Estimation of genetic parameters for Neospora canimum infection in Ontario dairy herds

Neospora caninum is a protozoan parasite that was first identified in dogs in 1988 and has subsequently been found to be an important cause of endemic fetal loss, and occasional abortion epidemics, in cattle around world. N. caninum is currently the single most common cause of abortion in Ontario cattle as about 15% of all bovine abortions detected in the Animal Health Laboratory, University of Guelph, are associated with this parasite. As a result, the parasite can be an important determinant of profitability in dairy cattle. Blood samples were collected from Ontario dairy herds in 1998, 1999 and 2000, Fifty-six herds bled in 1998 belonged to the Ontario Sentinel Herd Project and were selected specifically for study of udder health. Of 86 herds bled in 1999 for a case-control study, 28 herds had one or more abortion in the previous year that was diagnosed as due to N. caninum on the basis of fetal histopathology, 30 herds had one or more abortion in the previous year that was diagnosed as not due to N. caninum, and 28 herds had N. caninum seroprevalence that was less than 7%. Fifty herds bled in 2000 had N. caninum seroprevalence that was =10% in 1998 or 1999. Sera were prepared from all blood samples and stored at -70°C prior to analysis. A total of 12016 sera samples were obtained from 9723 Holstein cows on 125 herds. All of the collected sera were assayed for antibodies to N. caninum using a kinetic ELISA at the California Animal Health & Food Safety Laboratory System, University of California, Davis. The overall prevalence of N. caninum antibodies was 11.2%, and the prevalence in individual herds varied from 0 to 70.4%. Among 3109 daughter-dam pairs from 117 herds, 619 positive dams produced 252 positive daughters, giving a vertical transmission rate of 40.7%. In contrast, there were 167 positive daughters from 2490 negative dams with a horizontal transmission rate of 6.7%. To estimate the heritability of susceptibility to N. caninum, the data were edited for identification of sires, dams and maternal grandsires, resulting in 8031 animals of 1463 sires in 125 herds. Five different models were used to analyze the data: 1) sire model, 2) animal model, 3) sire-dam model, 4) sire-maternal grandsire model, and 5) maternal effects model. Fixed effects consist of bleeding year-month, age (years) of animals, and herds. The estimated heritability of susceptibility to N. caninum ranged between 0.084 and 0.124, suggesting that the high vertical transmission rate (40.7%) from dams to daughters is due mainly to maternal environmental (contamination) effects. (Texte intégral