Host-parasite ecology of keds (Melophagus ovinus (L.)) relative to sheep breed and age on Wyoming rangeland

Abstract

Sheep keds (Melophagus ovinus (L.)), an obligate external parasite of economic concern, are a continual problem in high-elevation grazing situations in most temperate sheep production areas of the world. The influence of sheep breed, age, and the interaction of breed*age on host-parasite ecology, could inform integrated keds management strategies. We measured ked density and presence/absence on Rambouillet, Hampshire, and Suffolk ewes in Wyoming, USA to determine the role of breed, ewe age, and breed*age interaction on ked parasitism. Eighteen ewes per breed group were examined for ked presence and ked density with sub-groups within each breed of 1, 2, and 3+ years of age. We used a generalized linear mixed model (GLMM) to analyze the effect of breed, age, and the interaction of breed and age, on a stacked response variable ‘ked infestation’ that is a function of both ked presence and density. Ked infestations differed by ewe age (p=0.0153; F=4.55) and by ewe breed (p=0.0178; F=3.30). Ked infestation increased with ewe age, as 29% of yearling ewes had keds and 63% of three-year old ewes had keds. Ked infestation was the greatest on the fine-wool Rambouillet ewes (72% presence; 1.4 keds per ewe), and was lower on the medium-wool breeds (Hampshire ewes had 39% presence and 1.0 keds per ewe and Suffolk ewes had 22% presence and 0.3 keds per ewe). The breed*age interaction did not significantly influence ked infestation in our joint model of the stacked bivariate ked infestation response outcome (p=0.1292; F=1.75). The lack of a significant interaction is attributed to the relatively large variance around the mean for such small breed-age sub-groups and warrants additional study. Our results have implications for parasite management as 100% of three-year old Rambouillet ewes had keds present, but none of the yearling Suffolk ewes had any keds and only 29% of three-year old Suffolk ewes had keds present. The results from our study indicate keds do discriminate by breed perhaps due to an ecological adaptation for a wool-environment that is dense and comprised of a long staple – features characteristic of the Rambouillet ewes in our study. Integrated ked management strategies could manipulate this host-parasite ecology by selecting resistant breeds and/or strategically treating the most susceptible ewes. For example, this could be all Rambouillet ewe ages or only Suffolk ewes >2 years of age.