Transplacental toxoplasmosis in naturally-infected white-tailed deer: Isolation and genetic characterisation of Toxoplasma gondii from foetuses of different gestational ages

Clinical toxoplasmosis is most severe in congenitally-infected hosts. In humans, transmission of Toxoplasma gondii from the mother to the foetus is considered to be most efficient during the last trimester of pregnancy but clinical congenital toxoplasmosis is more severe if transmission occurs during the first trimester. However, there are no data on the rate of congenital transmission of T. gondii with respect to gestational age in any host during natural infection. In the present study, attempts were made to isolate T. gondii by bioassay in mice inoculated with tissues from foetuses of 88 naturally-exposed white-tailed deer from Iowa and Minnesota. Viable T. gondii was isolated from foetuses of six of 61 deer in early pregnancy (45-85 days of gestation) from Iowa and foetuses of nine of 27 deer from Minnesota in mid-gestation (130-150 days) of a gestational period of 7 months. The 15 T. gondii isolates obtained from foetal deer were PCR-restriction fragment length polymorphism genotyped using polymorphisms at 10 nuclear markers including SAG1, SAG2, SAG3, BTUB, GRA6, c22-8, c29-2, L358, PK1 and an apicoplast marker, Apico. Five genotypes were revealed, including the clonal Type II and III lineages, and three non-clonal genotypes. DNA sequencing analysis of representative isolates at loci SAG2, c22-8, L358 and PK1 revealed that the three non-clonal genotypes are closely related to the clonal Type I, II and III lineages. It is very likely that these non-clonal genotypes were derived from genetic crosses among the three clonal Type I, II and III lineages. The most common genotype was Type II, commonly found in humans in North America and Europe, suggesting the possible link of transmission from game animals to humans.