Drench resistance spreads through ewe milk, study finds

Advice on pre-lambing drenches could change now scientists understand the drug can transfer through ewe milk and select for resistant parasites in lambs.

A study looking at the effect of long-acting Moxidectin in New Zealand sheep found the drug in ewe milk almost until weaning and in lamb plasma until eight weeks old.

Drug level was sufficient to prevent 70% of susceptible brown stomach worm (Teladorsagia circumcincta) worm larvae from establishing but promoted the resistant gene pool, the AgResearch study found.

See also: New research highlights more risks associated with pre-lamb drenching of ewes

According to Professor Jacqui Matthews of the Moredun Institute, the publication was “worrying and relevant to the UK situation” as this species is the commonest UK roundworm with a resistance problem already being seen.

Sustainable Control of Parasites in Sheep (Scops) guidelines

A sustainable strategy compromises reduction in pasture contamination for lambs and avoiding high selection pressure for anthelmintic resistance. Two options are available:

  • Leave a proportion of ewes untreated to populate paddocks with eggs from unselected parasites
  • Treat early in post-lambing phase to ensure ewes become reinfected before immunity is fully restored

She added: “This is important because a ‘lambing’ wormer treatment is commonly applied by UK sheep farmers around lambing because of the relaxation in immunity in ewes to this worm and thus to stop the ewes contaminating pasture.

“It also important because moxidectin is one of our few tools for the control of sheep scab and overuse may lead to resistant selection in mites.”

Farmers are advised to consult Scops guidelines and speak to their veterinarian or suitably qualified person and take into account the need to balance control of disease without exerting strong selection pressure for drug resistance, she told Farmers Weekly.

Lead scientist on the study, Dr Dave Leathwick, said New Zealand studies had shown the practice of pre-lambing drenching as “high risk” and frequently not cost-effective.

Dr Leathwick added: “Farmers ought to consider the future when they’re using a practice that has a 50/50 chance of losing them money and is shown to select for resistant parasites on their farm.

“You are effectively selecting for drug resistant parasites in lambs without treating them; you were simply treating its mother six to eight weeks earlier.”