Disease persistence and apparent competition in a three-host community: an empirical and analytical study of large scale wild populations.

1. We investigated the effects of three types of host on the persistence of a tick-borne virus, using the grouse-hare-deer-tick-louping ill virus system of upland Britain. Each host differed in its interaction with the vector and pathogen. Grouse amplify virus only, deer amplify vector only and hares amplify both. Grouse alone suffer high virus-induced mortality.

2. An analytical model of the system was parameterized using empirical data from two wild populations with different community structures. By changing relative host densities we examined the conditions under which the virus would persist and considered the possibility of parasite-mediated competition between hosts.

3. Although deer alone and grouse alone were unable to maintain louping ill virus, a deer-grouse community usually allowed virus persistence because grouse transmitted virus while deer maintained the tick population. Since virus reduces grouse populations this is a type of apparent competition, and is unusual because deer do not amplify the virus.

4. At very high deer densities, the opposite effect could occur, whereby virus died out because of 'wasted' infected tick bites on deer, that do not transmit virus (the dilution effect).

5. In a hare-grouse two-host system virus usually persisted because hares amplified both the vector and virus (through non-viraemic transmission). Thus, apparent competition may occur between mountain hares and grouse.

6. The addition of a third host type increased the likelihood of disease persistence. Hares added to the deer-grouse system rendered the dilution effect unlikely because of additional virus amplifiers. Deer added to the hare-grouse system meant virus almost always persisted because they amplified the vector.