relatives). Unsporulated oocysts are shed in the cat’s feces The number 1. Although oocysts are usually only shed for 1-2 weeks, large numbers may be shed. Oocysts take 1-5 days to sporulate in the environment and become infective. Intermediate hosts in nature (including birds and rodents) become infected after ingesting soil, water or plant material contaminated with oocysts The number 2. Oocysts transform into tachyzoites shortly after ingestion. These tachyzoites localize in neural and muscle tissue and develop into tissue cyst bradyzoites The number 3. Cats become infected after consuming intermediate hosts harboring tissue cysts The number 4. Cats may also become infected directly by ingestion of sporulated oocysts. Animals bred for human consumption and wild game may also become infected with tissue cysts after ingestion of sporulated oocysts in the environment The number 5. Humans can become infected by any of several routes:

  • eating undercooked meat of animals harboring tissue cysts The number 6.
  • consuming food or water contaminated with cat feces or by contaminated environmental samples (such as fecal-contaminated soil or changing the litter box of a pet cat) The number 7.
  • blood transfusion or organ transplantation The number 8.
  • transplacentally from mother to fetus The number 9.

In the human host, the parasites form tissue cysts, most commonly in skeletal muscle, myocardium, brain, and eyes; these cysts may remain throughout the life of the host. Diagnosis is usually achieved by serology, although tissue cysts may be observed in stained biopsy specimens The number 10. Diagnosis of congenital infections can be achieved by detecting T. gondii DNA in amniotic fluid using molecular methods such as PCR The number 11.

Geographic Distribution

Serologic prevalence data indicate that toxoplasmosis is one of the most common of humans infections throughout the world. A high prevalence of infection in France has been related to a preference for eating raw or undercooked meat, while a high prevalence in Central America has been related to the frequency of stray cats in a climate favoring survival of oocysts and soil exposure. The overall seroprevalence in the United States among adolescents and adults, as determined with specimens collected by the third National Health and Nutrition Examination Survey (NHANES III) between 1988 and 1994, was found to be 22.5%, with a seroprevalence among women of childbearing age (15 to 44 years) of 15%.

Clinical Presentation

The diagnosis of toxoplasmosis may be documented by:

  • Observation of parasites in patient specimens, such as bronchoalveolar lavage material from immunocompromised patients, or lymph node biopsy.
  • Isolation of parasites from blood or other body fluids, by intraperitoneal inoculation into mice or tissue culture. The mice should be tested for the presence of Toxoplasma organisms in the peritoneal fluid 6 to 10 days post inoculation; if no organisms are found, serology can be performed on the animals 4 to 6 weeks post inoculation.
  • Detection of parasite genetic material by PCR, especially in detecting congenital infections in utero.
  • Serologic testing is the routine method of diagnosis.

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  • Page last reviewed November 29, 2013
  • Page last updated November 29, 2013
  • Content source: Global Health - Division of Parasitic Diseases and Malaria
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