Bovine neosporosis: the development of the horizontal infection

Since 1998, serological testing was performed at least once annually on all the breeding cows and heifers in a herd of 70 dairy cows chronically infected with Neospora caninum and which presented with abortions associated with this parasite. A blood sample was taken before colostrum was consumed in all heifers born since 1998. Results indicated that the mode of parasite transmission was horizontal. During a four-year period, 66 new infections were found in females that were parasite-free at birth. This represented an annual rate of 20.6%. The percentage of abortion was significantly higher in chronically infected animals (16%) than in cows testing negative (5.3%). Among the 37 heifers infected after birth, 15 were seronegative in the following 14 months (40%). An explanation for the massive number of seronegative animals is given in the article. Among these 37 heifers, 27 went to full-term. One Out Of two of their calves were infected, mainly from cows that thereafter tested positive (p < 0.05). These observations should be taken into account when deciding protocols for culling due to Neospora infection and when interpreting results of serological testing in bought-in heifers.