The application of immunological, molecular and epidemiological approaches to the study of Neosporosis in cattle
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A prospective study was undertaken on a 260-head dairy herd in NSW, Australia to determine the modes of transmission, prevalence, effect on abortion and reproduction parameters and the impact on milk production of Neospora caninum. A test-and-cull method of control was also evaluated. The possible infectivity of milk from N. caninum infected cows was also investigated using a mouse model. As ELISAs form an important part of diagnosing N. caninum infection, studies were undertaken to evaluate two serum and one milk ELISA. Analysis was performed for each of the ELISAs to determine cut-off thresholds, Se and Sp. The milk ELISA was subsequently used to determine the overall prevalence of N. caninum in NSW dairy cows. In the prospective study, 11.4% of the herd’s cattle were seropositive to N. caninum by ELISA and the dominant route of transmission was vertical as the majority of the infected cattle were related and only a few seropositive cattle were born from seronegative dams (i.e. reflecting postnatal transmission). As 90% of offspring born from seropositive cows were also seropositive this suggests a high vertical transmission rate. Neospora caninum was found to be a major cause of abortion as cows seropositive to N. caninum had a 13-fold higher risk of abortion then seronegative cattle. Early foetal loss was also predicted to be associated with N. caninum infection as seropositive cows required a significantly greater number of inseminations and took longer to conceive than seronegative cattle. BVDV and IBR alone were not associated with causing abortion in this herd. This is the first report of an effective control strategy for N. caninum by either culling seropositives or not breeding from seropositive cows. This method was effective in reducing the number of infected cattle and was feasible due to the low prevalence of N. caninum on the farm and thus did not place too high a financial burden on the farmer. Neospora caninum DNA was also detected by PCR on milk samples from seropositive cows. This is the first Australian report demonstrating N. caninum DNA in milk. Of the serum ELISA evaluated, one was determined to have high Se and Sp at the cut-off recommended by the manufacturer while the other required modification of the cut-off value to gain the same high Se and Sp. After choosing the most suitable milk dilution, the milk ELISA was determined to have high Se and Sp of 97%. Using this ELISA the prevalence of N. caninum in NSW dairy cattle was determined to be 21.1%. Neospora caninum was found to be a significant cause of abortion and also foetal loss in the study herd and was vertically transmitted efficiently. Control efforts using a test-and– cull approach were successful without placing economic hardship on the farmer. The evaluation of several ELISAs was useful as these can be used in diagnosing infection. In particularly the milk ELISA will now make sampling easier so enabling whole herd sampling by the farmer. This ELISA may be of particular use in test-and- cull programs or in epidemiological studies. The concept of detecting N. caninum DNA in milk could be of great importance to the dairy industry as it suggests a new route of infection. Although milk extracts from seropositive cows were not infective to SCID mice in this study this should be investigated further.
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