The spread of bovine tuberculosis in Canadian shared pastures: Data, model, and simulations

Abstract

Bovine tuberculosis (bTB), caused by Mycobacterium bovis, is a chronic disease typical of cattle. Nonetheless, it can affect many mammals including humans, making it one of the most widespread zoonotic diseases worldwide. In industrialized countries, the main pathways of introduction of bTB into a herd are animal trade and contact with infected wildlife. In addition, for slow-spreading diseases with a long latent period such as bTB, shared seasonal pastures might be a between-herd transmission pathway, indeed farmers might unknowingly send infected animals to the pasture, since clinical signs are rarely evident in early infection. In this study, we developed a dynamic stochastic model to represent the spread of bTB in pastures. This was tailored to Canadian cow-calf herds, as we calibrated the model with data sourced from a recent bTB outbreak in Western Canada. We built a model for a herd with seasonal management, characterized by its partition into a group staying in the main facility and the remaining group(s) moving to summer pastures. We used this model to estimate the time of first introduction of bTB into the herd. Further, we expanded the model to include herds categorized as high-risk contacts with the index herd, in order to estimate the potential for disease spread on shared pastures. Finally, we explored two control scenarios to be applied to high-risk farms after the outbreak detection. Our results showed that the first introduction likely happened three to five years prior to the detection of the index herd, and the probability of bTB spreading in pastures was low, but not negligible. Nevertheless, the surveillance system currently in place was effective to detect potential outbreaks.

Descriptive analysis of horse movement networks during the 2015 equestrian season in Ontario, Canada

“…Several studies of wildlife contact networks have demonstrated the importance of seasonal behavioural factors on contact patterns; for example, the increased contact frequencies during the breeding season of raccoons [43] and variability of badger contact patterns between summer and winter [44]. Seasonality has also been observed in livestock movement and contact patterns, such as the influence of the commercial breeding season on horse movements in New Zealand [14] and the contribution of shared summer pastures to infectious disease transmission in cattle [45]. Thus, seasonality, whether commercially or environmentally driven, can be an important driver for disease dynamics.…”

Microbiological and molecular monitoring for bovine tuberculosis in the Polish population of European bison (Bison bonasus)

“…BTB testing is also important in the EB population because wildlife often share pastures with livestock, and this has been found to contribute to the spread of mycobacteria [38,39,40]. Such sharing clearly presents a risk for EB which may come into contact with potentially infected cattle; conversely, infected EB can also pose a zoonotic risk for humans by coming into contact with livestock.…”

Identifying likely transmissions in Mycobacterium bovis infected populations of cattle and badgers using the Kolmogorov Forward Equations

“…The purpose of the following model is to describe the probability of pair infected hosts to be in a given state of infection progression, assuming that a directional transmission event occurred between the two. We specify a three-state Susceptible-Exposed-Infectious (SEI) model as being appropriate to the epidemiology of M. bovis infection dynamics 45,[51][52][53][54][55][56] . Here, Susceptible individuals become Exposed (but not infectious) after a successful transmission from an Infectious individual, and then move to an Infectious state after the latency period.…”