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  • Infection confirmed by isolation of Toxoplasma gondii or identification of tachyzoites in tissue or body fluids.

Primary infection

  • Fever, malaise, headache, sore throat.

  • Lymphadenopathy.

  • Positive IgG and IgM serologic tests.

Congenital infection

  • Follows acute infection of seronegative mothers and leads to CNS abnormalities and retinochoroiditis.

Infection in immunocompromised persons

  • Reactivation leads to encephalitis, retinochoroiditis, pneumonitis, myocarditis.

  • Positive IgG but negative IgM serologic tests.


T gondii, an obligate intracellular protozoan, is found worldwide in humans and in many species of mammals and birds. The definitive hosts are cats. Humans are infected after ingestion of cysts in raw or undercooked meat, ingestion of oocysts in food or water contaminated by cats, transplacental transmission of trophozoites or, rarely, direct inoculation of trophozoites via blood transfusion or organ transplantation (eFigure 35–10). Toxoplasma seroprevalence varies widely. It has decreased in the United States to 20–30% or less, but it is much higher in other countries in both the developed and developing worlds, where it may exceed 80%. In the United States, T gondii is estimated to infect 1.1 million persons each year, with resultant chorioretinitis developing in 21,000 and vision loss in 4800.

eFigure 35–10.

Life cycle of Toxoplasma gondii. The only known definitive hosts for T gondii are members of family Felidae (domestic cats and their relatives). Unsporulated oocysts are shed in the cat’s feces image. 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 image. Oocysts transform into tachyzoites shortly after ingestion. These tachyzoites localize in neural and muscle tissue and develop into tissue cyst bradyzoites image. Cats become infected after consuming intermediate hosts harboring tissue cysts image. 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 image. Humans can become infected by any of several routes:

  • eating undercooked meat of animals harboring tissue cysts image.

  • 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) image.

  • blood transfusion or organ transplantation image.

  • transplacentally from mother to fetus image.

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 image. Diagnosis of congenital infections can be achieved by detecting T gondii DNA in amniotic fluid using molecular methods such as PCR image. (From Global Health, Division of Parasitic Diseases and Malaria, CDC.)

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