e (Gibbons 1994). Parasitic virulence frequently changes over coevolutionary time, but the length of parasite-host association does not account for the virulence of the parasite. Transmission has been identified as the factor which determines the level of parasitic virulence (Read and Harvey 1993). TRANSMISSION AND THE DIRECTION OF MODULATION Herre's (1993) experiment with fig wasps (Pegoscapus spp.) and nematodes (Parasitodiplogaster spp.) illustrates the effect of transmission mode on parasitic virulence. When a single 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 to the fecundity of the host upon which it had arrived. When a fig was inhabited by several foundress wasps, horizontal transmission between wasp families was possible. In the figs inhabited by a single foundress wasp, Herre found that less virulent species of the nematode were successful, while in figs containing multiple foundress wasps, more virulent species of the nematode were successful. Greater opportunity to find alternate hosts resulted in less penalty for lowering host fecundity. More virulent nematodes had an adaptive advantage 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. Certain parasites avoid impairing their host's fecundity by becoming dormant within maternal tissue. Toxocara canis larvae reside in muscles and other somatic tissues of bitches until the 42nd to 56th day of a 70-day gestation, when they migrate through the placenta, entering fetal lungs where they remain until birth (Cheney and Hibler 1990). A high proportion of puppies are born with roundworm infection, which can also be ...
