Transmission of classical swine fever virus within herds during the 1997-1998 epidemic in The Netherlands

In this paper, we describe the transmission of Classical Swine Fever virus (CSF virus) within herds during the 1997-1998 epidemic in the Netherlands. In seven herds where the infection started among individually housed breedi ng stock, all breeding pigs had been tested for antibodies to CSF virus sho rtly before depopulation. Based upon these data, the transmission of CSF vi rus between pigs was described as exponential growth in time with a paramet er r, that was estimated at 0.108 (95% confidence interval (95% CI) 0.060-0 .156). The accompanying per-generation transmission (expressed as the basic reproduction ratio, R-0) was estimated at 2.9. Based upon this characteris ation, a calculation method was derived with which serological findings at depopulation can be used to calculate the period in which the virus was wit h a certain probability introduced into that breeding stock. This model was used to estimate the period when the virus had been introduced into 34 her ds where the infection started in the breeding section. Of these herds, onl y a single contact with a herd previously infected had been traced. However , in contrast with the seven previously mentioned herds, only a sample of t he breeding pigs had been tested before depopulation (as was the common pro cedure during the epidemic). The observed number of days between the single contact with an infected herd and the day of sampling of these 34 herds fi tted well in the model. Thus, we concluded that the model and transmission parameter was in agreement with the transmission between breeding pigs in t hese herds. Because of the limited sample size and because it was usually unknown in wh ich specific pen the infection started, we were unable to estimate transmis sion parameters for weaned piglets and finishing pigs from the data collect ed during the epidemic. However, from the results of controlled experiments in which R-0 was estimated as 81 between weaned piglets and 14 between hea vy finishing pigs (Laevens et al., 1998a. Vet. Quart. 20, 41-45; Laevens et al., 1999. Ph.D.Thesis), we constructed a simple model to describe the tra nsmission of CSF virus in compartments (rooms) housing finishing pigs and w eaned piglets. From the number of pens per compartment, the number of pigs per pen, the numbers of pigs tested for antibodies to CSF virus and the dis tribution of the seropositive pigs in the compartment, this model gives aga in a period in which the virus most probably entered the herd. Using the fi ndings in 41 herds where the infection started in the section of the finish ers or weaned piglets of the age of 8 weeks or older, and of which only a s ingle contact with a herd previously infected was known, there was no reaso n to reject the model. Thus, we concluded that the transmission between wea ned piglets and finishing pigs during the epidemic was not significantly di fferent from the transmission observed in the experiments, (C) 1999 Elsevie r Science: B.V. All rights reserved.