THE PREVALENCE OF BOVINE VIRAL DIARRHEA VIRUS-INFECTION IN A POPULATION OF FEEDLOT CALVES IN WESTERN CANADA

The prevalence of bovine viral diarrhea virus (BVDV) infection was exa mined in a population of 5129 recently weaned steer calves entering a large feedlot in central Saskatchewan from September to December 1991. Serum samples were collected within 24 h of arrival at the feedlot fr om every fifth calf processed and again 96 d postarrival. A microtiter virus isolation test was used to determine the prevalence of calves v iremic with BVDV on entry to the feedlot. An enzyme-linked immunosorbe nt assay (ELISA) which detects antibody against glycoprotein 53 of the BVDV was used on paired sera to determine the seroconversion risk dur ing the first 96 d in the feedlot. A virus neutralization (VN) test fo r BVDV was conducted on a sub-sample of paired sera to measure agreeme nt in determination of seroconversion risk with the ELISA. A polymeras e chain reaction (PCR) test which detects BVDV was used to determine i f cattle were acutely viremic when treated for disease. The estimated prevalence of persistently infected calves in this population was <0.1 %. The seroconversion risk for BVDV was 27% (236/864) according to the ELISA and it varied from 0 to 63% among the 20 pens sampled. Accordin g to the VN test, the seroconversion risk for BVDV was 40% (132/327) a nd it varied from 0 to 100% among the 11 pens tested. The agreement be tween the ELISA and VN tests in seroconversion risk to BVDV was very p oor (kappa = 0.15 +/- 0.039 SE). The prevalence of acute virema in cal ves treated at the feedlot hospital was low at 4% (6/149). Although th e prevalence of persistent infection was low, serological tests sugges ted a high risk of seroconversion to BVDV. Unfortunately, the detailed epidemiology of acute BVDV infection in this feedlot could not be des cribed because of the lack of agreement between the two serological te sts. The two tests classified different individuals and groups of calv es as having been infected with BVDV during the first 96 d in the feed lot. While the benefits of the ELISA for BVDV may seem considerable, t he dynamics of the titer change and their interpretation subsequent to natural exposure to BVDV, need to be better defined before these test s can be used with confidence in future feedlot studies.