Review

Neospora caninum-induced abortion in cattle

Abstract

Neospora caninum is a protozoal parasite implicated as a leading cause of abortion in some dairy and beef herds. The bovine is an intermediate host for the parasite, and therefore N. caninum is not transmitted horizontally from infected individuals in the herd to non-infected individuals. Transmission can take place vertically from dam to fetus during pregnancy when tachyzoites from maternal tissues enter the bloodstream and cross the placenta to cause fetal infection. Once infected, an animal is considered to be infected for life and a female will pass the infection to a high percentage of her offspring. Although fetal infection in some herds is common, abortion following fetal infection is a fairly rare event, with risk of abortion being greatest in the first pregnancy following infection and decreasing thereafter. Vertical transmission of the parasite results in an endemic abortion pattern within family lines. N. caninum can also be passed from the definitive host, the canine, to cattle, presumably via contamination of feed or water with oocyst-infected faeces. Canines become infected by consuming N. caninum-infected tissue. Horizontal transmission of the parasite results in an epidemic abortion pattern which can cause a high percentage of susceptible females to abort. Laboratory tests are able to identify maternal, fetal, or calf infection with the parasite, but because of the low percentage of abortions in infected dams in endemically-infected herds, predictive value of N. caninum infection is low for abortion causation, and statistical tests to attribute risk are needed to infer abortion causation.