Immunization in prepubertal cattle with Neospora caninum live tachyzoites failed to prevent fetal infection in pregnant cattle after experimental heterologous challenge

HECKER YP; FIORANI F.; MALDONADO J.; DORSCH, M.A.; CIRONE, K.M.; CHEUQUEPÁN F.; COLQUE CARO L.; CAMPERO L.M.; ZAMORANO P.; CANTON G.J.; MOORE D.P.

Encuentro; In te r n a t i o n a l v i r t u a l ApicoWplexa meeting; 2020

Red Apicowplexa

The aim of the present study was to evaluate the safety and protective efficacy against vertical transmission in heifers inoculated with live tachyzoites from NC-Argentina LP1 strain at their puberty and challenged during pregnancy with a heterologous strain. Fifteen pregnant Angus heifers were involved in the present study: four animals inoculated subcutaneously (SC) with 1×106 live tachyzoites of NC-Argentina LP1 strain before reaching puberty (Hecker et al. 2019) and challenged intravenously (IV) with 1×108 live tachyzoites with NC-1 strain at 210 days of gestation (Group A); four animals received PBS SC before reaching puberty and were challenged IV with 1×108 live tachyzoites with NC-1 strain at 210 days of gestation (Group B), four animals inoculated SC with 1×106 live tachyzoites of NC-Argentina LP1 strain before reaching puberty (Hecker et al. 2019) and not challenged (Group C); and three animals have been received PBS SC before reaching puberty and were not challenged (Group D). Serum, placenta, peripheral blood mononuclear cells (PBMC) and umbilical cord samples were collected from dams and their calves at delivery before colostrum intake. The presence of antibodies in dams and calves was evaluated by indirect fluorescence antibody test, indirect ELISA and Immunoblot. Parasite DNA detection was assessed by nested PCR. Serological results showed that 4/4 dams and 3/4 newborn calves from Group A were seropositive to N. caninum at birth. In addition, one fetus from this group was aborted, but fetal fluid of this animal was seronegative to N. caninum and no parasitic DNA was detected in his tissues. In Group C, 4/4 dams and 1/4 newborn calves were seropositive to N. caninum at birth. All dams and calves from group B were N. caninum-seropositive and all dams and calves from group D remained N. caninum-seronegative. N. caninum DNA was amplified in two dams (placenta and PBMC) and one calf (umbilical cord) from Group A. In addition, N. caninum DNA was not detected in any placenta, PBMC or umbilical cord samples analyzed from Group B, C and D. The results of serology and PCR in heifers inoculated before puberty and challenge (Group A) showed that the specific immune response developed in the young animal (Hecker et al. 2019) was unable to prevent the congenital transmission after experimental challenge with a heterologous strain. However, some degree of protective immune response against recrudesce of infection in dams inoculated before puberty could have been induced because although recrudescence happened, the congenital infection with NC-Argentina LP1 strain was evidenced by serology only in one calf in animals not challenge (Group C). Although anti-N. caninum antibodies were detected in dams and calves of group B and C, no N. caninum DNA was described in these groups. This fact could be associated to a low parasite load associated with a loss of virulence in the NC-1 strain, as has been described in previous works. Although the inoculation with live tachyzoites of NC-Argentina LP1 isolate in 6-month-old female calves generated specific cellular immune responses with specific antibody levels that decreased at day 120 PI (Hecker et al. 2019), it was unable to protect the fetuses against an heterologous challenge. Based on the findings of this study, our research group will continue characterizing the evolution of the immune response during pregnancy in dams. In addition, it would be deepening the studies in calves from Group A to determine if the infection in these animals was for the strain used as vaccine inoculum or strain used as challenge strain.