Toksoplasmosis

01Pathophysiology

Toxoplasma gondii is an obligate intracellular protozoan parasite with a life cycle that depends on both definitive and intermediate hosts. The definitive host is the domestic cat, in which sexual reproduction takes place within the intestinal epithelium. Cats shed oocysts in their feces, which become infectious after sporulating in the environment over 1 to 5 days. Humans become infected through three primary routes: ingestion of oocysts from contaminated soil, water, or cat litter; ingestion of tissue cysts (bradyzoites) in undercooked or raw meat (particularly pork and lamb); and transplacental (vertical) transmission from a newly infected mother to the fetus.

Once ingested, the parasite invades the intestinal epithelium and disseminates hematogenously as tachyzoites, the rapidly dividing, actively destructive form. In an immunocompetent host, the cell-mediated immune response (primarily CD4+ and CD8+ T lymphocytes along with interferon-gamma) controls the infection and forces the parasite into a dormant state. The tachyzoites convert to bradyzoites and form tissue cysts, which can persist for the lifetime of the host, most commonly in the brain, skeletal muscle, and myocardium. This is why the host remains seropositive (IgG+) indefinitely.

The critical concept for exam purposes is reactivation. When the immune system weakens, particularly when the CD4 count drops below 100 cells/μL in HIV/AIDS, dormant bradyzoites reconvert to tachyzoites. Because the parasite has a strong tropism for the central nervous system, reactivation most commonly manifests as toxoplasmic encephalitis. The tachyzoites cause local tissue necrosis and an inflammatory response, producing the characteristic ring-enhancing lesions visible on neuroimaging. The ring of enhancement represents the surrounding inflammatory and vascular reaction at the border of the necrotizing abscess.

In congenital toxoplasmosis, vertical transmission occurs when a pregnant woman acquires a primary infection during pregnancy. The risk of transmission increases with gestational age (higher in the third trimester), but the severity of fetal disease is inversely related (worse in the first trimester). The tachyzoites cross the placenta and cause widespread tissue destruction in the developing fetus, with a particular predilection for the eyes and brain, resulting in the classic triad of chorioretinitis, hydrocephalus, and diffuse intracranial calcifications.

02Classification and Clinical Manifestation

03Diagnostic Workup

The clinical scenario that exam questions build around is an HIV-positive patient with a CD4 count below 100 who presents with subacute headache, focal neurological deficits, and fever. The diagnostic approach proceeds in layers.

Step 1: Serology. Order Toxoplasma IgG antibodies. A positive IgG indicates prior exposure and the presence of dormant tissue cysts that can reactivate. In the correct clinical context (immunosuppressed, focal neuro deficits), positive IgG strongly supports the diagnosis. A negative IgG makes toxoplasmic encephalitis very unlikely and should redirect the differential toward primary CNS lymphoma or other etiologies. IgM testing is not useful in reactivation disease because reactivation does not typically produce a new IgM response.

Step 2: Neuroimaging. CT head with contrast is the best initial imaging study. The hallmark finding is multiple ring-enhancing lesions with surrounding vasogenic edema, preferentially located in the basal ganglia, thalamus, and corticomedullary junction. If CT is equivocal, proceed to MRI with gadolinium, which is more sensitive and better delineates the number and characteristics of lesions.

Step 3: Empiric treatment trial. In a patient with a compatible clinical presentation, positive Toxoplasma IgG, and multiple ring-enhancing lesions, the standard approach is to start empiric anti-toxoplasma therapy and reassess in 10 to 14 days. Clinical improvement and reduction in lesion size on repeat imaging confirms the diagnosis without the need for biopsy. This empiric trial approach is a frequently tested concept.

Step 4: Brain biopsy. If the patient fails to improve after 10 to 14 days of empiric therapy, the next step is stereotactic brain biopsy to obtain a tissue diagnosis. This step is essential to differentiate toxoplasmosis from primary CNS lymphoma, which is the main alternative diagnosis. Biopsy is the gold standard but is invasive and therefore not performed first-line when the clinical picture is classic.

For congenital toxoplasmosis: Diagnosis involves Toxoplasma-targeted IgM and IgA antibodies in the neonate (IgG may reflect passive maternal transfer and is not diagnostic), PCR of amniotic fluid for prenatal diagnosis, and cranial imaging (ultrasound or CT) showing diffuse calcifications and hydrocephalus.

