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Toxoplasma gondii (Toxoplasmosis) (Pathogen)

Organism:
This organism is closely related to other coccidia and has similarities to malarial parasites, is a true pathogen, and causes toxoplasmosis.  Both the tachyzoites (crescent shaped, 2-3 µm wide by 4-8 µm long) and cyst forms containing the bradyzoites (may contain few to hundreds of organisms, may reach 100 µm long and 7-10 µm wide) can be found in clinical specimens.

toxoplas1 toxoplas2 toxoplas3
Tachyzoites Bradyzoites Tachyzoites in Bone Marrow

Life Cycle:
Oocysts in cat feces transmitted to mammals/birds; trophozoites, cysts in meat; sexual forms again develop in GI cells of the cat.

Acquired:
Fecal-oral transmission via oocyst form from members of the Felidae family (house cat, most common); poorly cooked meats and water contaminated with oocysts.  Tissue transplantation, blood transfusion, and congenital infections are known, as well.

Epidemiology:  Worldwide
Human infection can be acquired through ingestion or handling of infected meat or through ingestion of infective oocysts, which can remain viable within cool, moist soil for a year or longer. Certainly, hand washing is highly recommended when there has been potential exposure to the oocysts. It is recommended that meat be cooked so that the internal temperature reaches 150°F (66°C). The question of the house cat always arises; indoor cats fed on dry, canned, or boiled food are unlikely to be infected. However, cats that go outside and have access to other infected animals (birds or mice) may shed oocysts in their feces; therefore, other preventive measures, including changing the litter box daily and disinfecting the pan with boiling water, have been recommended. Also, the feces should not be placed onto the soil but, rather, should be disposed of either in the toilet or within bags.

Clinical Features:
Large numbers of people who are serologically active suggest the majority of infections are benign, with few or no symptoms (e.g., cold or light case of the flu).  The most severe symptoms are seen with congenital, transplacental infections or infections in the compromised patient. Congenital infections (acquired in first or second trimester) include:  retinochoroiditis, cerebral calcification, and possibly hydrocephalus or microcephaly.  CNS involvement may not appear until several years later.  Infections acquired postnatally can be categorized as:  (1) lymphadenitis, fever, headache, myalgia, (2) typhus-like exanthemous form with myocarditis, meningoencephalitis, atypical pneumonia, and possibly death, (3), CNS involvement, usually fatal, and (4) retinochoroiditis, which may be severe.  Most common symptoms in adults are local or generalized lymphadenopathy (nodes of the neck).  In immunocompromised patients, CNS symptoms are common.
Chorioretinitis in immunocompetent patients is generally due to an earlier congenital infection. Patients may be asymptomatic until the second or third decade; at that point, cysts may rupture with lesions then developing in the eye.  The number of people who develop chorioretinitis later in life is unknown but may represent over two‑thirds. Also, up to 30% of patients relapse after treatment.  Chorioretinitis is usually bilateral in patients with congenitally acquired infection, and is generally unilateral in patients with recently acquired infection.

Clinical Specimen:
Various serologic procedures are recommended (very complex diagnostic algorithms – serology results may be difficult to interpret; biopsy specimens, buffy coat cells, CSF, BAL

Laboratory Diagnosis:
Serologies; can also use PCR, biopsy specimens, buffy coat cells, CSF fluid, or isolation in tissue culture

Organism Description:
Tachyzoite:  Crescent shaped – may resemble smaller gametocyte of Plasmodium falciparum.
Cyst:  Cysts containing bradyzoites may be somewhat elongate.

Laboratory Report:  Toxoplasma gondii – serology results are reported

Treatment:
Garcia, L.S. 2007.  Diagnostic Medical Parasitology, 5th ed., ASM Press, Washington, D.C.

Control:
Improved cooking of meats, adequate disposal of cat fecal waste, adequate washing of contaminated litter boxes, hand washing when potentially exposed to oocysts

Comments:
Serologic testing in congenital infections often requires multiple specimens, and procedures, as well as the need for experienced interpretation.