unt for the virulence of the parasite.Transmission has been identified as the factor which determines the level ofparasitic virulence (Read and Harvey 1993). TRANSMISSION AND THEDIRECTION OF MODULATION Herre's (1993) experiment with figwasps (Pegoscapus spp.) and nematodes (Parasitodiplogaster spp.)illustrates the effect of transmission mode on parasitic virulence. When asingle female wasp inhabited a fig, all transmission of the parasite was vertical,from the female to her offspring. The parasite's fitness was intimately tied tothe fecundity of the host upon which it had arrived. When a fig was inhabitedby several foundress wasps, horizontal transmission between wasp familieswas possible. In the figs inhabited by a single foundress wasp, Herre foundthat less virulent species of the nematode were successful, while in figscontaining multiple foundress wasps, more virulent species of the nematodewere successful. Greater opportunity to find alternate hosts resulted in lesspenalty for lowering host fecundity. More virulent nematodes had an adaptiveadvantage when host density was high and horizontal transmission was possible. When host density was low, nematodes which had less effect on host fecundity ensured that offspring (i.e. future hosts) would be available.Low virulence is characteristic of many vertical transmission cycles. Certainparasites avoid impairing their host's fecundity by becoming dormant withinmaternal tissue. Toxocara canis larvae reside in muscles and other somatictissues of bitches until the 42nd to 56th day of a 70-day gestation, when theymigrate through the placenta, entering fetal lungs where they remain until birth(Cheney and Hibler 1990). A high proportion of puppies are born withroundworm infection, which can also be transmitted from bitch to puppy bymilk (Cheney and Hibler 1990). If host density is low, a highly evolvedvertical transmission cycle (which exhibits low virulence in the parent) ensuresthe survival of ...
