The 1997-1998 classical swine fever epidemic in The Netherlands - a survival analysis

The aim of this analysis was to characterise the temporal pattern of infect ion during the 1997/98 classical swine fever (CSF) epidemic in The Netherla nds and hence identify and quantify risk factors for infection in different enterprise types and areas. Survival analysis and Cox proportional hazards regression were used to describe the epidemic. Substantial differences in temporal survival patterns (herd breakdown rate) were found between areas w here different control policies operated. Factors with a significant influe nce on the infection hazard of individual herds included: sow numbers as a percentage of total sows and fatteners (HR = 3.38 for mixed herds (0.1-60% sows) vs. fattening herds (0% sows) and HR = 2.74 for breeding herds (60-10 0% sows) vs. fattening herds), the number of 'transport contacts per month' (>0.3 vs. <0.3; HR = 4.11), pig density (pigs/ m(2)) in the area (HR1000 ( pigs) 1.48) and herd size (HR100 (pigs) = 1.01). Pre-emptive slaughter in an area appeared to be associated with lower subse quent disease levels. Higher frequency of transport contacts for welfare sl aughter during the epidemic, however, well regulated and controlled, was as sociated with a substantially higher risk of becoming infected. The positiv e association of a higher pig density with CSF indicates the potential impo rtance of local spread as a factor in disease transmission and emphasizes t hat dilution of the pig population can contribute to reduction in CSF occur rence. This analysis suggests however, that if pre-emptive slaughter can pr omptly be applied effectively in an area after initial diagnosis, pig densi ty is then not a significant factor. Mixed and breeding herds had a higher probability of becoming infected than fattening herds, possibly due to diff erent types and frequencies of inter-herd contacts. These contacts continue to some extent during the epidemic, despite the standstill of animal movem ents. (C) 1999 Elsevier Science B.V. All rights reserved.