Trophically transmitted parasites and the conservation of small populations: Raccoon roundworm and the imperiled allegheny woodrat

Disease threats to endangered species are most commonly caused by exotic microparasites accidentally introduced into naive populations by humans. The case of raccoon roundworm ( Baylisascaris procyonis ) and the Allegheny woodrat ( Neotoma magister ) is an exception to this general rule and, as such, may be useful in the identification of other macroparasite disease threats in the future. B. procyonis is an intestinal nematode of raccoons ( Procyon lotor ) that is highly pathogenic in intermediate hosts. It is hypothesized to have caused or contributed to the extirpation of the Allegheny woodrat from the northern parts of its range. I tested this hypothesis by reintroducing Allegheny woodrats into four historically occupied sites with varying natural levels of environmental contamination with B. procyonis eggs. Populations were monitored via radiotelemetry and trapping for 22 months. Woodrat populations in less contaminated release sites survived significantly longer than populations in highly contaminated sites, and there was a strong negative correlation between the number of raccoon latrines and woodrat population persistence. This case demonstrates several points relevant to the conservation of small populations: trophically transmitted parasites ( in which prey species serve as intermediate hosts to the parasites of their predators ) can be significant sources of mortality in intermediate host populations; native parasites should not be ignored as potential disease threats; and parasites of human-adapted wild animals are likely to threaten rare and endangered species. This case also illustrates a previously unrecognized route through which host density can be decoupled from disease dynamics and provides evidence of apparent competition in nature.