Sheep scab is a highly contagious ectoparasitic disease that is caused by
infestation with the mite, Psoroptes ovis (Acari: Psoroptidae). The disease is an
important welfare issue due to the clinical symptoms induced, which include intense
pruritis and severe exudative dermatitis. The host response is typical of an immediate
hypersensitivity reaction however, there has been little definition of the antigens
present in the mites to date. The main aims of this study were to identify the
allergens associated with the sheep scab mites, and to determine the main antigens
The proteases present in the water-soluble (SI) and membrane associated
(S2) P. ovis extracts were characterised because of their allergenic potential by
analogy with other organisms, particularly house dust mites. This was conducted
using a combination of substrate gels and protein degradation assays. Substantial
protease activity was present with cysteine proteases dominating, although aspartyl
and metallo-proteases were also present. These proteases were found to degrade
many substrates found in the mites' natural habitat, e.g. gelatin, collagen, fibronectin,
and haemoglobin. The proteases were not inhibited by antibody from animals with a
current infestation, however, this does not rule out the possibility that these proteases
are targets for the host immune response.
A complementary DNA (cDNA) library was prepared from mixed stage
mites in a lambda TriplEx2 vector, for the dual purpose of immunoscreening and
expressed sequence tag (EST) analysis. Immunoscreening was carried out using sera
from animals which had been immunised and protected against mite challenge with
mite extracts and 48 cDNA clones were selected. Of these, six were recognised in a
differential immunoscreen using sera from strongly protected animals compared with
sera from weakly protected animals. Subsequent sequence analysis established that
these clones had homology to myosin or myosin-based proteins.
The EST analysis of 500 randomly selected cDNA clones identified many
novel proteins and this study focused attention on the allergens, proteases and
antioxidant enzymes. Eight different allergens were identified including homologues
of the major house dust mite (Dermatophagoides spp.) allergens. The group 2
allergens, e.g. Der f2, were most abundant in the EST dataset. Cathepsin B and L
cysteine proteases were identified as well as several antioxidant enzymes, e.g.
glutathione S-transferase, superoxide dismutase and thioredoxin peroxidase, which
may help to protect the mite from toxic free radicals released in the host lesion.
This study has shown that there are a range of proteases present in P. ovis
mites. Eight putative allergens and three classes of antioxidant enzymes were also
identified. These results provide an insight into the physiology of the mite, and the
aetiology of the disease.