Question
Toxoplasmosis
Answer
Toxoplasmosis is caused by Toxoplasma gondii, an obligate intracellular protozoan parasite, with cats as the definitive host and a wide range of warm-blooded animals (including humans) as intermediate hosts.
Transmission: ingestion of oocysts from cat faeces-contaminated soil/food/water; ingestion of undercooked meat containing tissue cysts; transplacental (congenital) transmission — of major clinical importance.
Clinical significance:
- Immunocompetent hosts — usually asymptomatic or mild (occasionally a mononucleosis-like illness with lymphadenopathy).
- Immunocompromised hosts (particularly AIDS) — reactivation of latent infection causes toxoplasma encephalitis, an AIDS-defining opportunistic infection, presenting with focal neurological deficits, seizures, and ring-enhancing brain lesions on imaging.
- Congenital toxoplasmosis — primary maternal infection during pregnancy can cause severe fetal disease: chorioretinitis, hydrocephalus, intracranial calcification, and other malformations, particularly severe if acquired in early pregnancy.
Laboratory diagnosis: serology (IgM/IgG ELISA) — IgM indicates recent infection, IgG avidity testing helps date infection timing (important in pregnancy); PCR on amniotic fluid for suspected congenital infection; imaging (CT/MRI showing ring-enhancing lesions) plus response to empirical treatment for toxoplasma encephalitis in AIDS; histopathology/tissue biopsy demonstrating tachyzoites in selected cases.
Treatment: pyrimethamine plus sulfadiazine (with folinic acid to reduce marrow toxicity) is the standard regimen for active disease.