The first evidence of sheep scab mite resistance to injectable treatments was confirmed in 2018, although the extent of this issue is not known. The good news is that sheep scab mites which are resistant to injectable treatments have been demonstrated as susceptible to plunge dipping with diazinon. This allows us to eliminate the resistant sheep scab mites from the flock entirely.
Injectables ML treatments are also used to target gastrointestinal roundworms. Using injectables to target scab, when there is a low worm burden which does not require treatment, may also speed up the development of roundworm resistance. With this in mind, when determining which product to utilise, consideration should be given to how this will impact the roundworm population. If the roundworm burden is low, and does not require treatment, consider utilising plunge dipping which only targets external parasites.
If you utilise any product for treatment, and clinical signs persist more than 2 weeks after treatment, report the issue to the prescriber and manufacturer for guidance. Do not administer an additional treatment without seeking guidance.
Source: Doherty, E., Burgess, S., Mitchell, S. and Wall, R. (2018), First evidence of resistance to macrocyclic lactones in Psoroptes ovis sheep scab mites in the UK. Veterinary Record, 182: 106-106. https://doi.org/10.1136/vr.104657