Epidemiology of subclinical ovine psoroptic otoacariasis in Great Britain.

Seven sheep flocks with subclinical psoroptic otoacariasis were investigated. Psoroptes species mites were isolated from 3.1 per cent of the 2676 sheep examined and the prevalence in the flocks ranged between 1.3 and 23.9 per cent, with the highest infestations in pedigree flocks. Unilateral or bilateral infestations were found in sheep of all ages. Adult and shearling rams, with prevalences of 21.5 and 14.2 per cent, respectively, were the most affected and two lambs, two and eight days old, were the youngest to be infested. There was no evidence of vertical transmission. The infestation affected several breeds of sheep. No other known hosts for Psoroptes species present on the premises were shown to be infested. Non-parasitic for-age mites were also isolated from the ear canals of the sheep and may have contributed to the clinical signs which, in adult sheep, ranged from aural haematomas/fibrosis (cauliflower ears), and violent head shaking and ear rubbing leading to excoriation and wounding of the ear and base of the ear. The signs in lambs included plaques of scab (often bloody) on the external ear cleft, excoriation of the base of the ear, ear scratching with the hind feet and inflammation of the external aspects of the horizontal canal. In all cases the internal pinnae were clear of typical psoroptic scabs. Psoroptes mites were isolated from 28.6 per cent of the damaged ears and from 7.8 per cent of the undamaged ears. There was no evidence of classical sheep scab in any of the flocks. Plunge dipping in diazinon, propetamphos or flumethrin, or the use of synthetic pyrethroid pour-on preparations and an oral drench of ivermectin had little effect because ear mite infestations were detected in the flocks after these treatments. An injection of ivermectin at 200 micrograms/kg bodyweight effectively eradicated the infestations when it was administered to these infested flocks.