Epidemiology of Neospora caninum Infection in Cattle Evaluation of diagnostic tests and herd studies

Frössling, J. 2004. Epidemiology of Neospora caninum infection in cattle. Evaluation of diagnostic tests and herd studies. Doctor’s dissertation ISSN 1401-6257, ISBN 91 576 6670 9 The intracellular parasite Neospora caninum is an important cause of abortion in cattle. The aims of this thesis were to characterize N. caninum dynamics in infected herds and to validate diagnostic tests for individualand herd-level testing. The thesis is based on four separate studies. To validate an N. caninum iscom ELISA, sera from 244 Swedish dairy cattle were analysed. These were also analysed by an IFAT, which was considered an imperfect reference test. Estimates of sensitivity and specificity at different cut-offs were therefore obtained using a latent class method (Gibbs sampling). The best over all cut-off was 0.20 where sensitivity and specificity were 99% and 96%, respectively. Optical densities ≤0.15 and ≥0.55 were suggested limits to rule out and rule in infection. A three-year study of an American beef herd demonstrated how the avidity iscom ELISA is used to describe the duration of infection. The avidity pattern gradually changed as the peak shifted towards a higher avidity. It was concluded that postnatal infection was first frequent but decreased during the following years. The high avidity values at the end of the study confirmed that the majority of cows were chronically infected and that their calves were congenitally infected. Both increased and decreased prevalences of N. caninum were seen when 15 Swedish dairy herds were investigated in a longitudinal study. These changes were not related to the prevalence that each herd had when first sampled. Avidity measurements and investigation of serological status of related animals revealed that most seropositive individuals were chronically infected and that vertical transmission was the dominant transmission route. Test results from three groups of herds (total n=244) were used in an evaluation of the iscom ELISA when used to analyse bulk milk. Accuracy was estimated for different cut-off levels by using a standard cross-tabulation technique and also, in order to adjust for milk yield covariates, a logistic regression model. There was a relationship between within-herd prevalences and bulk milk optical densities. Herds with test results >0.20 are likely to be infected and herds with results <0.05 will have a within-herd prevalence of at most 10-15%.