Abstract
In four lambs that had developed resistance to sheep keds, resistance broke down in two of them within 2 weeks of being injected daily for a month with long-acting adrenocorticotrophic hormone (ACTH) and, in one, 3 weeks after oral treatment twice daily with acetylsalicylic acid (ASA). The second lamb given ASA twice daily for 6 weeks, remained resistant. The first three treated lambs regained their resistance when the treatments were discontinued. Daily injections of cortisone for 4 and ACTH for 6 weeks were given to separate groups of five resistant lambs each. Hormone doses were adjusted on the basis of thrice weekly absolute eosinophil counts. Both treatments broke the resistance, which was regained when cortisone treatment ceased but after only four weeks of treatment with ACTH. The role of the pituitary-adrenal system in modifying ked resistance in lambs is discussed. It is concluded that the basic annual ked population cycle can be affected by physiological or environmental stress such as pregnancy or undernourishment.
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