Persistence of Mycobacterium avium subspecies paratuberculosis in rabbits: the interplay between horizontal and vertical transmission.
Abstract
Paratuberculosis (Mycobacterium avium subspecies paratuberculosis; Map) is a widespread and difficult disease to control in livestock populations and also has possible links to Crohn's disease in humans. Rabbits (Oryctolagus cuniculus) have been identified recently as the key wildlife species in terms of paratuberculosis transmission to the wider host community. Here, we test the hypothesis that Map can persist in rabbit populations for extended periods of time in the absence of any external source of infection. A spatially explicit stochastic simulation model of a generic host-disease interaction was developed to quantify the interplay between vertical and horizontal routes of transmission needed for the persistence of Map in rabbit populations and to test the hypothesis. The model was parameterized based on empirical studies on rabbit population dynamics and on rabbit-to-rabbit routes of Map transmission. Predictions from the model suggest that any disease with susceptible-infected (SI) dynamics without disease-induced mortality can persist within a rabbit population in the absence of vertical transmission, providing the horizontal transmission coefficient, β, is greater than approximately 0.012. The inclusion of any vertical transmission reduces the value of β that is necessary for infection to persist. Paratuberculosis persists in rabbit populations at all values of the horizontal and vertical transmission parameters in the range estimated from the field data and in many cases at all values within 95% confidence intervals around this range. The persistence of Map infection in rabbit populations in the absence of external sources of infection suggests that they may act as a reservoir of infection for sympatric livestock. Synthesis and applications. Our findings, in combination with the ubiquitous distribution of rabbits in the United Kingdom and elsewhere, suggests that if Map becomes established in rabbit populations they are likely to provide widespread and persistent environmental distributions of infection and thus disease risk to livestock and potentially humans. Where local rabbit populations are infected they should be included in any future paratuberculosis control strategies. Because eradication of rabbits is often not a realistic option, control strategies should include reducing interspecific transmission risk from rabbits to livestock via the faecal-oral route.