Ensefalitis Toksoplasma

01Pathophysiology

Toxoplasma gondii is an obligate intracellular protozoan parasite with a definitive host in cats and intermediate hosts in humans and other warm-blooded animals. Humans acquire primary infection through ingestion of oocysts from contaminated soil or cat feces, or through tissue cysts in undercooked meat (especially pork and lamb). In immunocompetent individuals, primary infection is controlled by cell-mediated immunity, and the organism encysts in tissues, most notably in skeletal muscle and the brain, remaining dormant indefinitely.

The critical concept for the exam is reactivation. In the setting of profound CD4+ T-cell depletion (below 100 cells/μL), the immune surveillance that keeps these tissue cysts dormant fails. The bradyzoites within the cysts convert to rapidly dividing tachyzoites, which invade and destroy surrounding brain parenchyma. This causes a necrotizing encephalitis with an intense inflammatory response, forming ring-enhancing abscesses. The center of each lesion is necrotic tissue, the enhancing ring represents the active inflammatory border with leaky vasculature (hence contrast uptake), and the surrounding hypodensity represents vasogenic edema.

The predilection for the basal ganglia and the gray-white matter junction is a high-yield anatomical detail. These regions have high blood flow, which correlates with the hematogenous seeding of tachyzoites. The mass effect from the abscesses and surrounding edema directly explains the focal neurological deficits (hemiparesis from corticospinal tract compression, aphasia from temporal lobe involvement) and seizures (from cortical irritation). The subacute tempo of symptoms, evolving over days to weeks rather than hours, distinguishes this from a vascular event.

02Classification and Clinical Manifestation

Toxoplasma encephalitis does not have a formal staging system, but clinical presentations can be organized by the dominant pattern:

The hallmark presentation tested on exams is the triad of headache, focal neurological deficits, and fever in a patient with advanced HIV. However, the presentation can be subtle: some patients present only with progressive confusion or personality changes before focal signs develop.

03Diagnostic Workup

The workup begins with neuroimaging. A contrast-enhanced MRI is the best initial test for any HIV patient presenting with new focal neurological deficits or seizures. The classic finding is multiple ring-enhancing lesions with surrounding edema, favoring the basal ganglia and gray-white junction. CT with contrast is an acceptable alternative but is less sensitive for detecting smaller lesions and posterior fossa pathology.

Next, check the serum anti-Toxoplasma IgG. A positive result confirms prior exposure and latent infection, which is the prerequisite for reactivation. Over 95% of patients with TE will have a positive IgG. A negative IgG makes TE much less likely and should push you toward alternative diagnoses, particularly primary CNS lymphoma. This is a commonly tested distinction.

The CD4+ count provides context. TE almost exclusively occurs when the CD4 count drops below 100 cells/μL. If the CD4 count is above 200 and the patient is not on other immunosuppressive therapy, reconsider the diagnosis.

The most important diagnostic concept for the exam is the empiric treatment trial. In clinical practice, brain biopsy carries procedural risk, so the standard approach for a patient with the classic clinical-radiographic picture (multiple ring-enhancing lesions + positive Toxoplasma IgG + CD4 < 100) is to start empiric anti-Toxoplasma therapy and reassess with repeat imaging in 10 to 14 days. Improvement on repeat imaging confirms the diagnosis without biopsy.

Stereotactic brain biopsy is the gold standard but is reserved for patients who fail to improve after 2 weeks of empiric therapy, have a negative Toxoplasma IgG, or have a single lesion (which raises concern for CNS lymphoma). A question that describes a patient failing empiric therapy at 2 weeks is prompting you to answer "brain biopsy."

If a lumbar puncture is performed (and it is safe to do so, given the risk of herniation with mass lesions), CSF Toxoplasma PCR can support the diagnosis, but its sensitivity is only moderate. A negative PCR does not rule out TE.

