the parasite population. High virulence is characteristic ofhorizontal transmission cycles. In Herre's (1993) experiment, more virulentparasites were favored when host density was high and reduction of hostfitness was permissible. Certain parasites benefit from reduced host fitness,particularly parasites borne by insect vectors (Esch and Fernandez 1993) andparasites whose intermediate host must be ingested by another organism tocomplete the parasitic life cycle. By immobilizing their host, heartworm(Dirofilaria immitis) and malaria (Plasmodium spp.) increase the likelihoodthat mosquitoes will successfully ingest microfilaria or gametocytes along witha blood meal. Heartworm infestation causes pulmonary hypertension in dogs(Wise 1990), resulting in lethargy and eventual collapse (Georgi and Georgi1990). Host immobility increases the opportunities for female mosquitoes tofind and feed upon hosts (Read and Harvey 1993). Infected dogs have largenumbers of D. immitis microfilaria in their circulatory systems, again increasingthe likelihood of ingestion by the insect. Many infected dogs eventually die from heartworm, but in the process the parasite has ensured transmission.Similar debilitating effects have been observed in tapeworm-sticklebackinteraction; infected sticklebacks must swim nearer the water's surface due toan increased rate of oxygen consumption caused by the parasite (Keymerand Read 1991). Parasitized sticklebacks are more likely to be seen andeaten by birds, the next host in the life cycle. Many horizontally transmittedparasites manipulate specific aspects of host behavior to facilitatetransmission between species. Host fitness is severely impaired in suchinteractions. The digenean D. spathaceum invades the eyes of sticklebacks,increasing the likelihood of successful predation by birds (Milinski 1990). D.dendriticum migrate to the brains of infected ants, causing them touncontrollably clamp their jaws onto blades of grass, ensur...
