This study examined the relationships among louse density, pruritis and dermal response to insect antigens in sheep infested with Bovicola ovis. Polypay and Columbia ewes were allocated to two groups, infested and naive, and louse densities and pruritis were monitored for 15 months. Ten months after the initial infestation, all sheep were tested for hypersensitivity on the midside and ears by intradermal injection of soluble extracts of B. ovis, Stomoxys calcitrans and Musca autumnalis. The areas of skin reactions were measured at 20 min. 1, 3 and 24h after injection and skin thickness was measured at 24 h. Louse densities on Polypays were approximately 10 times greater than on Columbus, and pruritis was correlated with louse numbers at most inspections. Most pruritic behaviour was directed to the sides of infested sheep. Wheal and flare reactions developed rapidly to all extracts in both infested and naive ewes. Reactions to louse extract were larger in infested than naive sheep at all four times after injection. In the infested Polypays, reactions to louse extract were greater than to the fly extracts, but in naive sheep there was little difference among extracts. Reactions in naive Columbias were larger than in naive Polypays at 20 min, 1 and 3h, but had almost completely abated in both groups at 24h. Reactions in infested Columbias were greater than in infested Polypays at 20 min, but at 24h reactions in the Polypays were larger. Louse numbers and pruritis were correlated with wheal areas and skin thickness at 24h, but there was little relationship with the size of reactions at earlier times. These findings are consistent with the development of a hypersensitive response to B. ovis and suggest that dermal reactions to lice may influence sheep susceptibility.
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Acknowledgements-We thank Dan Brown for willing assistance and care of the experimental animals, and Doug McNamara for help with hypersensitivity testing. We also thank Drs B.E. Stromberg, T.E. Kurtii, U. Munderloh and M.T. Dahl for many helpful discussions. Financial support for the study was provided by an International Wool Secretariat Postgraduate Scholarship (P.J.) and a University of Minnesota Graduate School Dissertation Fellowship (P.J.). This is contribution 971170129 of the Minnesota Agricultural Experiment Station (Project No. 50).
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