Evaluation of the Association of Parasitism with Mortality of Wild European Rabbits Oryctolagus cuniculus (L.) in Southwestern Australia

Abstract

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.There is increasing interest in the role of infectious diseases in regulating their host populations. In order to regulate host populations, parasites must cause a decrease in survival, or fecundity, or both, and their abundance should increase with increasing host density at least over most of the range in abundance of their hosts (Scott and Dobson, 1989). Current population models for macroparasites contain assumptions that disease agents such as parasites increase host mortality (Anderson and May, 1979;Roberts et al., 1995), but there have been very few field studies that have tested this assumption (for discussion see Gulland [1995]).In 1992, a field experiment was established to measure the effect of reducing the fertility of female rabbits Oryctolagus cuniculus (L.) on rabbit abundance. The experiment was carried out in the southwest of Western Australia . Twelve discrete rabbit populations were created by a combination of rabbit-proof fences and maintenance of buffer zones. Breeding stops (warrens) were confined to areas of native shrub habitat adjacent to the pasture where rabbits fed. Thus, the experimental rabbits were living under essentially natural conditions. All rabbits were live-trapped at 4-6 weekly intervals that allowed for repeated nondestructive sampling of parasite abundance by fecal egg counts. In contrast, most parasitological studies of wild mammal populations involve the destructive sampling of hosts for the determination of actual parasite numbers by postmortem (e.g., Dunsmore and Dudzinski, 1968;Gulland, 1992). Fecal egg counts are effec...

Factors Influencing the Fecal Egg and Oocyst Counts of Parasites of Wild European Rabbits Oryctolagus cuniculus (L.) in Southern Western Australia

“…Climate is Mediterranean, with long dry summers during which time there is little breeding by rabbits. The annual average rainfall is 500 mm, concentrated in the cooler months of May to November, although some summer rainfall is normal (Twigg et al, 1998). Twelve discrete populations of rabbits were established to determine the level of female sterility that is required to cause a sustained reduction in rabbit abundance.…”

“…A field study set up to determine the effects of imposing 4 levels of female sterility (0%, 40%, 60%, 80%) on rabbit populations in southwestern Australia (Twigg et al, 1998) provided an opportunity to investigate further the effects of host age and sex, pregnancy, and season on parasitism by coccidia and nematodes. The 12 discrete free-ranging populations varied considerably in rabbit numbers particularly because some of the higher sterility regimes were successful in reducing population size (Hobbs et al, 1999). This variation enabled us to investigate the relationship between host population size and parasite abundance.…”

The future biological control of pest populations of European rabbits, Oryctolagus cuniculus

“…In support of our argument for examining sources of BCAs within Australia it emerges that two pathogenic forms of intestinal coccidiosis (Eimeria intestinalis and E. flavescens) have been recorded in western but not eastern Australia (Myers et al 1994;Williams et al 1995;Hobbs et al 1999). However, Hobbs and Twigg (1998) suggest that their reported absence from eastern Australia may be due to misidentification, and this suggestion needs to be tested.…”

“…This arid area may impede the movement of these parasites: in eastern Australia, Stodart (1968) reported that Eimeria infections were less intense, less frequent and caused by fewer species at a semiarid site than at wetter sites. Hobbs et al (1999) concluded that none of the Eimeria spp. present in Western Australia caused severe mortality.…”

“…present in Western Australia caused severe mortality. Nevertheless, given the caveats mentioned in Hobbs et al (1999), their evidence on this point was not clear cut and the possible benefits of releasing E. intestinalis and E. flavescens in eastern Australia should be investigated.…”