Experimental Toxoplasma gondii infections in pigs: Humoral immune response, estimation of specific IgG avidity and the challenges of reproducing vertical transmission in sows.

Ten pregnant sows were experimentally inoculated per os with T. gondii in order to investigate vertical and galactogenic transmission of the parasite and the evolution and maturation of the specific IgG humoral response in the sows and piglets. Five seronegative sows received 10T. gondii (CZ isolate clone H3) sporulated oocysts during late-pregnancy (Exp. 1), three sows received 10 oocysts during mid-pregnancy (Exp. 2) and three sows from Exp. 1 (and two seronegative sows) were re-inoculated with 10 oocysts during a further pregnancy (late-pregnancy) (Exp. 3). Besides, six 4.5 week-old piglets inoculated per os with 5×10 oocysts were also included in the serological investigations. All animals seroconverted (PrioCHECK Toxoplasma Ab porcine ELISA, Prionics, Switzerland) by 2-3 weeks post inoculation (wpi) and remained seropositive for at least 38 weeks or until euthanasia. Four chronically infected sows from Exp. 1 and 2 were serologically monitored during a further pregnancy and no reactivation, but a decrease of the antibody levels was observed at farrowing (Exp. 4). In all experiments, the specific IgG-avidity was initially low, increased during the course of infection and after re-inoculations. An avidity index (AI) ≥40% could be used to rule out recent infections (<8 weeks) in most (15 of 16) animals. In some piglets (18.6% of 70) delivered by inoculated sows (Exp. 1 and 2), maternal antibodies were still detectable at 2 months (but not by 3 months) of age, with constant high avidity values, comparable to those of the dams at farrowing. In all experiments, the sows remained asymptomatic and delivered non-infected offspring at term. A total of 208 normal and 5 stillborn piglets delivered by the inoculated sows (Exp. 1-4) tested serologically negative before colostrum uptake. Placentas (n=88) from all sows and tissues (brain, liver, lung, heart, and masseter muscle) from 56 delivered piglets were analysed histopathologically and by real-time PCR for T. gondii with negative results. Colostrum and milk samples from all sows were negative by real-time PCR for T. gondii DNA. In addition, no seroconversion was observed in 16 piglets from seronegative dams that were transferred to infected dams one day after birth to detect a possible infection through colostrum or milk during the suckling period. Although vertical transmission of T. gondii was demonstrated in naturally infected pigs, many factors involved in the outcome of vertical transmission and congenital toxoplasmosis in pigs are still unknown.