04Management & Treatment

Acute treatment of toxoplasmic encephalitis follows a three-drug regimen. Pyrimethamine inhibits dihydrofolate reductase (DHFR) in the parasite, blocking folate metabolism and DNA synthesis. Sulfadiazine inhibits dihydropteroate synthase, acting synergistically with pyrimethamine at two sequential steps in the folate pathway. The combination is bactericidal against tachyzoites.

The critical addition is leucovorin (folinic acid), which is given to prevent pyrimethamine-induced bone marrow suppression (megaloblastic anemia, pancytopenia). Pyrimethamine also inhibits human DHFR to a degree, and leucovorin provides the active folate form that bypasses this block in human cells without rescuing the parasite. This is a classic exam trap: leucovorin (folinic acid) must NOT be confused with folic acid. Folic acid requires DHFR for activation and would be ineffective because the enzyme is inhibited by pyrimethamine.

For patients with a sulfonamide allergy, clindamycin replaces sulfadiazine. Leucovorin is still required because pyrimethamine remains in the regimen.

Antiretroviral therapy (ART) should be initiated or optimized in all HIV patients with toxoplasmic encephalitis, as immune reconstitution is the definitive long-term treatment. After completing at least 6 weeks of acute therapy, patients step down to chronic maintenance therapy (secondary prophylaxis) at lower doses and continue until their CD4 count remains above 200 cells/μL for at least 6 consecutive months on ART.

Primary prophylaxis with TMP-SMX is indicated for all HIV-positive patients with a CD4 count below 100 who are Toxoplasma IgG-positive. Conveniently, TMP-SMX at prophylactic doses also provides coverage against Pneumocystis jirovecii pneumonia (PCP), which is indicated at CD4 < 200. This dual coverage is frequently tested.

Corticosteroids (dexamethasone) may be added if there is significant mass effect or cerebral edema, but they are not part of the standard regimen and should be used cautiously because steroids can also transiently reduce the size of CNS lymphoma lesions, potentially confusing the empiric treatment trial.

In pregnancy, the goal is to prevent or treat congenital infection. Spiramycin, a macrolide that concentrates in the placenta, is used in the first trimester to reduce vertical transmission; it does not reliably cross the placental barrier to treat an already-infected fetus. If amniocentesis confirms fetal infection after 18 weeks of gestation, the regimen is switched to pyrimethamine + sulfadiazine + leucovorin (pyrimethamine is avoided in the first trimester due to teratogenic risk).

05Differential Diagnosis & Distractors

06Traps & High-Yield Pearls

The single most common way students lose points on toxoplasmosis questions is by confusing it with primary CNS lymphoma. Both conditions present in HIV-positive patients with low CD4 counts and ring-enhancing lesions on brain imaging. The vignette will often hinge on two distinguishing features: the number of lesions (multiple favors toxo, solitary favors lymphoma) and the Toxoplasma IgG status (positive IgG supports reactivation and therefore toxoplasmosis; negative IgG argues strongly against it and points toward lymphoma). When the vignette describes an HIV patient with multiple ring-enhancing lesions and positive Toxoplasma IgG, the next best step is empiric anti-toxoplasma therapy, not biopsy.

The second major trap involves leucovorin vs. folic acid. The question may ask which agent should be co-administered with pyrimethamine, and folic acid will appear as a distractor. Leucovorin (folinic acid) is the correct answer because it does not require DHFR for activation and can rescue human bone marrow cells while leaving the parasite susceptible. Folic acid requires the DHFR enzyme that pyrimethamine is inhibiting, rendering it useless in this context.

A third commonly tested concept is the prophylaxis threshold. TMP-SMX prophylaxis for toxoplasmosis is initiated at CD4 < 100 in a patient who is Toxoplasma IgG-positive, whereas TMP-SMX for PCP begins at CD4 < 200. A single daily double-strength TMP-SMX tablet covers both, but the trigger points differ. If the question asks when to start toxoplasmosis prophylaxis, the answer is CD4 < 100 with positive IgG, not CD4 < 200.

Finally, for congenital infections, students frequently confuse the calcification patterns: toxoplasmosis causes diffuse, scattered intracranial calcifications, while congenital CMV produces periventricular calcifications. Both may present with chorioretinitis, but hydrocephalus and the distribution of calcifications help differentiate them.

The core competency being tested across toxoplasmosis questions is the ability to integrate clinical context (HIV status, CD4 count, serology) with imaging findings to correctly sequence the diagnostic and therapeutic approach, particularly the decision to treat empirically before pursuing invasive testing.