What is Drench Resistance?

Drench resistance is a change in the genes in a worm population which allows certain worms to survive exposure to drench.

This change comes from continued exposure of the population to the drench. Drench resistance is not “all or nothing”. A drench will range in efficacy, depending on how many genetically “improved” worms are present on your farm.

Drench resistance is a farm level problem
Resistant genes can be purchased with land – not just stock. At any one time most of the worm population is on the pasture.

Most often, one worm species on a farm is resistant to one drench family. However, properties where two or more species are resistant to one or more drench families are increasingly common.

Cross Resistance
Worms resistant to one member of a drench family carry genes that make it resistant to all drenches in that family. Changing products within a drench family is not an answer to a resistance problem.

How does drench resistance develop?
The use of drench “selects” for worms that survive treatment. These survivors (carrying genes for resistance) can then mate and produce offspring which carry resistance genes. Over time, the proportion of worms carrying resistant genes increases.

View an interactive illustration of how resistance develops.

What is the effect of drench resistance?
Reduced persistent activity and decreased parasite kill by one or more drench families. This leads to reduced weight gains, deaths and prolonged maintenance feeding.

What does it cost?
Trials have demonstrated significant reduction in liveweight of lambs treated with ineffective drenches^[7,8]. A conservative estimate^[8] of the effect of low level, “unseen” drench resistance can be used to predict costs to individual farms at $1,700 – $7,500 per 2,000 ewes per year.

What is reproductive advantage?.

What is meant by "refugia"?.