The classical swine fever epidemic 1997-1998 in the Netherlands: descriptive epidemiology

The objective of this paper is to describe the severe epidemic of classical swine fever (CSF) in The Netherlands in 1997-1998 under a policy of non-va ccination, intensive surveillance, preemptive slaughter and stamping out in an area which has one of the highest pig and herd densities in Europe. The primary outbreak was detected on 4 February 1997 on a mixed sow and fin ishing pig herd. A total of 429 outbreaks was observed during the epidemic, and approximately 700 000 pigs from these herds were slaughtered. Among th ese outbreaks were two artificial insemination centres, which resulted in a CSF-suspect declaration of 1680 pig herds (mainly located in the southern part of The Netherlands). The time between introduction of CSF virus (CSFV) into the country and diagnosis of CSF in the primary outbreak was estimate d to be approximately 6 weeks. It is presumed that CSFV was spread from The Netherlands to Italy and Spain via shipment of infected piglets in the beg inning of February 1997, before the establishment of a total stand-still of transportation. In June 1997, CSFV is presumed to be introduced into Belgi um from The Netherlands. Pre-emptive slaughter of herds that had been in contact with infected herds or were located in close vicinity of infected herds, was carried out aroun d the first two outbreaks. However, this policy was not further exercised t ill mid-April 1997, when pre-emptive slaughter became a standard operationa l procedure for the rest of the epidemic. In total, 1286 pig herds were pre -emptively slaughtered. (approximately 1.1 million pigs). A total of 44 out breaks (10%) was detected via preemptive slaughter. When there were clinical signs, the observed symptoms in infected herds wer e mainly atypical: fever, apathy, ataxia or a combination of these signs. I n 322 out of 429 outbreaks (75%), detection was bases on clinical signs obs erved: 32% was detected by the farmer, 25% by the veterinary practitioner, 10% of the outbreaks by tracing teams and 8% by screening teams of the vete rinary authorities. In 76% of the outbreaks detected by clinical signs, the farmer reported to have seen clinical symptoms for less than I week before diagnosis, in 22% for 1-4 weeks before diagnosis, and in 4 herds (1%) the farmer reported to have seen clinical symptoms for more than 4 weeks before diagnosis. Transportation lorries played a major role in the transmission of CSFV befo re the primary outbreak was diagnosed. It is estimated that approximately 3 9 herds were already infected before the first measures of the eradication campaign came into force. After the first measures to stop the spread of CSFV had been implemented, t he distribution of the most likely routes of transmission markedly changed. In most outbreaks, a neighbourhood infection was indicated. Basically, there were two reasons for this catastrophe. Firstly, there was the extent of the period between introduction of the virus in the region an d detection of the first outbreak. As a result, CSFV had opportunities to s pread from one herd to another during this period. Secondly, the measures i nitially taken did not prove sufficient in the swine- and herd-dense region involved. (C) 1999 Elsevier Science B.V. All rights reserved.