Effects of a lone star tick (Amblyomma americanum) burden on performance and metabolic indicators in growing beef steers

We examined the response of selected animal performance, endocrine, immune, and metabolic factors from 13 steers (254 ± 6.1. kg) with and without a lone star (Amblyomma americanum) tick burden during progressive days of the tick feeding cycle. Steers were randomly assigned to either non-treated controls or treated with 300 adult pair of A. americanum per animal. Animals were weighed and blood sampled on days -7, 0, 10, 14, 21, 28, and 35 relative to tick treatment. Tick treatment did not affect (P< 0.1) animal performance. Experimental day did (P< 0.05) affect body weight gain and dry matter intake. Tick treatment did not affect (P< 0.1) metabolic indicators. Experimental day affected (P< 0.05) IGF1 and lactate, tended to affect cortisol (P< 0.07), but did not affect (P< 0.1) glucose concentrations. Tick treatment did not significantly (P< 0.1) affect growth hormone receptor (GHR) mRNA in liver, but liver tissue from treated animals had numerically lower GHR mRNA than did tissue from control animals. Day had a significant (P< 0.05) effect on liver GHR mRNA. There was a significant treatment by day interaction (P< 0.05) for liver IGF1 gene expression, as IGF1 mRNA was reduced in tick-treated cattle versus control cattle on day 35. Overall, liver IGF1 gene expression was lower (P< 0.05) in tick than in control animals while there was no effect (P> 0.1) due to day. Within the tick-treated group, correlations were found between quantitative female tick feeding characteristics and host metabolic indicators. Feeding by adult female lone star ticks did cause acute stress in growing beef steers on a moderate plane of nutrition as indicated by some physiologic indicators. In particular there may be longer term effects on the somatotrophic axis in the liver which could affect subsequent (i.e. feedlot) performance. It is not known how these observed effects would be manifest under a lower plane of nutrition, as is common and may become more so within the current native range of A. americanum. Other acute effects due to tick feeding may have been masked by the effects of handling and invasive sampling. Non-invasive experimental procedures are called for in order to study the effects of a stressor such as arthropod infestation on grazing animals. Future research efforts will be aimed at non-invasively elucidating the effects of tick stress on grazing animals under various nutritional environments.