Interaction of pesticidal toxicity, parasites, and reversible anticholinesterase activity as stresses on growth rate in cattle infested with horn flies Haematobia irritans L.

Abstract

Differential rates of gain in groups of immature cattle exposed to horn fly attack were used to distinguish the interaction of pesticidal activity and nonpesticidal pharmacologically induced stress with hostparasite reaction. Group treatments included DDT and coumaphos sprays as pesticidal agents, which produce low and moderate bovine toxicities, respectively, at parasiticidal concentrations; water spray at high pressure as an intermittent but relatively innocuous physical suppressor of infestations; and eserine as an agent of intermittent pharmacologically induced stress superimposed on uncontrolled fly attack. Feeding activity of parasites retarded growth of hosts during the buildup but not during the decline of the infestation. Toxicity of coumaphos applied as a prophylactic treatment reduced potential for accelerated growth, especially in the compensatory period during and after decline in infestations. Anticholinergic stress induced by eserine amplified parasitic retardation of growth in unprotected animals for 6–9 weeks, after which animals adapted to the drug and corresponded more closely in weight with the untreated group. Intermittent reduction of flies with a water spray induced a high, stable rate of growth during the horn fly season. With DDT-protected animals as a baseline, growth rate under pressure of infestation was retarded by interacting stresses at the rate of 1, 8, 15.3 and 17.5 equivalent growing days, respectively, for animals subjected to water treatment, coumaphos spray, no treatment, and pharmacologically induced stress during a 98-day observation period.