Neospora caninum-associated abortion in cattle: the time of experimentally-induced parasitaemia during gestation determines foetal survival

The parasite, Neospora caninum is an important cause of abortion in cattle. It is transmitted vertically or horizontally and infection may result in a bortion or the birth of a live, healthy but infected calf at full-term. Onl y a proportion of infected cattle abort and the pathogenesis of abortion is not understood. Groups of cattle were infected with 10(7) N, caninum tachy zoites intravenously at different times relative to gestation. Intravenous inoculation was chosen to reproduce the putative haematogenous spread of N. caninum following either recrudescence of endogenous infection or de novo infection. In all cattle, infection was accompanied by high gamma -interfer on and lymphoproliferative responses, and a biased IgG, response indicating that iV. caninum infection is accompanied by a profound Th1 helper T cell- like response. Infection at 10 weeks gestation resulted in foetopathy and r esorption of foetal tissues 3 weeks after infection in 5 out of 6 cows. Inf ection at 30 weeks gestation resulted in the birth of asymptomatic, congeni tally-infected calves at full term in all 6 cows, whereas the 6 cows infect ed before artificial insemination gave birth to live, uninfected calves. Th ese results suggest that the reason some cows abort is related to the time during gestation when they become infected or an existing infection recrude sces.