Effect of Horn Flies on Vanilmandelic Acid Excretion of Dairy Cattle

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Abstract
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.degree. C. Animals were then exposed to about 500 horn flies/cow per day for 4 wk. On days 1 and 21 of exposure, 2 urine samples were obtained from each cow. Mean urinary values for cows were 13.3 .+-. 3.1 .mu.g/100 ml in the control period and 18.9 .+-. 3.4 .mu.g/100 ml during fly exposure. Increased vanilmandelic acid apparently reflects an increase in standing time and nervous activity associated with the physical disturbance due to the biting of the flies.