The influence of parasites on faecal glucocorticoid metabolite levels in raccoons: An experimental assessment in a natural setting

Parasites and glucocorticoid hormones interact and affect a variety of processes within vertebrates, such as immune system function and reproduction. The nature of the relationship between parasite infection and glucocorticoid levels has received relatively little attention among free-ranging animals and results of experimental research in natural settings are equivocal. We conducted a parasite-reduction experiment to determine if reductions in nematodes or ectoparasites affect levels of faecal glucocorticoid metabolites (FGM) in adult raccoons. Individual raccoons were randomly assigned to a parasite-reduction treatment (ivermectin injection and Frontline Plus^® application) or control group (saline injection) and recaptured within 30 days to assess treatment-related differences in parasitism and FGM levels. Treated animals had reduced nematode and ectoparasite communities. The most common and energetically expensive ectoparasite of raccoons in the region, the American dog tick, was reduced five-fold from an average of 19.3 ± 2.5 (se) to 3.4 ± 8 ticks per animal, and was unable to feed to repletion on treated animals. The prevalence of four out of seven nematode species was significantly lower in treated versus control animals; prevalence of these four nematodes ranged from 0 to 19% among treated animals and from 21 to 55% among control animals. The parasite infracommunity was also significantly reduced; the average number of nematode species per individual was 2.5 ± 0.3 in treated animals and 1.1 ± 0.2 in control animals, and the average number of ectoparasite species per individual was 2.3 ± 0.1 on treated animals and 1.1 ± 0.12 on control animals. No differences in FGM values were observed within individuals or between treatment and control groups following parasite-reduction treatments, indicating that the observed reductions in nematodes and ectoparasites had no effect on FGM levels of raccoons across the time frame of this study.