Histamine content of tick attachment sites and the effects of H1and H2histamine antagonists on the expression of resistance

The histamine content at attachment sites of Dermacentor andersoni larvae on tick-resistant guineapigs was significantly increased compared with the histamine content of similar sites on animals given an initial, sensitizing, tick infestation. Anti-histamines with specificity for type-1 and type-2 receptors were given individually or concurrently in an attempt to alter the expression of tick resistance. Type-1 or type-2 histamine receptor antagonists given individually did not alter the resistance response, but the concurrent administration of type-1 and type-2 anti-histamines significantly blocked the expression of resistance. None of the above treatment regimens, when given only during an initial infestation, altered the acquisition or subsequent expression of tick resistance.