Mechanisms underlying disease transmission between spatially separated animals

Abstract

Transmission of infections between spatially separated hosts is a common problem, not only during major outbreaks of livestock diseases, but also in many other settings such as the transmission of infectious diseases between plants and crops or in healthcare settings. During the last major epidemics of livestock diseases in the Netherlands and abroad, disease transmission events occurred despite movement bans and other (bio-)security measures, implying that indirect transmission plays a major role. A better understanding of indirect transmission is necessary to put in place evidence based bio-security measures against neighbourhood (indirect) transmission. To gain more insight in the mechanisms underlying indirect transmission a series of experimental studies combined with mathematical modelling were conducted of which the results are presented in this thesis. First the effect of acidification of drinking water on the transmission parameters of direct and indirect transmission of Campylobacter jejuni (C. jejuni) between broilers was studied. It was shown that acidified drinking water has an effect on indirect transmission but not on direct transmission of C. jejuni between broilers. The sender and receiver sub-process of indirect transmission was then studied in more detail and it was shown that a significant negative interaction effect between acidification of the sender and receiver sub-processes exists, indicating that there is no additional effect of acidification of the drinking water on both sides of the transmission process compared to acidified drinking water only on one side. To study the transport of the pathogen in the environment in more detail, a series of indirect transmission experiments was carried out and a model framework was developed to study indirect transmission between spatially separated hosts. These studies showed that indirect transmission of C. jejuni between broilers is best described by a multistage environmental route from sending to receiving animal, suggesting that indirect transmission occurs through progressive (but slow) contamination of the environment surrounding the source. Indirect transmission experiments where repeated with both C. jejuni and Escherichia coli and the results showed that for C. jejuni it takes much longer for the first effective (viable) bacterium to cross the small distance of approximately 75 cm than it does for Escherichia coli. A new modelling approach to study indirect transmission was developed guided by these indirect transmission experiments. This model is capable of accurately describing the pathogen dispersal process by a diffusive transport mechanism which includes pathogen mortality. Lastly, a range of dose-response models were compared and tested how well these fitted to the data from a dose-response experiment. Here it was shown that for interpolation purposes two relatively simple models are best capable of describing the data from the dose-response experiment. For extrapolation purposes, however, it was shown that from the models that were studied a model that abides by the independent action hypothesis is best.</p