Risk factors for bovine infection with verocytotoxigenic Escherichia coli in Ontario, Canada

Abstract

Risk factors for prevalent infection with verocytotoxigenic Escherichia coli (VTEC) were studied in a random sample of 1131 cows and 659 calves < 3 months of age on 100 dairy farms in southern Ontario. Faecal-culture supernatants from each animal were screened for verocytotoxicity using a Vero cell assay. Five E. coli isolates from positive samples were tested for verocytotoxin production using a test sequence consisting of (1) hybridization with DNA probes for Shiga-like toxin I (SLT-I) and SLT-II sequences, (2) neutralization of verocytotoxicity by a mixture of anti-SLT-I and anti-SLT-IIv antisera, and (3) amplification of SLT genes by a polymerase chain reaction procedure. Neutralization assays were performed on positive faecal-culture supernatants from which no positive isolates were obtained. Farm managers were interviewed using a standardized questionnaire to obtain information on farm and individual animal-level characteristics and management practices. Calves > 2 weeks of age were at significantly greater risk of infection than those under 2 weeks (OR = 2.0). Farm-level calf infection was negatively associated with herd size, the use of nipple bottles for feeding calves, the use of traditional tie-stall housing as opposed to loose housing or other methods, and the maintenance of a closed herd. Farm-level cow infection was negatively associated with herd size, the use of loose housing, the maintenance of a relatively open herd, and the use of milking parlours or pipelines as opposed to bucket-type milking machines. These variables provide potential markers for the VTEC infection status of dairy herds in the target population, but the causal mechanisms underlying these observed associations are unknown.