Moxidectin Resistance Confirmed in Sheep Scab Mites

Moxidectin Resistance Confirmed in Sheep Scab Mites from SAC Consulting
Confirmation of moxidectin resistance in Psoroptes ovis mites from four flocks in England and Wales has been published online this week in the Veterinary Record (Doherty et al., 2018).
SAC C VS has offered free testing for the diagnosis of sheep scab since 2002 and diagnoses peak at this time of year (Figure 1).
Almost 300 samples have been examined over the last five years with scab mites detected in 32%. 
In addition, free testing is to be offered in England and Wales until 31/3/18. 

Figure 1: Psoroptes ovis diagnoses by month 1/1/11 to 31/12/17

Accurately diagnosing the cause of itchy sheep is essential in order to select the correct treatment and avoid the overuse of macrocyclic lactone injections.
It is pertinent to note that 29 of the holdings that made a notification of sheep scab in 2016 treated their flock with moxidectin.  Doramectin was used by 37, 5 administered ivermectin, and 13 flocks dipped (Acknowledgement: Ian Bell, APHA).

It is highly likely that cross resistance will occur to the other macrocyclic lactones, and it is essential that any suspicion of treatment failure is investigated.  Fortunately the poor off-host survival of P. ovis mites means that, if resistant mites are detected, it should be possible to eradicate them from a flock by dipping effectively in diazinon.

Doherty, E., Burgess, S., Mitchell, S. & Wall, R. (2018) First evidence of resistance to macrocyclic lactones in Psoroptes ovis sheep scab mites in the UK.  Veterinary Record, vetrec-2017-104657;DOI:10.1136/vr.104657

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