A manipulative field experiment to examine the effect of Capillaria hepatica (Nematoda) on wild mouse populations in southern Australia.

A 12-month manipulative field study of the effect of a liver parasite, Capillaria hepatica, on mouse populations (Mus domesticus) was conducted in the Mallee wheatlands of northwestern Victoria. There were 2 untreated and 4 treated sites each consisting of a 16 km2 sampling zone. The parasite was released in September (spring) 1993 into an increasing mouse population which had a medium density (100-250 mice per ha). A third untreated site was monitored from January 1994. A simple but effective baiting method was developed and with the assistance of local farmers about 40,000 mice were dosed on the 4 sites; an estimated 5-7% of the population. During a second release, a month later, a further 20,000 mice were infected, boosting the level of infection to around 10%. Two months after the release approximately 30% of the population was infected. Thereafter, although there was effective transmission of the parasite, this was associated with a significant reduction in prevalence with time. The parasite reduced host survival by 5-10%, although this difference was not statistically significant, and had a minimal effect on the breeding and the rate of growth of mouse populations. Densities of 500-800 mice per ha were attained at each site in autumn 1994. Thus a spring release of the parasite into a rapidly increasing mouse population in the Victorian Mallee wheatlands did not prevent the population from increasing. The occurrence of very dry conditions following its release and the low but sustained transmission of the parasite indicate that we need to know more about factors that influence the survival and transmission of the parasite under field conditions.