Correct use of injectable macrocyclic lactones for the control of sheep scab is vital.
Prof Richard Wall, from the University of Bristol, explains that the recent study published in Veterinary Parasitology demonstrated scab mites collected from sheep that apparently had not responded to treatment on-farm were resistant to all three ML compounds: moxidectin, ivermectin and doramectin.
He adds the study also found considerable variation in response, suggesting resistance alone was not the only factor affecting their ability to work, with inappropriate treatment application and management the most likely reasons.
Prof Wall says: “The lack of susceptibility to MLs was not seen universally and, in some cases, reported lack of response to treatment was likely to have been due to improper application or management of sheep after treatment.
“This is an important finding, because inappropriate treatment or inadequate application may result in sub-optimal residues and further hasten selection for resistance.
“The ideal long-term approach to scab management on a farm would be to eliminate all mites by appropriate treatment of sheep, for example with organophosphate [OP], and then maintaining scabfree stock by applying appropriate biosecurity and quarantine measures.”
However, he adds that if ML resistance spreads, which is possible with long-distance sheep movements, greater reliance on OP dip alone will inevitably impose selection pressure on this compound.
Correct plunge dipping is one of the best ways to remove scab and will remove ML-resistant mites, meaning these actives will potentially be successful on the farm in future years.
However, for those farmers unable to dip due to inadequate facilities or the cost, then using the correct injectable and only when necessary, is important.
Zoetis Vet Dr Dave Armstrong says: “It is vital to remember all MLs are also wormers so ensure their use is necessary and avoid repeat doses in any given year.”
Dr Armstrong explains most MLs do not offer persistent protection against sheep scab. This means if you use a non-persistent ML injection, you need to take suitable management steps to avoid reinfestation.
He advises treating all sheep on the same day and after treatment, sheep need to be moved to a scab free pasture that has not contained sheep for at least three weeks.
This is because the scab mites can live in the environment for up to 17 days so it is important to avoid contact with other potentially infested sheep.
How to prevent scab from entering your flock
■ Always assume the sheep have scab
■ Keep animals in quarantine for at least three to four weeks
■ Treat or test the quarantined animals. This can involve blood testing all the animals in quarantine after two weeks to see whether they have been in contact with sheep scab, injection with macrocyclic lactones (ML), and plunge dipping using an organophosphate (OP) (diazinon) dip
■ Have a robust plan for control of sheep scab that you have discussed with your animal health adviser