Identifying disease reservoirs in complex systems: mountain hares as reservoirs of ticks and louping-ill virus, pathogens of red grouse

Abstract

1. We examined the role of mountain hares in the louping-ill virus/Ixodes ricinus tick system to determine whether hares were reservoirs of these pathogens for red grouse. A field experiment, which involved reducing mountain hare densities was undertaken and changes in tick abundance, louping-ill virus seroprevalence and red grouse densities recorded. 2. Hares were found to be important hosts for all stages of ticks at two study sites and, where sheep were frequently treated with acaricide, hares fed the greatest proportion of adult ticks. Hare densities were reduced at the experimental site between 1993 and 2001 but remained relatively constant at a control site. Both nymph and larvae tick burdens on red grouse chicks declined over this period to very low levels at the treatment site, but not at the control site. The estimated size of the tick population at the treatment site decreased by more than 99% by 1999. 3. Louping-ill prevalence, as measured by antibody prevalence in shot young red grouse, also declined at the treatment site, lagging behind the tick decline by approximately 2 years. The number of young grouse produced per hen grouse at this treatment site increased in comparison to a second control area. However, no change was found in summer grouse densities and thus we cannot demonstrate parasite-mediated competition. 4. A deterministic SIR-type model describing the dynamics of this pathogen/vector/host system was constructed and used to examine the role of mountain hares in louping-ill virus and tick dynamics. The model predicted a decline in tick numbers and seroprevalence as hare densities reduced. The inclusion of hares as both tick hosts and vector-host-vector transmitters of louping-ill virus gave the best fit to the observed data.