Quantification of the transmission of classical swine fever virus between herds during the 1997-1998 epidemic in The Netherlands

In this study, we describe a method to quantify the transmission of Classic al Swine Fever Virus (CSFV) between herds from data collected during the 19 97-1998 epidemic in the Netherlands. From the contacts between infected her ds and the serological findings shortly before depopulation, we estimated t he week of virus introduction and the length of the period over which the h erd emitted virus for each CSFV-infected herd. From these data, we estimate d the infection-rate parameter beta (the average number of herds infected b y one infectious herd during one week) and the herd reproduction ratio, R-h (the average total number of secondary outbreaks caused by one infectious herd, i.e. in its entire infectious period), using a SIR-model for differen t sets of CSF control measures. When R-h > 1, an epidemic continues to grow . On the other hand, when R-h < 1 an epidemic will fade out. During the phase before the first outbreak was diagnosed and no specific me asures had been implemented, beta was estimated at 1.09 and R-h at 6.8. In the subsequent phase infected herds were depopulated, movement restrictions were implemented, infected herds were traced forward and backward and the herds in the protection and surveillance zones were clinically inspected by the veterinary authorities (regional screening). This set of measures sign ificantly reduced beta to 0.38. However, R-h was 1.3 and thus still >1. Con sequently, the number of outbreaks continued to grow. After a number of add itional measures were implemented, the value of R-h was reduced to 0.5 and the epidemic came to an end. These measures included pre-emptive slaughter of herds that had been in contact with infected herds or were located near an infected herd, increased hygienic procedures, replacement of transports of pigs for welfare reasons by killing of young piglets and a breeding ban, and regional screening for CSF-infected herds by local veterinary practiti oners. (C) 1999 Elsevier Science B.V.. All rights reserved.