The population biology of parasite-induced changes in host behavior

Although changes in the behavior of infected hosts do occur for pathogens with direct life cycles, they are most commonly recorded in the intermediate hosts of parasites with complex life cycles. All the changes in host behavior serve to increase rates of transmission of the parasites between hosts. In the simplest case the changes in behavior increase rates of contact between infected and susceptible conspecific hosts; in more complex cases fairly sophisticated manipulations of the host's behavioral repertory are achieved. Three topics are dealt with in detail: 1) behavior of the insect vectors of such diseases as malaria and trypanosomiasis; 2) intermediate host of helminths whose behavior is affected in such a way as to make them more susceptible to predation by the definitive host in the life cycle; and 3) behavior and fecundity of molluscs infected with asexually reproducing parasitic flatworms. In each case an expression is derived for R₀, the basic reproductive rate of the parasite when first introduced into the population. This is used to determine the threshold numbers of definitive and intermediate hosts needed to maintain a population of the pathogen. In all cases, parasite-induced changes in host behavior tend to increase R₀ and reduce the threshold number of hosts required to sustain the infection. The population dynamics of the interaction between parasites and their hots are then explored using phase plane analyses. This suggests that both the parasite and intermediate host populations may show oscillatory patterns of abundance. Both asexual reproduction of the parasite within a host and parasite-induced reduction in host fecundity may be stabilizing mechanisms when they occur in the intermediate hosts of parasite species with indirect life cycles. -from Author