Quantification of underlying mechanisms of classical swine fever virus transmission

Abstract

Classical swine fever (CSF) is an exotic viral disease in most European countries. Occasionally, outbreaks occur due to re-introduction of the virus. During these outbreaks, virus transmission between herds occurs via direct contact between infected and susceptible pigs, or via indirect transmission routes like swill feeding, artificial insemination, or contaminated mechanical vectors. During the 1997-1998 epidemic in the Netherlands, in approximately 50% of the cases, no route of transmission could be identified. Because most of these infected herds were situated close to already infected herds, they were called neighbourhood infections. Several authors found that the likelihood of occurrence of a neighbourhood infection decreased, when the distance to the primary infected herd increased. Based on this relationship, models have been developed that evaluate control measures like preventive depopulation of herds or emergency vaccination. The application of these control measures has far-reaching consequences. Preventive depopulation of pig herds in close vicinity to an infected herd can result in the destruction of large numbers of uninfected pigs. Furthermore, vaccination could hamper international trade, which can result in high economic losses. Because our knowledge of the underlying mechanisms (transmission routes) of the neighbourhood infections still has considerable gaps, the effect of more specific control measures is difficult to quantify and evaluate. The research in this thesis focused on the quantification of underlying mechanisms of CSFV transmission and studying their contribution to transmission. This comprised the quantification of virus excretion by infected pigs, survival of the virus in the environment, and susceptibility of pigs. Furthermore, the effect of these parameters on transmission was studied in transmission experiments and in a risk model. It was shown that the higher the virulence of the strain, the more virus was excreted by infected pigs during their entire infectious period. An exception were pigs infected with a moderately virulent strain that developed the chronic form or died in the (sub)acute phase, because they can shed virus for a very long time (high excreting pigs). These high excreting pigs may play a crucial role in the transmission of CSFV due to these high amounts of virus excreted. Furthermore, studies in this thesis described for the first time the detection and quantification of CSFV in the air of rooms housing infected pigs. Although this is a hypothesized route of transmission in neighbourhood infections, previous attempts to isolate the virus from the air failed. This information can be used for further modelling to assess the importance of this transmission route. All knowledge obtained in this thesis on virus excretion of pigs, but also on survival of CSFV in the environment and susceptibility of pigs, was used in a risk model to estimate the probability of infection of susceptible pigs via different secretions and excretions. With additional data, this model can be used in the future to assess the efficacy of specific intervention methods.