Factors associated with the introduction of classical swine fever virus into pig herds in the central area of the 1997/98 epidemic in the Netherlands

A matched case-control study of 135 infected and 99 uninfected pig herds fr om the central area of the 1997 to 1998 epidemic of classical swine fever ( CSF) in The Netherlands was undertaken to identify factors associated with the introduction of the virus. The herds were matched on the basis of herd type and the shortest geographical distance between pairs of herds. Data on management, hygienic measures, experiences during the depopulation of an i nfected nearest neighbour, and the frequency of contact with professionals and other agencies were collected by means of a questionnaire taken by pers onal interview. There were no significant differences between the infected and uninfected herds in the median total number of contacts per year with p rofessionals and other agencies either with or without contact with the pig s. On the basis of a multivariable analysis, five variables were found to b e significantly associated with an increased risk of infection: (1) the pre sence of commercial poultry on the premises; (2) visitors entering the pig units without wearing an overcoat or overalls and boots supplied by the far m; (3) the driver of the lorry transporting pigs for the Pig Welfare Dispos al Scheme (PWDS) using his own boots instead of boots supplied by the farm; (4) herds of moderate size (500 to 1000 animals) and very large herds (>70 00 animals) were at greater risk than small herds (<500 animals); and (5) a n aerosol, produced during high-pressure cleaning of the electrocution equi pment used to kill the pigs on a neighbouring infected herd less than 250 m away was carried by the wind on to the premises. Two variables were signif icantly associated with a decreased risk of CSFV-infection: (1) more than 3 0 years of experience in pig farming; and (2) additional cleaning of the lo rries used to transport pigs for the pesos before they were allowed on to t he premises. In the opinion of the cooperating farmers, airborne transmissi on of the virus and its transmission during the depopulation of an infected neighbour were among the most important routes of infection.