Response of hypersensitized guinea pigs to the feeding of Amblyomma americanum ticks.

Guinea pigs acquired resistance to the feeding of immature Amblyomma americanum ticks after a single infestation. The initial larval and nymphal feedings resulted in cumulative mortalities of 27.2 and 28.9%, respectively. The second and third larval infestations yielded cumulative mortalities of 50.5 and 52.4%, compared to 18.9 and 24.4% cumulative mortality from controls. Second and third nymphal feedings resulted in 75.7 and 73.4%, and ticks from controls exhibited 36.6 and 34.7% cumulative mortality. The development of host resistance adversely affected tick feeding and resulted in abnormally coloured ticks and a protracted moulting period with a marked decrease in moulting ability. Histological examination of tick feeding sites on guinea pigs challenged twice revealed greater cavity and lesion sizes in hosts fed upon by nymphs than larvae. The primary cell type within these feeding sites were basophils at 12 h post-attachment, but by 72 h post-attachment eosinophils were dominant. The upper epidermal layer adjacent to both larval and nymphal feeding ticks had accumulations of basophils with a few eosinophils. Lymphocytes may have been involved in the resistance response because local lymph nodes in resistant hosts were much larger than in uninfested and singly infested hosts. Some of these nodes had secondary germinal centres.