Drench resistance is a major problem to sheep producers for two main reasons: it is costly, and it’s mostly invisible.
Without testing, it’s difficult to gauge how well a drench has worked—even if signs are obvious in a mob, drenching generally kills enough worms to give an apparent cure. In any case, it usually takes some years before resistance becomes severe, and incremental decreases in drench effectiveness are hard to detect. It is easy to continue to use drenches that are below par, with an unrecognised but increasing economic penalty.
Using drenches that are no longer fully effective has several effects on worm control, and therefore sheep profitability:
Immediate effects: It’s obvious that when resistant worms survive a drench, they continue to affect sheep. In extreme cases, where few worms are killed, signs of worm disease can continue with little pause. Drench resistance was first suspected in Australia back in the 1970s, when sheep deaths due to Barbers Pole worm re-commenced only a couple of weeks after drenching. The white drench involved had killed very few worms.
More usually, obvious worm disease is the tip of the iceberg. Far larger economic losses are caused by small surviving burdens that reduce growth rates and cause ill-thrift. It may be weeks or longer before numbers build up, and obvious effects indicate the need for treatment.
Failure of longer-term worm control: most “strategic” worm control programs aim to reduce the number of worm larvae on the pasture during seasons when few survive, so it will take longer before worm numbers build up once seasonal conditions change. “Summer drenching” is a classic example: the more worms that survive a drench in summer, the larger the burdens that develop in autumn, and the more serious the winter worm risk. Even a small number of resistant worms surviving summer drenches can de-rail an otherwise effective program.
Side-effects: where more “scour” worms survive drenches than expected, continued worm infections increase the percentage of sheep with significant dags, and hence the need for crutching and the risk of blowfly strike. Extra treatments due to the more rapid re-development of worm problems are another cost, both in drench and time.
Increased resistance levels: Continuing to use a drench after significant resistance has developed is a one-way trip. As the percentage of resistant worms that survive a drench increases, the rate of resistance development accelerates. The earlier that a drop in drench performance is detected, the more likely it is that resistance management strategies can prevent a total loss of use of the particular drench group.
As noted, the impact of drench resistance is hard to see immediately after a drench, but controlled investigations have clearly shown the numerous effects—and confirmed the mostly invisible nature of the problem.
A study in Western Australia in the mid-1990s tracked the health and production of weaner Merinos over the course of a year when the same worm control program was followed, but with drenches that were either 65% or 85% effective and which were compared to a 100%-effective drench.
Over the year, significant production and disease effects due to the “scour worms” occurred in the sheep given the 65%-effective drench, in comparison to the 100% group:
Overall, this represents a loss of well over 10% in sheep profitability—but the low drench-effectiveness groups didn’t "look" all that bad. Without the comparison groups, the potential loss would have been hard to pick.
The effects related to the 85%-effective drench were smaller, but still negative (about 3–5%), but with virtually no visible signs.
Most sheep producers are well-aware that drenches should be as close to 100% effective as possible. Few continue to use the older drench types, which are now rarely fully-effective—this includes the white and clear drenches, and in most locations, ivermectin.
“Invisible” drench resistance often occurs with drenches that still work on many or most properties, such as abamectin and moxidectin. In some cases, even the “triple combination” drenches (abamectin with other drench types) may be affected by resistance. However, there is no easy way to predict whether or not particular drenches remain effective on a particular property.
A drench resistance test (DrenchTest) that includes several drench types will quickly show the options available. Even without a full test, however, it’s easy to check for drench effectiveness by comparing worm egg counts before and after a drench. Dung samples can be taken when yarded for drenching, and then at random from the paddock 10–14 days later. (See the DrenchCheck-Day10 test)
Even if a drench is only a few percentage less effective than the full “100%” at present, it will almost always become increasingly worse unless a resistance management program is followed. Using partially-effective drenches could result in a significant cost in sheep production cost, that goes unnoticed for some time.