transmitted from bitch to puppy by milk (Cheney and Hibler 1990). If host density is low, a highly evolved vertical transmission cycle (which exhibits low virulence in the parent) ensures the survival of the parasite population. High virulence is characteristic of horizontal transmission cycles. In Herre's (1993) experiment, more virulent parasites were favored when host density was high and reduction of host fitness was permissible. Certain parasites benefit from reduced host fitness, particularly parasites borne by insect vectors (Esch and Fernandez 1993) and parasites whose intermediate host must be ingested by another organism to complete the parasitic life cycle. By immobilizing their host, heartworm (Dirofilaria immitis) and malaria (Plasmodium spp.) increase the likelihood that mosquitoes will successfully ingest microfilaria or gametocytes along with a blood meal. Heartworm infestation causes pulmonary hypertension in dogs (Wise 1990), resulting in lethargy and eventual collapse (Georgi and Georgi 1990). Host immobility increases the opportunities for female mosquitoes to find and feed upon hosts (Read and Harvey 1993). Infected dogs have large numbers of D. immitis microfilaria in their circulatory systems, again increasing the 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-stickleback interaction; infected sticklebacks must swim nearer the water's surface due to an increased rate of oxygen consumption caused by the parasite (Keymer and Read 1991). Parasitized sticklebacks are more likely to be seen and eaten by birds, the next host in the life cycle. Many horizontally transmitted parasites manipulate specific aspects of host behavior to facilitate transmission between species. Host fitness is severely impaired in such interactions. Th...
