A SEROLOGICAL SURVEY OF BOVINE SYNCYTIAL VIRUS IN ONTARIO - ASSOCIATIONS WITH BOVINE LEUKEMIA AND IMMUNODEFICIENCY-LIKE VIRUSES, PRODUCTIONRECORDS, AND MANAGEMENT-PRACTICES

Of the 920 cows tested, 56.7% showed antiretroviral serological reacti vity. Prevalence rates (95% confidence interval) of antiretroviral ant ibodies among individual dairy cows in Ontario were: BIV 5.5% (4.0-7.0 ), BLV 25.7% (22.9-28.6), and BSV 39.6% (36.4-42.8). The following per centages of cows showed serological reactivity against the specified r etroviruses: BIV 2.3%, BLV 14.0%, BSV 27.5%, BIV and BSV 1.3%, BIV and BLV 0.9%, BLV and BSV 9.9%, BIV and BLV and BSV 0.9%. These rates of seropositivity are similar to those found in other countries. Serologi cal test results were not adjusted for sensitivity and specificity. Th e prevalence rates of antibodies to the three retroviruses (BIV, BLV, and BSV) were significantly different, but no associations were observ ed between specific retroviral serological test results among individu al cows. The prevalence rates of BIV and BSV seropositivity were const ant across Ontario, whereas, there was a significant trend for the pre valence rate of BLV seropositivity to decrease going from southwestern to eastern Ontario; cows in eastern Ontario had approximately half th e prevalence rate of those in southwestern Ontario. Cows that were ser opositive for BSV were significantly older than BSV seronegative cows. There was no association between culling rate and BSV serology. Signi ficant negative associations were found with winter or summer housing of calves separate from adults and summer outdoor exercise for dry cow s. The use of calf hutches in the summer had a significant positive as sociation with BSV seropositivity. Regression analyses were done to as sess the association of retroviral (BIV, BLV, and BSV) seropositivity on calving interval, milk somatic cell count, and milk production. Ser ological test results for BIV, BLV, and BSV were entered into all mode ls and all models were adjusted for intra-cluster (intraherd) correlat ion. Herd size and age were found to be important confounding variable s. BIV seropositivity was not associated with any changes in productio n using this approach, however when considered in isolation BIV seropo sitivity remained associated with decreased milk production. BLV serop ositivity was significantly associated with longer calving intervals a nd higher somatic cell counts in older cows. As well, in older cows, B SV seropositivity was significantly associated with higher milk produc tion.