Toxoplasmosis Clinical Spectrum
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Direct answer
Toxoplasmosis is an infection caused by the protozoan Toxoplasma gondii, and its severity depends on the host's immune status and on when a pregnant woman is infected. In immunocompetent adults it is usually a self-limited lymphadenopathy, while in congenital infection and in people with low CD4 counts it causes serious disease. Pyrimethamine combined with sulfadiazine and folinic acid is the standard treatment for active disease.
What you must remember
- Definitive host: the cat, which sheds oocysts in faeces; humans acquire infection by ingesting oocysts, undercooked meat containing tissue cysts, or unwashed contaminated produce.
- Immunocompetent adults: usually asymptomatic or a self-limited cervical lymphadenopathy with mononucleosis-like features, and treatment is generally not needed.
- Nodal histology: reactive follicular hyperplasia with clusters of epithelioid histiocytes and monocytoid B cells, without the architectural effacement seen in lymphoma.
- Congenital triad: chorioretinitis, hydrocephalus and intracranial calcification; transmission rises with gestational age, while the severity of fetal disease is greatest after early maternal infection.
- Ocular disease: retinochoroiditis, usually from congenital infection or reactivation, with a necrotic focus that often lies next to an old pigmented scar.
- Cerebral disease: reactivation in profound immunosuppression causes multiple ring-enhancing lesions on imaging, typical of advanced HIV with low CD4 counts.
- Treatment: pyrimethamine with sulfadiazine plus folinic acid, because folinic acid limits the bone marrow suppression caused by pyrimethamine.
- Diagnosis: IgM and IgG serology with IgG avidity testing to judge timing in pregnancy, and PCR on amniotic fluid after about 18 weeks of gestation.
A clinical spectrum sorted by host and setting
Toxoplasmosis looks different depending on who is infected and where the parasite lodges. The table sets the four common presentations side by side.
| Setting | Typical picture | Key investigation | Management |
|---|---|---|---|
| Immunocompetent adult | Painless cervical nodes, fever, fatigue | IgM and IgG serology; node histology | Usually self-limited; treat if severe or ocular |
| Congenital infection | Chorioretinitis, hydrocephalus, calcification | Serology, amniotic fluid PCR, neonatal imaging | Pyrimethamine, sulfadiazine, folinic acid |
| Acquired ocular disease | Unilateral retinochoroiditis with vitritis | Clinical examination, serology | Treat active lesions with the same drug combination |
| Advanced HIV, low CD4 | Ring-enhancing brain lesions, seizures | CT or MRI, serology, PCR where available | Empirical therapy, then prophylaxis at low CD4 counts |
The pattern to remember is that immunocompetent disease is mild and nodal, while immunosuppressed disease is cerebral and dangerous. Prophylaxis with cotrimoxazole is used in HIV when the CD4 count is low and Toxoplasma serology is positive. Ocular disease can recur, so follow-up matters even after the acute phase settles.
Where the exam tries to catch you
Toxoplasmosis questions tend to test serology interpretation, the folinic acid rule and the ring-enhancing lesion trap.
- Trap: treating any positive IgG as active infection Correct: IgG shows past exposure, and IgM with low avidity helps date recent infection, which matters in pregnancy.
- Trap: giving pyrimethamine without folinic acid Correct: folinic acid reduces marrow suppression, and blood counts still need monitoring during therapy.
- Trap: assuming congenital toxoplasmosis is always severe Correct: infants infected late in pregnancy are often asymptomatic at birth, although later sequelae can still appear.
- Trap: biopsying a ring-enhancing lesion in AIDS before a trial of therapy Correct: empirical anti-toxoplasma treatment is usually tried first, because primary CNS lymphoma is the main alternative.
Frequently asked questions
Which investigation dates an acute toxoplasma infection in pregnancy?
IgM serology combined with IgG avidity testing helps date an acute toxoplasma infection in pregnancy. Low avidity suggests recent infection, while high avidity points to infection acquired several months earlier.
What is the classic congenital toxoplasmosis triad?
The classic triad in congenital toxoplasmosis is chorioretinitis, hydrocephalus and intracranial calcification. Severe congenital disease can also present with a generalised picture of hepatosplenomegaly, jaundice and thrombocytopenia.
Why is folinic acid given along with pyrimethamine?
Folinic acid is given with pyrimethamine to reduce bone marrow suppression, since pyrimethamine inhibits folate metabolism in the host as well as in the parasite. Blood counts still need regular monitoring during treatment.
Which lymph node histology favours toxoplasmosis over lymphoma?
Reactive follicular hyperplasia with clusters of epithelioid histiocytes, along with monocytoid B cells, favours toxoplasmosis. Lymphoma usually shows effacement of the nodal architecture, which the toxoplasma pattern does not.
How is toxoplasma lymphadenopathy managed in an immunocompetent adult?
Toxoplasma lymphadenopathy in an immunocompetent adult is usually self-limited and often managed by observation. Treatment is considered for severe, prolonged or ocular disease, and the drug choice follows the same principles as for active toxoplasmosis.