Studies on the Life Cycle, Infectivity and Clinical Effects of Capillaria hepatica (Bancroft) (Nematoda) in Mice, Mus musculus

The life cycle, infectivity and clinical effects of Capillaria hepatica, a nematode parasite of the liver of Mus rnusculus, were investigated as a prelude to assessment of the potential of the organism to suppress populations of this economic pest. Eggs of the parasite are deposited in the liver of the host and are not released during the life of that animal. C. hepatica was not transmitted by a trans-seminal, transplacental or trans-mammary route. A high rate of transmission of C. hepatica (prevalence 71.7% at 145 days) occurred in an enclosed breeding colony of BALB/c mice after the founders of the colony were fed infected mouse liver. After ingestion of infected liver by mice, the majority of eggs were released unembryonated to the environment, via the faeces, within 40 h. Eggs have specific temperature, moisture and oxygen requirements for embryonation. We postulate that transmission of C. hepatica is dependent principally upon cannibalism and/or necrophagy for release of eggs, upon the mouse burrow and the host population density for embryonation and survival of eggs, and upon grooming behaviour within the burrow for ingestion of embryonated eggs. C. hepatica established, and produced clinical signs, in 11 strains of laboratory mice and in wild mice. Hepatomegaly and splenomegaly were characteristic features of infection, and death occurred in some animals. The slight variation in susceptibility to infection among genetically diverse mice suggests that mice may not have the genetic flexibility to rapidly develop resistance to C. hepatica. The parasite has been recorded only in M. rnusculus, Rattus norvegicus and R. rattus in urban areas in Australia. However, native murids Rattus fuscipes and Pseudomys australis, and a marsupial Trichosurus vulpecula, are susceptible to experimental infection. Suspected C. hepatica infection has been found only in native murids near Atherton, Qld. The absence of this conspicuous infection in free-ranging native mammals elsewhere in Australia suggests that they do not come into contact with embryonated eggs of C. hepatica, a situation compatible with the postulated transmission cycle. The life cycle of C. hepatica appears unique among helminth parasites of mammals, in that transmission and hence survival of the parasite is dependent upon death of the definitive host. This parasite may have the potential to suppress mouse population density.