Toxoplasma gondii, an intracellular protozoan that lies dormant in infected individuals and rarely reemerges unless there are significant deficits in cell-mediated immunity.
Although toxoplasmosis may affect multiple organ systems (eg, pulmonary, ocular), encephalitis is by far the most common manifestation.
| Toxo encephalitis | |
|---|---|
| Symptoms | • Headache |
| • Confusion | |
| • Fever | |
| • Focal neurologic deficits/seizures | |
| Diagnosis | • AIDS with CD4 count <100/mm3 |
| • Positive Toxoplasma gondii IgG | |
| • MRI: Multiple ring-enhancing brain lesions (with a preference for the basal ganglia) | |
| Treatment | • Sulfadiazine & pyrimethamine (plus leucovorin to prevent hematologic side effects) for several weeks |
| • Antiretroviral initiation within 2 weeks if not on it already (once patients are clearly tolerating toxoplasma therapy) | |
| • Prophylaxis: TMP-SMX (CD4 count <100/mm3) |
There is no reliable test for toxoplasmic encephalitis. The diagnosis is generally made by the presence of suspicious clinical symptoms, positive T gondii IgG serology, and characteristic central nervous system findings on MRI.
Primary prophylaxis against toxoplasmosis is highly effective at preventing the condition in those with advanced AIDS. Therefore, all patients with HIV undergo laboratory evaluation (T gondii IgG testing) for exposure; those with positive serology who have a CD4 count <100/mm3 require primary prophylaxis with trimethoprim-sulfamethoxazole (TMP-SMX), which reduces the risk of toxoplasmosis to 0%-2%.
Patients on antiretroviral treatment can usually discontinue TMP-SMX prophylaxis when their CD4 count is >200/mm3 for 3 months (and there is adequate viral suppression). TMP-SMX is also used for primary prophylaxis against Pneumocystis pneumonia for those with a CD4 count <200/mm3.