Postnatal Neospora caninum transmission and transient serologic responses in two dairies.

Cattle on two typically managed drylot dairies were serologically monitored from birth through year 1 to year 4 of life to characterise congenital and postnatal Neospora caninum transmission. Of the 456 calves enrolled, 284 were classified as N. caninum negative and 172 were classified as N. caninum positive. Ninety-six percent of congenitally infected calves were seropositive for all samples tested. Seven (4%) of the 172 congenitally infected animals had a period that persisted for 9 to 18 months when they were seronegative; however, all returned to seropositive status by 25 months of age. In N. caninum-negative calves, colostral antibody decayed by 128 days, with an estimated half-life of 19.6 +/- 5.2 days. Of the 284 calves classified as negative, 18% had sporadic, isolated responses to N. caninum, typically between 29 and 35 months of age, without subsequent seroconversion or infection. During the study, 17 animals seroconverted and remained seropositive throughout the follow-up. Thirteen of the seroconversions occurred in the neonatal period; however, in nine of 10 where dam status was available, the dam was N. caninum positive, suggesting late gestation congenital infection rather than postnatal infection. Seroconversion was detected in an additional four animals, between 13 and 22 months of age. The estimate of postnatal infection rate was less than 1% per year despite a high N. caninum seroprevalence in the herds, and the presence of potential definitive and intermediate hosts on the dairy throughout the study. The extremely low rate of postnatal infection, as well as the lifelong persistence of congenital infection, emphasises the importance of congenital transmission in maintaining N. caninum infection in dairies.