Evaluation of the association of parasitism with mortality of wild European rabbits Oryctolagus cuniculus (L.) in southwestern Australia.

Abundances of the parasitic nematodes Trichostrongylus retortaeformis and Passalurus ambiguus, and 8 Eimeria species were estimated by fecal egg and oocyst output in 12 discrete free-ranging populations of wild rabbits (Oryctolagus cuniculus) in southwestern Australia. Comparisons of parasite egg and oocyst counts were made between those rabbits known to have survived at least 2 mo after fecal samples were collected and those rabbits that did not survive. There were significant negative relationships between parasite egg and oocyst counts and survival when all age groups and collection periods were pooled for several species of coccidia and for T. retortaeformis. However, when the same comparisons were made within rabbit age groups and within collection periods, there were very few significant differences even where sample sizes were quite large. The differences indicated by the pooled analysis for coccidia were most likely due to an uneven host age distribution with respect to survival, combined with an uneven distribution of the oocyst counts with rabbit age. The result for T. retortaeformis was similarly affected but by a seasonal pattern. Parasitism by nematodes and coccidia did not appear to be an important mortality factor in these rabbit populations, at least at the range of host densities we examined. This suggests that other factors must have been responsible for the observed pattern of density-dependent regulation in these rabbits.