Effect of horn flies on vanilmandelic acid excretion of dairy cattle

This study measured a physiological effect of known horn fly (Haematoba irritans L.) population densities on dairy cattle. Urinary excretion of the catecholamine metabolite, 3-methoxy-4-hydroxy mandelic acid, was an indicator of physiological response to the parasites. Six lactating Holstein cows were acclimated to the test room prior to a 3-wk control at 21 C. Animals were then exposed to approximately 500 horn flies per cow per day for 4 wk. On days 1 and 21 of exposure, two urine samples were obtained from each cow. Mean urinary values for cows were 13.3 +/- 3.1 mug/100 ml in the control period and 18.9 +/- 3.4 mug/100 ml during fly exposure. We believe that increased vanilmandelic acid reflects an increase in standing time and nervous activity associated with the physical disturbance due to the biting of the flies.