Relationships Among Fecal Lungworm Loads, Fecal Glucocorticoid Metabolites, and Lamb Recruitment in Free-Ranging Rocky Mountain Bighorn Sheep

Abstract

Most wild Rocky Mountain big-horn sheep (Ovis canadensis canadensis) in northern latitudes are infected with lungworms. Indirect effects of lungworms on bighorn sheep are unknown, but high pulmonary burdens might increase stress (i.e., elevated glucocorticoid levels), and chronic stress could in turn decrease fitness. We hypothesized that high lungworm burdens in Rocky Mountain bighorn ewes increase stress, thereby increasing lamb mortality. To test our hypothesis, one subherd of bighorn sheep in Custer State Park, South Dakota was provided a free-choice loose mineral mix containing the anthelmintic fenbendazole every six weeks from March 1999 to August 2000 to eliminate lungworms; another subherd served as the control. Daily, individually marked ewes were located telemetrically from the ground and uniquely marked animals were observed until they defecated. After the herd moved from the area, fecal samples were collected and stored at Ϫ23 C. A consistent number of samples per season per herd (xϭ16.56Ϯ3.99 samples) were collected. Fecal larval lungworm levels (LPG) in the treatment subherd were lower than levels in the control subherd; however, there was no difference in fecal glucocorticoid metabolite (FGM) levels between the two subherds. Fecal glucocorticoid metabolite levels varied by season in both subherds, with levels in winter lower than during the other three seasons. Lamb:ewe ratios were not different between the control and treatment subherds at the end of summer 1999. In contrast, the treatment group had a lower lamb:ewe ratio at the end of summer 2000 despite having lower LPG. However, this result was attributed to lower ewe production, not lower lamb survival. The LPG levels were not correlated with FGM concentrations; instead, FGM levels might reflect normal seasonal patterns. Other factors, including contagious ecthyma, were more important for determining lamb mortality than LPG and FGM levels during our study. We suggest further experimental work over a longer duration to address these relationships.

Ecology of Gastropod and Bighorn Sheep Hosts of Lungworm on Isolated, Semiarid Mountain Ranges in Utah, Usa

“…Levels of fecal glucocorticoid metabolites did not differ between a subherd of bighorn sheep females treated with an anthelmintic to decrease fecal LPG and a control subherd of females, suggesting no relationship between stress hormones and lungworm burdens in female bighorn sheep (Goldstein et al, 2005). Finally, sex differences in parasite susceptibility or resistance may be related to the immunosuppressive effects of sex steroids (Dobson and Meagher, 1996;Hoby et al, 2006) and/or the allocation of energy to reproductive effort in different seasons (Festa-Bianchet, 1989;Pelletier et al, 2005).…”

“…Median larvae counts are based on fecal samples collected May through August in each year. Although larval output in bighorn feces differs significantly between late winter/early spring and summer/fall (Uhazy et al, 1973;Goldstein et al, 2005), variation in fecal larvae counts between May and October was not significant (Uhazy et al, 1973). However, reported seasonal patterns differ among populations (Arnett et al, 1993); therefore, we tested for variation in fecal larvae counts among months (May-August) in each year.…”

“…Despite treatment with Ivermectin prior to release on Antelope Island in 1997 and on the Newfoundland Mountains in 2001, a high prevalence of lungworms was observed in bighorn sheep fecal samples collected from the two populations during this study. Ivermectin is not effective against all life stages of lungworms (Easterly et al, 1992;Goldstein et al, 2005). Consequently, lungworms were introduced to these isolated, semiarid ranges with the bighorn sheep.…”

“…The high variability of fecal larvae counts may obscure differences between groups of individuals (Wilson et al, 2002), making it difficult to interpret negative results. Despite these methodological problems, field studies have relied on these noninvasive, indirect measures to investigate parasitism and its relationship to population and behavioral ecology, particularly in wild ungulate populations (Festa-Bianchet, 1989;Goldstein et al, 2005;Pelletier et al, 2005;Acevedo et al, 2005).…”

“…As lungworm levels in bighorn sheep feces are reported, in many areas, to be highest in late winter/early spring (Samson et al, 1987;Festa-Bianchet, 1991a;Goldstein et al, 2005), we also collected fecal pellets in March 2002 from Antelope Island. The March fecal samples were included in the overall analysis of lungworm prevalence on Antelope Island.…”