Drench resistant worms in sheep – what is drench resistance?

Sheep, farming, husbandry, animal health, worms, internal parasites, drenching, drench resistance

By Dr Clive Dalton

If you are always drenching & dagging, then suspect there may be a drench resistance problem worth investigating

Drench resistance

• Drench resistance is a problem the worms have, and is not a problem with the drench.
• Veterinarians constantly remind us of this as there are plenty of drenches being used that are working perfectly well to control internal parasites. It’s easy to forget this and long may it last.
• Drench resistance is where the worms have developed a resistance to the chemicals in drenches so are not killed by them. This is a ‘genetic’ resistance so the worms have in fact become ‘super worms’ and can pass on their genes to future generations.
• When worms are resistant, at least 95% of them will survive drenching. If the drench works, then no more than 5% should survive if they are susceptible (non-resistant).

Four types of resistance
Four types of resistant worms have been identified. This may seem a bit academic but it could be useful to appreciate the situation on your farm. It’s not a simple picture as there is:

• Single resistance - where the majority of worms are resistant to one drench family. If more than one species of worms is resistant it is called ‘multigenic’ resistance.
• Dual resistance - where some species of worms are resistant to one drench family and other species are resistant to another drench family. BZs and Levs are the only drench families known to be implicated at this stage.
• Multiple resistance - where one or more worm species are resistant to more than one drench family - and possibly all three.
  

Key point: Whilst general resistance knowledge is valuable, it is critical each farmer gains an understanding of the situation on their property.

How big is the problem?

• The full picture is not known but a lot of work is going on to find out. We know it’s an increasing problem on farms that run cattle, sheep and goats.
• Estimates vary about the current situation. Generally it seems that on most sheep farms worm resistance to drench has developed to at least one drench family; and more than half have resistance to two.
• Results from a 2005 survey have shown that drench resistance in sheep to the three broad spectrum chemical families is 61% for Benzimidazoles (BZ or white drench), 31% for Levamisoles (Levs or clear drench), 34% for Ivermectins (Ectins) and 10% for Levs + BZs.
• The conclusion from this survey was that drench resistance is real and is widespread, and measures need to be taken to combat it and certainly prevent its spread.
• The major concern at the moment is that so many farmers don’t know the drench resistance status of the worms in their sheep, and they seem in no hurry to find this out.
• A survey showed that only about 20% of farmers have tested for worm resistance to the drench they are using, and half of these have only done one test so many are still relying on the old advice to prevent problems, and it won’t any more.
• The table below shows more information from the 2005 survey. It shows in a very clear way how drench resistance has appeared in all major internal parasites of sheep to all three broad-spectrum drench families. This is the sort of information farmers need to know for their particular farms, to manage their drenching programme.
  

Frequency of worm resistance found to different drench actives for some parasite species in sheep.

How has drench resistance developed?

• Anthelmintics were introduced in the 1960s and resistance to benzimidazole was first discovered around 1980. Warnings about the long-term consequences of using these new chemicals were aired by some scientists and veterinarians after that, but were ignored by most.
• In 1980 there was even concern voiced in Australia about anthelmintic medications and their bad effect getting through into the ecosystem after they had gone through highly-stocked sheep. Nothing was heard of it again!
• Worms that survive drenching mate with other fellow survivors, and produce mainly drench-resistant offspring which then have a ‘reproductive advantage’ because they have the place to themselves!
• The longer time resistant parasites enjoy a reproductive advantage, the faster drench resistance will develop, but the rate at which this happens varies widely because there are so many variables involved.
• The problem has developed by the ‘survival of the fittest’ among the resistant worms, and the speed of their genetic gain for resistance (to some chemicals) has been remarkable.
• Resistance to a chemical family has developed within a few years on some farms, but on others that have used the same chemical family for a similar length of time or longer and there is no sign of it yet.
• The more times an animal is drenched, the longer the drench-resistant worms will have a reproductive advantage.
• Once the worm population on a farm has become resistant to drench chemicals, this is permanent and worms will not become ‘susceptible’ again. This is the current belief in Australasia anyway.
• In the old days when farmers were advised to change drench families each year, we believed that after a rest period, the worms would become susceptible again. This practice is now classed as bad advice.
• In the 1970s and 1980s it was seen as good farm accounting practice to pay as little tax as possible. Buying drench was a great way to use up surplus cash so woolsheds were full of drums - which then had to be used up. The promotions that went along with the brands certainly encouraged bulk purchase.
• Monday was always a bad day for head shepherds. Their staff were keen to give their dogs a run as they’d been tied up all weekend and were going crazy for work. The head shepherds asked their bosses what was to be done and they had no plans either. The solution was easy - ‘drench the hoggets’. Then everyone was happy except the susceptible worms!
• When long-acting and persistent drenches came along, they were seen as a great idea, especially for ewes to reduce scouring and dagging. Long-acting products are now seen as a fast-track to creating drench-resistant worms.
• There is now growing acceptance that drenching adult ewes that are carrying resistant worms is also a fast-track to increasing drench resistance.
  

Key points:

• Every farm is different and making general assumptions is dangerous.
• In the Farm Case Study – notice that within each family of drench, not all the species of worms have the same level or resistance.
• So this again reinforces the point that you really must know the situation on your farm, and not just grab the first drench container from the shed that rattles before heading for the yards.
  

Disclaimer This material is provided in good faith for information purposes only, and the author does not accept any liability to any person for actions taken as a result of the information or advice (or the use of such information or advice) provided in these pages.