Drench resistance can occur in very dry areas and this is mainly due to:
Never assume that a drench treatment will completely kill worms in your goats. Drench resistance is a result of worms having genes that enable them to survive treatment. It is likely that these genes were present in some worms before a drench was ever used. Drench resistance is now very common and in many cases severe for some drench groups, making testing for drench effectiveness a vital component of a worm control program.
Drench groups are the ‘chemical families’ of drenches and some groups contain a number of drench actives. For example the Benzimidazole group has the following actives: fenbendazole, oxfendazole, albendazole. When resistance is present for one of these actives, it is likely present for all other actives within the same group.
Selection for drench resistance happens when worms in the goat are exposed to a drench. Initially, there may be very few worms that survive the treatment (perhaps as few as 1 in 100,000) but these resistant worms lay eggs and their offspring constitute an increasing proportion of the worm population. In this way each treatment causes an increase in drench resistance because only resistant worms survive to reproduce.
Resistance may develop faster with more drenching and use of persistent products. Drench resistance is unlikely to be reversible, so not using a drench for a while will not permanently result in the worm population becoming susceptible again. While ever drenches are being used, drench resistance cannot be prevented, but the rate at which it occurs can be greatly reduced.
The first step is to know what drenches are effective on your property.
Each property has its own drench-resistance profile based on its own drenching history and that of properties from which the goats were sourced. The profile of neighbouring properties can be quite different.
The extent of resistance is only known by testing. Obvious worm control failures may only occur when resistance is quite advanced.
In this region, a DrenchCheck is the preferred method to check individual drenches at any time. DrenchChecks should be considered when any drench is given and it is the most practical and cost-effective method of testing drenches in this region.
While a DrenchTest or Worm Egg Count Reduction Test (WECRT) is the most accurate test for drench resistance, this test is rarely feasible in this region as infections are often not high enough and when they are, they may be unexpectedly high and need swift treatment or are in lambs at weaning, which should not be put at risk in a DrenchTest.
This simple and inexpensive test gives an indication of drench effectiveness and whether it should be properly investigated using a DrenchTest.
The DrenchCheck involves two WormTests with larval differentiation
The results from the two WormTests are compared to gauge the extent that worm egg counts (sometimes based on the larval differentiations) have been reduced by the drench. Discuss the results with a worm control advisor.
Keeping drench-resistant worms out of your property is part of sustainable worm control.
Assume that purchased goats (and sheep) are carrying worms with some degree of drench resistance to one or more drench groups. See Drench groups and actives.
When using anthelmintic products in goats, a veterinary prescription is often required because:
While cattle drenches can be used at the label rates on goats in South Australia and sheep drenches on goats in Victoria, a veterinary prescription is still required for dose rates recommended for goats.
Use all 3 principles where possible.
They are equally important and greatly slow the development of drench resistance.
A small benefit can be gained by rotating drench groups providing you also rotationally graze stock across the property so that paddocks are exposed to sheep that have received different drenches. However, if you set-stock, drench rotation will not slow the development of drench resistance.
While not affecting resistance, it is essential to choose a drench with an appropriate withholding period (WHP) and export slaughter interval (ESI) according to the time left before the animals may go to slaughter, or their milk may be used for human consumption.
Search for drenches based on the worms or other parasites targeted, drench group or active and product name.
Follow all 5 principles where possible:
i. When the goats eventually leave these low worm-risk paddocks, treat them with an effective drench that is from a different group to the drench used when the goats first went onto the paddock. The aim is to remove any drench-resistant worms surviving in the sheep after the first drench.
ii. Ensure that the next time the paddock is grazed it is with a different mob of goats. This second mob should have a moderate to high worm burden and their last treatment must be different from the treatment used on the first mob that grazed the low worm-risk paddock. This will dilute drench-resistant worms already on the paddock with more susceptible worms that the second mob is carrying. Note that grazing with cattle will not dilute the proportion of drench-resistant worms, but they will decrease the total number of worm larvae on this paddock.
In this region the only time a persistent (also called long-acting treatment; an unregistered drench for goats) should be considered is in the summer rainfall areas where there has been a history of barber’s pole worm outbreaks and extensive flooding threatens to isolate and crowd goats for a number of weeks.
Fortunately, producers often have some days’ notice of large floods, so in a situation where goats are likely to be inaccessible for a month or more, the goats can be treated with a long-acting product before being moved to a safer paddock.
Do not use a long-acting drench more than once a year.