Risk factors associated with seropositivity to porcine respiratory coronavirus in Danish swine herds

Serological screening of swine herds in 1984 indicated that porcine respiratory coronavirus (PRCV) had been introduced into Denmark. To determine risk factors associated with the introduction of PRCV, a cross-sectional study of 408 Danish swine herds was carried out between May 1985 and June 1986. The association between herd-PRCV serological status and possible risk factors, obtained from a field questionnaire, was assessed by unconditional maximum likelihood logistic regression. An increasing herd size, location in the Jutland peninsula (compared with location on the island of Funen) (OR = 7.9 in a multivariable logistic regression model not including interaction terms), the presence of a slurry system (i.e. pigs living on a slatted floor) (OR = 4.6) and purchase of pigs (OR= 1.7) were significantly (P < 0.05) associated with seropositivity. Two significant interactions, both involving herd size, were subsequently identified. The PRCV serological status of neighbouring herds was found to be related, and closeness of a seropositive herd was associated with an increased risk of a herd being serologically positive. The results of this study indicate that herd size may be an important determinant of airborne transmission of PRCV infection, and that herd size may modify the effect of other risk factors.