The population biology of multispecies helminth infection
Vertebrate hosts are frequently infected with multiple helminth species. There is a body of experimental evidence to suggest that infection with one parasite species can have either an antagonistic or synergistic effect on another species such interactions may occur through parasite establishment, survival and fecundity. The extent to which such interactions are involved in the organization of helminth communities is largely unknown. Mathematical models based on Markov processes are used to explore two themes: 1) The effect of interspecific interactions on the joint distribution of helminth par asites in a population of hosts, and 2) conditions under which interacting species can coexist. To explore the former, models are formulated that describe the pro cess by which helminths of two species are acquired and lost in a cohort of ageing hosts. In these models, the interspecific interaction occurs at the point of parasite establishment within the host such that the rate of establishment depends on the current worm burdens of the two species. The results are used to highlight some of the difficulties associated with inferring interspecific interactions from ecological data. The relationship between competition and species coexistence is investigated us ing models of the long-term dynamics of interacting species. Models are developed in which there is a free-living larval stage whose population size is dependent on the size of the adult worm population. The models are analyzed using 'hybrid' and 'moment-closure' approximations the former involves replacing stochastic com ponents of the model with deterministic approximations, and the latter assumes a functional relationship between higher and lower order moments based on a specified distribution. The Lotka-Volterra model of competition is derived for the case where hosts are equally exposed to parasites of the same species. Coexistence of two compet ing species is promoted by heterogeneous host exposure to each parasite species, provided that the rates of exposure to the two parasite species are not perfectly, positively correlated, and provided that the degree of heterogeneity in host expo sure is similar for both species. In addition, it is shown that the conditions required for coexistence are the same regardless of whether competition occurs at the point of parasite establishment within the host or via parasite fecundity. These results are discussed within the context of helminth community ecology.