Inter- and intra-specific variation within the genus Psoroptes (Acari: Psoroptidae).

In this paper the taxonomy and relative importance of various species within the genus Psoroptes are reviewed with respect to inter- and intra-specific variations in cross infestivity and host specificity, site of infestation and morphological characters. Intra-specific variation within populations of the sheep scab mite, Psoroptes ovis will then be discussed critically with respect to virulence (the relative growth of the scab lesion and mite burdens), epidemiology, susceptibility to the systemic acaricide, ivermectin, and resistance to synthetic pyrethroid and organophosphate acaricides. Finally, it is suggested that the ear mite, P. cuniculi, and the sheep scab mite, P. ovis are variants of the same species. Information was collected from published literature and from the results of unpublished studies carried out at the VLA, Weybridge over the last 10 years.

The use of moxidectin for the prevention and treatment of psoroptic mange (scab) in sheep.

The introduction of systemic injectable products, belonging to the avermectin/milbemycin class of antiparasitic compounds, has provided an alternative to dips for sheep scab control. Their main advantages are that they are quicker and safer to use, cause less stress to the sheep (hence can be used in heavily pregnant ewes), do not require any special handling facilities for their use and do not present the same environmental concerns over disposal of spent product. Moxidectin, a second generation milbemycin, has recently been registered as an injectable product for the prevention and treatment of sheep scab. Laboratory and field studies carried out in the UK and Ireland have shown that two injections at a dose rate of 200 mg/kg bwt., 10 days apart, are 100% effective in the treatment of scab. Further studies have shown that a single injection at the same dose rate will protect against infestation or reinfestation for a period of at least 28 days. This long persistent activity has been evaluated in two large-scale field prophylactic trials, one in Ireland and one in the UK, involving over 16,000 sheep, in which it was demonstrated that a single injection of moxidectin is a safe and effective alternative to dips for routine prevention of scab.

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.

Alternative methods for the control of sheep scab.

For nearly 17 years, plunge dipping has been the only method of scab control in the UK under a Government-controlled eradication campaign. Dipping is inconvenient and has been incriminated in the post-dipping illness of stockowners and contractors, and the disposal of large volumes of used dipwash is a potential hazard to the environment. Upon the deregulation of sheep scab in July 1992, stockowners are no longer obliged to dip and will seek alternative methods of scab control, ie, systemic injectables or synthetic pyrethroid pour-ons. Plunge dipping still offers the only fully effective method of controlling scab and it has a broad spectrum of activity against other ectoparasites of sheep. The present systemic pour-ons and oral drenches have little effect against scab, but double injections of systemic acaricides, ie, ivermectin, show great promise. The efficacy of systemic injectables is related to the initial mite burden and the second injection must not be omitted if complete control is to be achieved. The available synthetic pyrethroid pour-ons do not cover the entire sheep with a standard dose of acaricide, and leave the ventral surfaces unprotected and the sheep open to reinfestation.

Failure of a single treatment with ivermectin to control sheep scab (Psoroptes ovis) on artificially infested sheep.

Ivermectin at 200 micrograms/kg bodyweight given either as a single subcutaneous injection or as an oral drench failed to eradicate Psoroptes ovis from artificially infested sheep. The oral drench reduced the mite populations by 43 per cent within 24 hours but no further significant decline was recorded over 38 days. The subcutaneous injection reduced the mite populations by 90 per cent after 10 days but live P ovis were present on all the treated animals 84 days after treatment. The efficacy of treatment was less the higher the initial mite burden. The injection had no effect on clinical sheep scab, and the disease continued to progress despite the mite mortality.

Enzyme-linked immunosorbent assay for the detection of antibodies to the sheep scab mite Psoroptes ovis.

An enzyme-linked immunosorbent assay has been shown to detect antibodies in the circulation of sheep infected with Psoroptes ovis. Strong positive reactions were obtained from 19 sheep with four-month-old infections. No cross reaction was observed with sera obtained from sheep infected with either Fasciola hepatica, Nematodirus battus, Ostertagia circumcincta or Damalinia ovis.

Flumethrin: a non-stripping pyrethroid dip for the control of sheep scab.

Flumethrin, a synthetic pyrethroid, used in the laboratory as a sheep dip (Bayticol) at a concentration of 55 ppm, eradicated sheep scab from 30 infested sheep and provided protection against infestation for at least seven weeks. Similar results with at least five weeks protection were obtained on monitor sheep dipped under field conditions with flocks of 525 and 870 sheep. The dip was prepared and replenished with 66 ppm flumethrin and the concentration did not strip.