Seventy-five per cent of 845 Norway rats examined in the Baltimore Zoo for Capillaria hepatica were infected. Nearly all adult rats and 65% of juveniles were infected. Only 8% of 299 infected rats were heavily infected. The prevalence and intensity of infection increased with the size of the host. There were no seasonal differences in infection rates among adults, but juveniles collected during spring had higher infection rates than those collected during winter. Prevalence of C. hepatica infection varied from one place to another. No correlation between infection rate, vegetative cover, soil type, monthly rainfall, mean daily temperature, or food habits of rats was found. The dynamics of rat populations are perhaps the most important factors in the maintenance and dynamics of C. hepatica infections. Rapid population turnover contributes to the rapid release of a great number of eggs into the environment and high recruitment rates provide sufficient numbers of susceptible hosts for the parasite to complete its cycle.

It has been generally accepted that infected fleas do not pass on Rickettsia mooseri, or indeed any other known pathogen, to their progeny. It is reported here that such transovarial transmission does occur in laboratory-infected Xenopsylla cheopis fleas. By means of the direct fluorescent antibody test, Rickettsia mooseri was observed in cells of the hemolymph of infected fleas. As many as 11 percent of the adults and 2.9 percent of the larvae of the generation reared therefrom, had demonstrable rickettsiae. Moreover, batches of the F1 fleas were capable of transmitting the infection to more than 18 percent of the rats they infested. The data support the contention that Xenopsylla cheopis fleas play an important role in the maintenance of murine typhus in rats in nature.

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The egg-releasing mechanism and transmission ecology of Capillaria hepatica among Norway rat populations of the Baltimore Zoo were studied from 1972 to 1974. Nearly all adult rats were infected, while 65% of juveniles had infections. The mean egg count per liver was calculated to be 457,783 (N = 39 livers) and ranged from 11,270 to 1,400,000 eggs per liver. Data from the present study suggest that cannibalism serves as a primary egg-releasing mechanism and is a source of infection within the burrows. Increased infection rates among juveniles in spring support the hypothesis of maintenance of C. hepatica infections within the burrow system through cannibalism. Predation was responsible for scattered foci of infection throughout the study area and considered as a secondary source of infection. Decomposition was an important egg-releasing mechanism in secondary foci and in the warmer season when insects were active. However, of 849 carrion-associated insects and soil invertebrates collected from around decomposing rats, eggs of C. hepatica were found in only two species of beetles. This suggests a minor role for insects and soil invertebrates as egg disseminators.

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