Babesia microti: Infectivity of parasites from ticks for hamsters and white-footed mice

To compare the infectivity of tick-transmitted Babesia microti for natural and experimental hosts, we permitted parasitized nymphal Ixodes dammini to feed on white-footed mice (Peromyscus leucopus) and hamsters. About nine-tenths of infested mice developed patent infection, but only about one-half of the hamsters become parasitemic. Tick-bitten mice become parasitemic more rapidly, but parasites became more abundant in hamsters than in mice. More than 100,000 B. microti were present in the salivary glands of nymphal ticks that remained attached to rodents for 60 hr. These parasites were collected, diluted, and inoculated into mice and hamsters. The mean infectious dose for both experimental hosts was in the range of 10,000 to 25,000 salivarian parasites. Compared to experimental hosts, natural hosts were more susceptible to tick-transmitted infection, became parasitemic more rapidly, but developed less intense parasitemias. Paradoxically, natural and experimental hosts were similarly susceptible to measured inocula of salivarian parasites.