The cost of resistance

Drench resistance is the acquired ability of a worm to survive in the presence of normally lethal levels of a drench. Originally resistant worms are rare in a population but a genetic mutation allows them to survive the drench and they pass on their resistant genes to their offspring.

Drench resistance represents a major problem facing the Australian sheep industry. With no new actives on the horizon and resistance to the major drench families growing steadily across the country, producers will need to explore new techniques to ensure sustainable worm control.

Australian woolgrowers spend 10% of their annual wool production on worm control. Drench resistance increases this cost to 30% because of less effective control, the need for additional testing and drenching, and losses in sheep with heavy worm burdens. While resistance is developing, producers may be experiencing significant subclinical production losses that are not easily seen. One of the challenges growers face is to equate these losses with developing resistance.

Which worms are a problem?

In winter rainfall areas of southern Australia, black scour (Trichostrongylus spp.) and small brown stomach worm (Ostertagia) are the major parasites that affect sheep. When it rains in summer, as it usually does in the coastal areas of Victoria and WA, Barber’s Pole worm (Haemonchus) can also become a serious problem. Drench resistance is a major issue in each of these parasite species.

“There could easily be 5–10 % loss in production, and that’s in terms of growth rate and potential wool production, which would be going on without the producer being aware.” Dr Colin Trengrove, Veterinarian, Naracoorte SA