Temporal development of ovine cutaneous hypersensitivity responses to Psoroptes ovis

Abstract

Clinical responses to bites of several haematophagous arthropods and to Sarcoptes scabiei develop in an orderly sequence: induction, only a delayed response, both immediate and delayed responses, only an immediate response, and desensitization. Wheal reactions to intradermal injections of Psoroptes spp.extract have demonstrated that Psoroptes spp. infestations of rabbits, cattle and sheep elicit immediate (IH) and delayed‐type (DTH) hypersensitivity responses. However, the temporal development of these reactions has not been reported. In the study described here, clinical and histological examination of responses provoked by intradermal injection of P. ovis whole‐mite extract (WME) were employed to investigate the temporal development of hypersensitivity responses of sheep infested with P. ovis. Five sheep were infested with P. ovis mites and five were used as uninfested controls. Intradermal tests were carried out 1, 3, 5, 7 and 9 weeks after infestation and at the same intervals in the controls. P. ovis WME and a diluent control were each injected intradermally at three sites on the flank. Wheal reactions were measured at 0.5, 6, 24 and 48 h after challenge. Punch biopsies were collected from antigen and diluent control sites 6, 24 and 48 h after challenge. Half the biopsy was fixed in 4% paraformaldehyde for estimation of dermal eosinophils and half in a zinc salts fixative to determine CD4+ counts. Statistical significance of results was determined by analysis of variance. In one animal, the infestation failed to become established; consequently, it was omitted from the results. Immediate hypersensitivity reactions were first observed in three of the four successfully infested sheep 7 weeks after infestation. One infested animal failed to develop an immediate reaction by 9 weeks, and none of the sheep developed a delayed wheal reaction. Histology demonstrated that at 9 weeks, P. ovis WME provoked an eosinophil‐rich, delayed reaction in three of the four infested sheep. This response was significantly greater than that seen 24 and 48 h after challenge at 7 weeks. In two sheep, the DTH response was preceded by an immediate wheal reaction. None of the control sheep developed immediate or delayed wheal reactions or histological evidence of a DTH response. At 9 weeks after infestation, mean CD4+ counts elicited by challenge with P. ovis WME were substantially, but not significantly, greater than at 1 week. The present study has demonstrated that IH and DTH responses are not detected until relatively late in the course of P. ovis infestation and that histology proved more reliable than wheal reactions in revealing the occurrence of the DTH reaction. In contrast to the classical response pattern, P. ovis infestation may first generate an IH response that is succeeded by a phase when both IH and DTH reactions occur, or first provoke a DTH response. Increased CD4+ counts after challenge with P. ovis WME at 9 weeks may indicate the involvement of CD4+ T cells in the genesis of the DTH response to P. ovis. Funding: Department of the Environment, Food and Rural Affairs; Scottish Executive.