Acquisition and Expression of Resistance by Bos indicus and Bos indicus x Bos taurus Calves to Amblyomma americanum Infestation

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Abstract
Purebred and crossbred Bos indicus calves were infested 1, 2, or 3 times with 10 female and 5 male A. americanum. Resistance was acquired by both the purebred and the crossbred calves after 1 infestation and resulted in statistically significant decreases in the percentages of females that engorged, the mean weights of engorged females, and the mean weights of egg masses. Comparisons between breeds of the percent of female ticks that engorged during the first and second infestations indicate that purebred B. indicus expressed a stronger acquired resistance to A. americana more readily than did crossbred animals. Calves of both genetic compositions displayed similar levels of resistance during a 3rd exposure. All tick-exposed and control animals were skin tested with salivary gland extracts of A. americanum, A. cajennense and Dermacentor andersoni. Control, uninfested calves, did not display significant cutaneous reactivity to these extracts. All calves that had been infested had immediate, 30-min, 5-h and delayed, 24-h, skin reactions to Amblyomma spp. antigens. Reactions to D. andersoni salivary antigens in tests of both purebred and crossbred calves with acquired resistance to A. americanum suggest that Amblyomma spp. salivary gland antigens might have cross reactive moieties with a salivary extract prepared from D. andersoni. Peripheral blood lymphocyte in vitro responsiveness to Amblyomma spp. antigens was detected in purebred calves after a 1st, 2nd, and 3rd infestation, indicating the presence of cells of the immune system capable of recognizing and undergoing blast transformation in response to tick salivary components. Only 1 crossbred animal reacted to Amblyomma spp. salivary gland antigen. Cross reactions detected by in vitro lymphocyte proliferation reinforced evidence from skin tests that A. americanum and A. cajennense salivary antigens share components.