Studies were conducted to investigate the immunopathogenesis of cutaneous lesions in sheep scab. The initial studies examined
the nature and temporal development of the lesional infiltrate and growth during experimental primary and challenge infestations
of sheep with Psoroptes ovis. These demonstrated that granulocyte infiltration was dominated by eosinophils, accompanied by
mast cell hyperplasia and degranulation and dermal edema, features characteristic of IgE-mediated Type I hypersensitivity
reactions. In primary infestations eosinophil counts and epidermal and dermal pathology were maximal 9 weeks after infestation
while in challenge infestations eosinophil counts fell rapidly 3 weeks after challenge, an observation consistent with failure of the
mites to become established. Importantly, within 24 hours of primary infestation P. ovis had provoked an intense eosinophil
infiltrate and marked degenerative and proliferative epidermal pathology. Immunohistochemistry of primary infestations
revealed a massive lesional infiltration of CD4+ and CD45RA+ cells that was accompanied by smaller numbers of y§ T cells and
dendritic cells. In primary infestations, lesional growth was exponential during the first 7 weeks and then plateaued. In
challenge infestations lesional growth was significantly reduced (p< 0.001), an observation indicative of the development of a
substantial protective immunity.
Alterations in haematology and serology were investigated in parallel with the initial studies of lesional histopathology. In
primary infestations eosinophilia developed in all sheep and basophilia in 6/14 sheep but after challenge infestation basophilia was
not detected and eosinophilia was recorded in only one animal. Analysis of isotype-specific antibodies indicated that P. ovis
elicited antigen-specific IgG, IgM and IgE but apparently not IgA responses and that challenge infestations provoked a significant
(p<0.01-0.03) amnestic IgE but not IgG or IgM antibody response. Detection of P. ovis antigen-specific IgE provided further
evidence of the involvement of immediate hypersensitivity in the immunopathogenesis of cutaneous lesions but a significant rise
in antigen-specific IgE level was not detected until late (7 weeks) in the course of primary infestation. SDS-PAGE/Western blots
demonstrated that P. ovis antigens/allergens >100kD were labelled most consistently by IgG/IgE antibodies and that the number
of antigens/allergens recognised and the intensity of labelling increased with time after infestation.
Intradermal skin tests demonstrated the occurrence of immediate hypersensitivity, late phase (LPR) and delayed-type
hypersensitivity (DTH) reactions to P. ovis whole-mite extract. Histopathology ofthe LPR and DTH indicated that both
responses, but particularly the DTH, were dominated by eosinophils. Prausnitz-Kustner tests demonstrated the IgE dependence
ofthe LPR and that the DTH was not transferred by a serum factor. Examination of the temporal development of these
hypersensitivity responses indicated that they were not manifested until late in the course of infestation.
This research has demonstrated that P. ovis provokes an intense eosinophil dominated innate response, an IgE-mediated Type I
reaction and eosinophil dominated late phase and non-classical DTH reactions and that in addition, it generates a significant
protective immune response.