# Toxoplasma Gondii > Toxoplasma gondii for MBBS Microbiology: cat as definitive host, tissue cysts, congenital triad, IgG avidity dating and pyrimethamine-sulfadiazine therapy. - Canonical URL: https://prepelephant.com/topics/mbbs/microbiology/toxoplasma-gondii - Exam / course: MBBS · Subject: Microbiology - Publisher: PrepElephant (https://prepelephant.com) — Prepared and reviewed by the PrepElephant Academic Review Team - First published: 2026-10-02 - Last updated: 2026-10-02 - How to cite: "Toxoplasma Gondii", PrepElephant, https://prepelephant.com/topics/mbbs/microbiology/toxoplasma-gondii ## Direct answer Toxoplasma gondii infects a large share of humanity yet troubles few: an obligate intracellular coccidian whose cats serve as definitive hosts shedding oocysts in faeces, whose tissue cysts (bradyzoites) wait in meat, and whose tachyzoites circulate in acute infection. Immunocompetent hosts meet it as silent seroconversion or cervical lymphadenopathy; danger concentrates at the two ends of immunity — the fetus, where first-trimester infection threatens chorioretinitis, hydrocephalus and intracranial calcification, and the AIDS patient with CD4 under 100, in whom reactivated cysts produce ring-enhancing encephalitis. Diagnosis rests on IgM and IgG ELISA refined by avidity and PCR of amniotic fluid; treatment is pyrimethamine plus sulfadiazine with folinic acid, or spiramycin in early pregnancy. ## What you must remember - **Life cycle:** sexual enteroepithelial cycle in cats shedding unsporulated oocysts that sporulate in soil; asexual cycle in intermediate hosts — tachyzoites disseminate in acute infection, bradyzoites encyst in brain, muscle and eye for decades. - **Routes to humans:** undercooked meat with tissue cysts, food or water contaminated with oocysts, cat litter and gardening soil, transplacental tachyzoites, plus rare transfusion and transplant routes. - **Immunocompetent disease:** silent seroconversion or cervical lymphadenopathy with fatigue and low fever, self-limiting without treatment in the non-pregnant. - **Congenital infection:** transmission risk rises with gestational age while severity falls — first-trimester infection is least frequent yet most devastating; the triad is chorioretinitis, hydrocephalus and intracranial calcification, convulsions completing Sabin's tetrad, with strabismus, blindness and deafness following. - **AIDS and immunosuppression:** CD4 below 100 reactivates brain cysts into multifocal ring-enhancing lesions with fever, focal deficits and seizures — a defining opportunistic infection of advanced HIV, including in Indian cohorts. - **Diagnosis:** IgM and IgG ELISA screening refined by IgG avidity — low avidity suggests acquisition within roughly three to four months; the Sabin-Feldman dye test remains the historic reference standard; PCR on amniotic fluid from 18 weeks, cerebrospinal fluid, blood or aqueous humour. - **Treatment:** pyrimethamine plus sulfadiazine plus folinic acid (to rescue marrow) for congenital, ocular and AIDS disease; spiramycin for maternal infection before fetal involvement is confirmed; cotrimoxazole as the practical alternative and for maintenance in AIDS until immune recovery. - **Prevention in pregnancy:** cook meat thoroughly, wash vegetables, wear gloves while gardening, and delegate litter box changing — owning a cat is not the risk; handling its litter is. ## Prenatal serology that needs interpretation A primigravida at ten weeks screens IgM and IgG positive for toxoplasma, and panic threatens both clinic and couple. The reflex order is IgG avidity: high avidity places infection before conception and the pregnancy continues reassured; low avidity keeps recent infection live, so serology repeats document rising IgG and falling IgM, and amniotic fluid PCR from 18 weeks searches for fetal infection directly. If PCR is negative, spiramycin continues as placental prophylaxis to term; if positive, the regimen crosses to pyrimethamine-sulfadiazine — treating fetus as well as mother — with ultrasound surveillance and neonatal vision-hearing follow-up for a year. The AIDS mirror-case runs different arithmetic: CD4 60, new focal seizures, multiple ring-enhancing lesions. Empirical pyrimethamine-sulfadiazine with folinic acid is treatment and diagnostic trial together — shrinkage on repeat imaging supports toxoplasmosis, stasis pushes toward biopsy for lymphoma. Maintenance continues until CD4 holds above 200 for six months, and cotrimoxazole prophylaxis for Pneumocystis adds partial anti-toxoplasma cover — a prescribing coincidence examiners enjoy explained. ## How the exam frames it The TORCH list is the frame, and toxoplasma is its treatable star — naming which congenital infections are treatable in pregnancy (toxoplasmosis, syphilis) earns the discriminating mark. Image questions contrast toxoplasma's scattered calcifications with cytomegalovirus's periventricular ones, and ring-enhancing lesions bring the toxoplasma-lymphoma-tuberculoma triangle. The cat counselling classic: oocysts are shed for one to two weeks, once in a lifetime, usually by kittens — indoor adult cats are low-risk and the litter tray is the specific exposure. Serology traps recur: IgM alone overcalls recent infection because it can persist a year — hence the avidity anchor — and the Sabin-Feldman dye test is the named gold standard vivas demand. Indian framing adds honesty — seroprevalence is substantial, yet antenatal toxoplasma screening is not national policy, so diagnosis rides on suspicious findings rather than programme mandate. ## Frequently asked questions ### Which host is definitive for Toxoplasma gondii, and why? The cat and other felids, where the sexual cycle produces oocysts shed in faeces — the only hosts that excrete the environmental form. ### What are tissue cysts and where do they lodge? Bradyzoite-filled cysts in brain, skeletal muscle and eye, persisting for the host's lifetime and reactivating when cell-mediated immunity falls. ### What is the congenital triad of toxoplasmosis? Chorioretinitis, hydrocephalus and intracranial calcifications — with convulsions completing the classical tetrad in severe newborn disease. ### How does IgG avidity help date infection? Low-avidity IgG indicates acquisition within roughly three to four months, while high avidity excludes recent acquisition — decisive in prenatal serology. ### What is the treatment regimen, and why folinic acid? Pyrimethamine plus sulfadiazine (or cotrimoxazole) with folinic acid to rescue the marrow; spiramycin covers maternal infection before fetal involvement is proven.