RISK-FACTORS FOR BOVINE INFECTION WITH VEROCYTOTOXIGENIC ESCHERICHIA-COLI IN ONTARIO, CANADA

Risk factors for prevalent infection with verocytotoxigenic Escherichi a coli (VTEC) were studied in a random sample of 11 31 cows and 659 ca lves < 3 months of age on 100 dairy farms in southern Ontario. Faecal- culture supernatants from each animal were screened for verocytotoxici ty using a Vero cell assay. Five E. coli isolates from positive sample s were tested for verocytotoxin production using a test sequence consi sting of (1) hybridization with DNA probes for Ship-like toxin I (SLT- I) and SLT-II sequences, (2) neutralization of verocytotoxicity by a m ixture 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 whi ch no positive isolates were obtained. Farm managers were interviewed using a standardized questionnaire to obtain information on farm and i ndividual animal-level characteristics and management practices. Calve s >2 weeks of age were at significantly greater risk of infection than those under 2 weeks (OR = 2.0). Farm-level calf infection was negativ ely associated with herd size, the use of nipple bottles for feeding c alves, the use of traditional tie-stall housing as opposed to loose ho using or other methods, and the maintenance of a closed herd. Farm-lev el 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 ma chines. These variables provide potential markers for the VTEC infecti on status of dairy herds in the target population, but the causal mech anisms underlying these observed associations are unknown.