Immunization against sheep scab: preliminary identification of fractions of Psoroptes ovis which confer protective effects.

In an attempt to enrich for potentially protective Psoroptes ovis antigens, three separate vaccine trials were conducted in which groups of sheep were immunized three times with various fractions of a soluble extract of P. ovis mites using QuilA as adjuvant. These groups, as well as controls that received adjuvant only, were challenged with P. ovis, and protective immunity was assessed by measuring lesion areas and conducting mite counts 4 and 6 weeks later. All fractions stimulated high titre serum antibodies. Most conferred some protection on sheep with active disease, although there was considerable variation between sheep in all groups, including the controls. Some fractions were more protective than the extract itself, suggesting that the protective components had been concentrated. Indeed the best fraction, obtained by ion exchange chromatography, followed by a gel filtration step, slowed lesion growth to less than a third by 6 weeks after challenge and reduced mite numbers by more than 13 times compared to control sheep vaccinated with QuilA only. However, as judged by polyacrylamide gels, the polypeptide profile of this fraction was still complex, indicating that further work is required to identify the protective components.