Thallium-201 SPECT or FDG-PET scanning can help distinguish TE from CNS lymphoma when imaging is ambiguous. TE lesions are metabolically inactive ("cold"), while lymphoma lesions are metabolically active ("hot").

04Management and Treatment

Acute stabilization begins immediately once the clinical and radiographic picture supports TE. The first-line regimen is the triple combination of pyrimethamine, sulfadiazine, and leucovorin (folinic acid). This is a frequently tested drug combination. Pyrimethamine and sulfadiazine work synergistically to block sequential steps in folate synthesis within the parasite. Pyrimethamine inhibits dihydrofolate reductase, and sulfadiazine inhibits dihydropteroate synthase. Leucovorin is given not to help kill the parasite, but to rescue the patient's bone marrow from the myelosuppressive effect of pyrimethamine. This is a commonly tested rationale: leucovorin prevents pancytopenia without interfering with the antiparasitic effect because Toxoplasma cannot utilize folinic acid.

For patients with a sulfonamide allergy, replace sulfadiazine with clindamycin (600 mg every 6 hours). Pyrimethamine and leucovorin remain in the regimen.

Corticosteroids (typically dexamethasone) are added only when there is significant mass effect or impending herniation. They are not given routinely because steroids can also improve CNS lymphoma, which would confound the diagnostic treatment trial.

ART (antiretroviral therapy) should be initiated within approximately 2 weeks of starting TE treatment. Delaying ART too long worsens outcomes, but starting too early in the setting of a large intracranial inflammatory process carries a risk of immune reconstitution inflammatory syndrome (IRIS), which can paradoxically worsen cerebral edema.

After completing at least 6 weeks of full-dose induction therapy with documented clinical and radiographic improvement, the patient transitions to chronic maintenance therapy (secondary prophylaxis) at lower doses. This continues until the patient achieves sustained immune reconstitution, defined as a CD4 count above 200 cells/μL for at least 6 consecutive months while on effective ART. If the CD4 count subsequently falls again, prophylaxis must be restarted.

Primary prophylaxis is also high-yield. HIV patients with a CD4 count below 100 and a positive Toxoplasma IgG should receive TMP-SMX (trimethoprim-sulfamethoxazole), which conveniently also provides prophylaxis against Pneumocystis jirovecii pneumonia (PJP). This is the same drug and same dose used for PJP prophylaxis (one double-strength tablet daily), making it a two-for-one preventive strategy that examiners like to test.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common way students lose points on Toxoplasma encephalitis questions is confusing it with primary CNS lymphoma. Both occur in HIV patients with very low CD4 counts, and both can produce ring-enhancing lesions on imaging. The exam will embed distinguishing clues in the vignette: multiple lesions and a positive Toxoplasma IgG point to TE, while a single periventricular lesion with a negative Toxoplasma IgG or a positive CSF EBV PCR points to PCNSL. When neither diagnosis is clear from the initial workup, the next best step is always the empiric anti-Toxoplasma treatment trial, not an immediate biopsy.

Another frequent trap involves the role of leucovorin. Students sometimes confuse leucovorin with folic acid or believe it enhances the drug's antiparasitic activity. Leucovorin is folinic acid given purely to protect the host's bone marrow from pyrimethamine toxicity. Toxoplasma cannot use exogenous folinic acid, so the rescue does not diminish treatment efficacy.

Watch for questions where a patient is already on TMP-SMX prophylaxis. If a patient is adherent to TMP-SMX and still develops ring-enhancing brain lesions, TE becomes much less likely, and the differential should shift strongly toward CNS lymphoma. This is a subtle but testable point.

Finally, remember the timing rules. Empiric therapy should show clinical improvement within 2 weeks and radiographic improvement on repeat imaging. Failure to improve at that checkpoint is the trigger for stereotactic brain biopsy. Students sometimes select biopsy too early (before a treatment trial) or too late (continuing empiric therapy beyond 2 weeks without improvement). The exam is testing your ability to sequence the diagnostic and therapeutic steps correctly.