Some physiological responses associated with reduced wool growth during blowfly strike in Merino sheep

Objective To examine the effect on wool growth of physiological changes associated with experimental flystrike in superfine Merino wethers. Design An animal house study comparing experimentally fly struck sheep with non-infected control groups that were pair fed or fed ad libitum. Procedure Sheep in the blow fly larval challenge group were each artificially infected with 500 first instar larvae per day for 8 days. Infections were terminated with insecticide on day 10. For all sheep, live weight change, feed intake and body temperature were monitored and blood collected for analysis of haematology, cortisol, serum haptoglobin, serum amyloid A and the proinflammatory cytokines, TNFa, IL-1b, IL-6 and IL-8. Wool growth and staple strength were examined 4 months after infection. Results Experimental fly strike included moderate fever, depressed feed intake, and elevated cortisol, IL-6, serum amyloid A and haptoglobin. Staple strength was significantly decreased in struck sheep and did not differ between wool from sites adjacent to and remote from strike lesions. Conclusions The results suggest that depression in feed intake alone is not sufficient to account for the decline in staple strength seen during fly strike. Cortisol, IL-6 and metabolic consequences of the acute phase response are likely to be major contributors to the systemic effects of blowfly strike on wool.