Abstract
As the size of livestock farms in The Netherlands is on the increase for economic reasons, an important question is how disease introduction risks and risks of onward transmission scale with farm size (i.e. with the number of animals on the farm). Here we use the epidemic data of the 1997–1998 epidemic of Classical Swine Fever (CSF) Virus in The Netherlands to address this question for CSF risks. This dataset is one of the most powerful ones statistically as in this epidemic a total of 428 pig farms where infected, with the majority of farm sizes ranging between 27 and 1750 pigs, including piglets. We have extended the earlier models for the transmission risk as a function of between-farm distance, by adding two factors. These factors describe the effect of farm size on the susceptibility of a ‘receiving’ farm and on the infectivity of a ‘sending’ farm (or ‘source’ farm), respectively. Using the best-fitting model, we show that the size of a farm has a significant influence on both farm-level susceptibility and infectivity for CSF. Although larger farms are both more susceptible to CSF and, when infected, more infectious to other farms than smaller farms, the increase is less than linear. The higher the farm size, the smaller the effect of increments of farm size on the susceptibility and infectivity of a farm. Because of changes in the Dutch pig farming characteristics, a straightforward extrapolation of the observed farm size dependencies from 1997/1998 to present times would not be justified. However, based on our results one may expect that also for the current pig farming characteristics in The Netherlands, farm susceptibility and infectivity depend non-linearly on farm size, with some saturation effect for relatively large farm sizes.
Highlights
During the last two decades the mean size of pig herds has increased in many European countries, including The Netherlands
The best description c = 5, with the estimated parameter values of Table 2, confirms that between-farm transmission decreases with increasing between-farm distance
We note that the fit of c = 1 is not significantly different from that of c = 5, as the difference between the two Akaike’ Information Criterion (AIC)’s is smaller than 2 (DAIC,2.0) [18]
Summary
During the last two decades the mean size of pig herds has increased in many European countries, including The Netherlands. In 1997 around 14.5 million pigs were present in The Netherlands, housed in 21,500 farms, leading to an average number of 674 pigs (including piglets) per farm [1]. There is a public debate about the desirability of large farms, concerning animal welfare and disease aspects. For this discussion about the advantages and disadvantages of large farms, it is important to know the consequences of a large farm size for the risk of epidemics of contagious animal diseases
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