Deb Maxwell (Sheep CRC) and Lewis Kahn (University of New England)
October 2012

Selection for drench resistance happens when worms in a sheep are exposed to a drench treatment. Some worms can survive certain drench active ingredients as they have genes for drench resistance. There may initially be a few resistant worms, say one worm in 100,000 worms or even less. Some worms present may be partly drench-resistant: they can survive lower (sub-lethal) but not full doses of the drench treatment. Worms that survive treatment continue to produce eggs that give rise to infective larvae on a pasture that are now resistant to the active ingredient. These are eaten by sheep and so the worm life cycle continues. In this way, each treatment causes an increase in the proportion of the worm population that is either partly or fully drench-resistant.

There are a number of ways to slow development of drench resistance by using combination drench treatments. Combination treatments (also called multi-active treatments) expose worms to more than one active ingredient at the same time and the chance of a worm being resistant to both actives is much lower than for any single active ingredient.

It is critical to understand exactly what a “combination” drench is, because some drenches might give the appearance of being a “combination” when, in fact, they are a “mixture” of drugs treating different worms.

Drench mixtures and combinations are different

Some drenches contain more than one anthelmintic active ingredient, that is, the chemical that is responsible for killing worms. These are often referred to as multi-active drenches.
However, the active ingredients within a multi-active product may not target all of the same worms.

Therefore, multi-active drenches can be considered to be:

• Combinations.
• Mixtures.
• Both a combination and a mixture.

A combination contains two or more active ingredients that each targets the same worms. This gives an increased chance that worms resistant to one active are killed by the other active. This can be useful as many farms have worms resistant to more than one active.

• Combination example: products that combine levamisole and fenbendazole both target scour worms, barber’s pole worm and other roundworms.

A mixture contains two or more active ingredients, but the actives target different worms. These give the convenience of a single drench when quite different worms are targeted, however, they should be considered “single-active” against each worm.

• Mixture example: products containing praziquantel and ivermectin are targeting only tapeworm with the praziquantel and only roundworms (Scour, Barber’s Pole etc) and some lesser parasites with the ivermectin.

A product that is both a mixture and a combination contains two or more active ingredients, where some worms are affected by only one active, but other worms are affected by two or more actives.

• Combination and mixture example: products containing levamisole, fenbendazole and praziquantel are combinations against roundworms due to the action of the first two active ingredients, but are only a single active (i.e. praziquantel) against tapeworm.

NOTE: Drench resistance is common on many farms, therefore one or more of the active ingredients within a combination or mixture that you use could be ineffective on your farm. You should conduct DrenchTests and/or DrenchCheck-Day10s to know what drench actives are effective on your property.