Paper II — 2024 December (Supplementary) (2019 Scheme) — Q13
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Paper II
2024 December (Supplementary) (2019 Scheme) · 100 marks · 180 min
Question
How is Toxoplasma gondii transmitted. Add a note on the laboratory diagnosis of congenital Toxoplasma
infection.
Q134 marksShort Answers
Answer
Transmission of Toxoplasma gondii: ingestion of undercooked/raw meat containing tissue cysts (bradyzoites); ingestion of food/water contaminated with oocysts shed in cat faeces (the definitive host); congenital (transplacental) transmission from an acutely infected mother to the fetus; and, less commonly, via blood transfusion or organ transplantation from an infected donor.
Laboratory diagnosis of congenital toxoplasma infection:
Maternal screening: IgM and IgG serology during pregnancy — IgM indicates recent/acute infection; IgG avidity testing helps date the infection (low avidity suggests recent infection within the past few months, high avidity suggests infection acquired more than several months earlier), important since only primary maternal infection during pregnancy poses significant fetal risk.
Prenatal diagnosis: PCR detection of T. gondii DNA in amniotic fluid (obtained via amniocentesis), the standard method for confirming fetal infection when maternal primary infection is suspected/confirmed.
Neonatal diagnosis:
IgM/IgA antibody in neonatal serum — since maternal IgM/IgA do not cross the placenta, their presence in the infant confirms congenital infection.
Persistence or rise of IgG titre beyond the expected decline of passively transferred maternal antibody (which typically wanes over months) supports the diagnosis.
PCR on neonatal blood/CSF/urine.
Clinical/radiological correlation — cranial imaging for hydrocephalus/intracranial calcifications, and ophthalmological examination for chorioretinitis, supporting the diagnosis alongside laboratory confirmation.
Follow-up serology in the infant over the first year of life helps distinguish true congenital infection (persistent/rising IgG) from passively transferred maternal antibody (which should decline and disappear).
