Serological investigation of an outbreak of Neospora caninum-associated abortion in a dairy herd in southeastern United States

An outbreak of Neospora caninum-associated abortion occurred in a South Car olina dairy wherein greater than 10% of the herd aborted over a 4-month per iod. Of the total number of cows at mid-late gestation, nearly 40% (28/71) aborted while the remaining 60% (43/71) gave birth to normal calves, Immuno histochemical examination of brain tissue from a subset of aborted fetuses confirmed N, caninum as the causative agent of abortion in these animals. A variety of serological assays, including indirect fluorescence antibody te st (IFAT), recombinant enzyme-linked immunosorbent assay (rELISA), ISCOM-EL ISA, avidity ELISA, and Neospora agglutination test (NAT), were used to eva luate sera collected during the outbreak from 240 cows for antibodies to N, caninum. IFAT and ISCOM-ELISA testing showed that nearly 80% of the dairy cows had antibodies to N. caninum. NAT and rELISA had similar levels of ser opositivity relative to IFAT and ISCOM-ELISA, but the percentage of positiv e sera was dependent on the cut-off value chosen. As indicated by kappa coe fficient statistical analysis, ISCOM-ELISA and IFAT exhibited the highest l evel of agreement in identifying N. caninum-positive and -negative cows. A decrease in the percentage of seropositive cows as determined by ISCOM-ELIS A and IFAT with increasing cow age was noted. However, no significant diffe rence was observed between cow age and abortion status. In addition to thes e tests, an avidity ELISA was performed on all sera with high (greater than or equal to0.4) ISCOM-ELISA readings. Avidity index (AI) increased with ti me post-abortion suggesting that most abortions were due to recent N. canin um infection. Of the cows at risk for abortion, the mean serological AI of aborting cows was significantly lower(P < 0.05) than mean serological AI of non-aborting cows. Published by Elsevier Science B.V.