Identification of neospora caninum and its impact on reproductive performance in Thai dairy herds

Abstract

The aims of this study were (1) to identify N. caninum (NC) in the aborted fetuses and to establish an isolation guideline and (2) to investigate the impact of NC on the reproductive performance and abortion risk in dairy cows of NC-seropositive herds. NC tachyzoites were detected, by IHC, in the brain of one of 22 fetuses and its dam was NC seropositive by IFA. The other fetuses and their dams were seronegative. For isolation guideline, the brains of 4 seropositive pregnant cows and their calves, after parturition, were used for direct culture in vero cells and bioassay in hamsters (Mesocricetus auratus). Four groups (4 treated and 2 controls each) of hamsters were treated with the brain homogenates of 2 cows and their calves. All cows, calves and hamsters showed no clinical illness until they were euthanized. Selected tissues of cows, calves and hamsters were tested by HE, IHC and PCR. At calving, sera of 3 cows and precolostral blood of their calves were seronegative. The sera of one cow and its calf were seropositive (IFA titer of 1:100 each and ELISA value of 69.3% and 73.1% respectively). A tissue cyst was detected in the brain of this calf by IHC and Neospora DNA was also PCR-amplified but not from the brain of its dam. The parasites were not detected in the other tissues of all cows, calves and hamsters. Parasite DNA was also amplified from brains of two of four hamsters treated with the brain homogenates of IHC positive calf. The DNA was not amplified from the other dam-calf pairs and hamsters. Partial sequencing of the NC DNA showed the 99% identities of 225 bp with the reference sequence of NCl strain. It was designated as Thai-B1. Parasite isolation was not successful. It was concluded that vertical transmission may be much lower than previous reports, as evidenced by serology, IHC and PCR. Also chronically infected cows may have seronegative conversion and the antibody titers of their calves at parturition may be much lower than the other reports. The hamsters (M. auratus) may not be susceptible enough to N. caninum. Whether to use the isolation method in this guideline needs further clarification. This is the first report of Neospora-infection in aborted fetus and congenitally infected calf in Thailand. Holstein Friesian crossbred cows (total: 216) from 12 NC-seropositive farms in Nakhon Pathom were used to study the impact of Neospora infection on the reproductive performance. Blood samples were collected monthly for 10 months. The records of AI, calving and abortions were obtained from the Ratchaburi AI center and farm records. Herd seroprevalence was 12.9% (range 4.76 to 30%). Reproductive performances in seropositive and seronegative cows were the same (P > 0.05). But the performances were considerably lower then the other reports. Highly significant differences in the reproductive performance among herds were found (P = 0.0001 to 0.0005). The risk of abortion was also not associated with the Neospora seropositivity of the cows (RR = 2.25;95%CI = 0.64 to 7.9). It was concluded that in herds with relatively low seroprevalence, Neospora seropositivity had no impact on the reproductive performances of the dairy cows. The inferior performance of the cows and large variation among herds indicate the need for the check of management and/or other factors for the improvement.