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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 human 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%. In a more recent evaluation using data from NHANES 2009-2010, the overall age-adjusted T. gondii antibody seroprevalence among persons > 6 years of age was 12.4%, and among women 15–44 years of age was 9.1%.

Clinical Presentation

Acquired infection with Toxoplasma in immunocompetent persons is generally an asymptomatic infection.  However, 10% to 20% of patients with acute infection may develop cervical lymphadenopathy and/or a flu-like illness.  The clinical course is usually benign and self-limited; symptoms usually resolve within a few weeks to months.  In rare cases ocular infection with visual loss can occur.  Immunodeficient patients often have central nervous system (CNS) disease but may have retinochoroiditis, pneumonitis, or other systemic disease.  In patients with AIDS, toxoplasmic encephalitis is the most common cause of intracerebral mass lesions and is thought to usually be caused by reactivation of chronic infection.  Toxoplasmosis in patients being treated with immunosuppressive drugs may be due to either newly acquired or reactivated latent infection.

Congenital toxoplasmosis results from an acute primary infection acquired by the mother during pregnancy.  The incidence and severity of congenital toxoplasmosis vary with the trimester during which infection was acquired.  Because treatment of the mother may reduce the incidence of congenital infection and reduce sequelae in the infant, prompt and accurate diagnosis is important.  Many infants with subclinical infection at birth will subsequently develop signs or symptoms of congenital toxoplasmosis.  Ocular Toxoplasma infection, an important cause of retinochoroiditis in the United States, can be the result of congenital infection, or infection after birth.  In congenital infection, patients are often asymptomatic until the second or third decade of life, when lesions develop in the eye.

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  • Page last reviewed July 6, 2015
  • Page last updated July 6, 2015
  • Content source: Global Health - Division of Parasitic Diseases and Malaria
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