Epidemiology of Neospora caninum Infection in Dairy Cattle in Thailand

The protozoan parasite Neospora caninum is causing bovine abortion worldwide. The overall aim of this thesis was to gain a better understanding about N. caninum infection in dairy cattle in Thailand both at the individual animal and the herd level. Specifically, the aims were to investigate variations of N. caninum antibodies in milk of individual cows during lactation, to demonstrate the use of bulk milk antibody testing and its application, and to characterize N. caninum dynamics of infection in Thai dairy herds. The thesis is based on four separate studies. The levels of N. caninum antibodies in milk of 15 infected cows varied considerably during 18 months although they were consistently considered positive. Cows of all lactation groups had a higher milk antibody at calving compared to at later months after calving, but the only significant difference was in the first lactation. Serum and milk antibody levels were always lower in first lactation than in second and later lactations. The results showed that individual milk can be an alternative material to demonstrate presence of N. caninum antibodies in lactating cows. The bulk milk antibody levels in a cross-sectional sample of 11 herds ranged between 0.04 and 0.89 and the seroprevalences varied between 0% and 46%. Herds with higher bulk milk antibody levels showed a trend of higher portion of seropositive cows although there was no strong relationship between the bulk milk antibody level and the within-herd seroprevalence. Forty-six percent of the 220 bulk milk samples from nine milk collection centres were judged positive and the herd prevalence varied between milk collection centres. Repeated bulk milk antibody testing of 418 dairy herds was evaluated and the herd N. caninum status was established at three consecutive samplings during one year. Herd status at either of the first two samplings was used to predict herd status at the last sampling, and was also interpreted in combinations. Using combinations gave higher predictability of a herd’s Neospora status than a single test. One hundred and thirty-six were considered negative, and one hundred and thirty-four herds were positive throughout the study. It was concluded that repeated bulk milk testing at regular intervals provided better information about herd N. caninum-antibody status than a single test. The results also showed that the infection is prevalent in northeast Thailand, but that a herd can keep a negative infection status. When 11 dairy herds were investigated repeatedly during four years, the overall percentage of antibody-positive cattle was constant and varied only between 10% and 13%. However, the within-herd seroprevalence differed substantially between herds. Vertical transmission, i.e. from dam to calf, was the most frequent route of transmission. The proportions of individual animals that changed from being seronegative to seropositive and from being seropositive to seronegative between the years were 3.9-4.6% and 19-39%, respectively, although most animals had consistent serological status throughout the study. Some herds can thus keep free from N. caninum infection without farmers taking control measures.