ing ingestion bysheep (Esch and Fernandez 1993, Combes 1991). Infection of a mammalianbrain by rabies (Lyssavirus spp.) alters the host's behavior, increasing thechance of conflict with other potential hosts, while accumulation of rabiesvirus in the salivary glands ensures that it is spread by bites (Krebs, J. W. etal. 1995). Horizontally transmitted parasites which target nervous tissueincrease transmissibility by modifying the host into a suicidal instrument oftransmission. Transmission factors determining parasitic virulence are thespatial element in a spatial-temporal dynamic. Host density directlydetermines the virulence of parasites which depend upon a single host species(Herre 1993). Virulence may be increased when transmission necessitates insect vectors or consumption of the primary host by another species.Virulence varies inversely with the distance between potential hosts; thisdistance is magnified when it is measured between different species. THEEQUILIBRIUM MODEL It has been proposed that there is acoevolutionary arms race between parasite and host, as the former seeks tocircumvent the defensive adaptations of the latter (Esch and Fernandez1993). In this view, parasitic virulence is the result of a dynamic stalematebetween host and parasite. This exemplifies the red queen hypothesis, whichpredicts continued stalemate until the eventual extinction of both species.Benton (1990) notes that the red queen hypothesis ignores the potential forcompromise in such a system. Snails (Biomphalaria glabrata) resistant toSchistosoma mansoni are at a selective disadvantage due to the costsassociated with resistance (Esch and Fernandez 1993). A high level ofvirulence persists in the system because the snail cannot afford to mount anadequate defense. The arms race hypothesis assumes that the host populationcan successfully counter increasing parasitic virulence with resistance over anextended period of time. Although an arms race may be sust...
