New Zealand Sheep Husbandry - Using Refugia to control worms

By Dr Clive Dalton

Refugia – what’s it all about?
The word ‘refugia’ seemed to appear out of the blue about 5-6 years ago, and was used by veterinarians and parasitologists when talking to farmers about worms (internal parasites) in sheep and in cattle.

Finding out about the bad news

• In a survey of sheep and beef farms by Meat & Wool around 2006, it was discovered that worms had become resistant to a range of anthelmintic drenches, and that some urgent action was needed to stop the rot!
• A massive bureaucratic panic resulted with one of the main results being the formation of a programme called ‘Wormwise’ to advise farmers what to do to control worms, and especially how to stop drench resistance increasing.
• Wormwise is still in existence, but with much less panic now as despite dire predictions that no new drenches were in the pipeline, at least two are now on the market which are promoted as solving the resistance problem.

We should have known!

• Drench resistance in worms should not have come as a surprise, as a few lone voices about 30 years ago warned this could happen if we became totally reliant on the new drugs (at that time) to kill worms.
• The worms would outsmart the humans through the simple process of ‘the survival of the fittest’, so that those that became resistant to these chemicals would survive and multiply.
• But then of course – nobody wanted to think too far ahead and about the downside of using chemicals to kill worms, as everybody involved was making money from their widespread use – the big international pharmaceutical companies who made them, the vets and distributors who sold them, and the farmers who used them.
• It was easy (too easy) to believe that predicted problems were only conjecture. It only took 30 -40 years to prove it was fact!

Refugia arose from the insect world

• It was when parasitologists started to describe how worms started to develop resistance that the word refugia appeared, as similar concerns were expressed about insects about 40 years ago when chemicals like DDT were used extensively to kill them. It was a particular concern with mosquitoes in Africa.
• It was predicted and proven true that a population of insects resistant to chemicals could keep on increasing, and one way to stop this was to have a ‘refuge’ where ‘susceptible’ insects could survive (by not spraying) to mate with the resistant ones and thus slow the rate of total resistance in the population.
• Exactly the same situation was described by parasitologists as a possible way to slow up the rate of drench resistance happening in worms in sheep and cattle.

Refugia was a concept
Refugia was and still is a concept – and is by no means a proven solution. It was the only good news that parasitologists could come up with at the time, and it sounded reassuring – to some.

How is refugia supposed to delay drench resistance in worms?

• In the worm life cycle, male and female worms mate in the gut and produce eggs, which go out in the faeces on to the pasture where they live as larvae, then crawl up the grass again and be eaten by the host for the cycle to continue.
• Anthelmintic drenches kill all the worms (some kill the eggs too) that are susceptible to the particular chemical, so they can’t mate and lay eggs.
• So when resistant males mate with resistant female worms, their offspring are not killed and they produce resistant offspring.
• To slow up all worms becoming resistant, we need drench resistant worms to mate with susceptible worms, so a proportion (unknown) of their offspring will be killed by drenching.
• So the more susceptible worms that are in a ‘refugia population’ on the farm, the slower will be the rate of descent toward total drench resistance.

How to keep a refugia population of worms?

• This done by not drenching all the animals in the mob – which used to be best practice before drench resistance was identified.
• Best practice now is to leave ‘a proportion’ of the mob undrenched.
• The size of this proportion is not easy to define – figures like 5, 10, or 15% are suggested, or you should not drench the biggest animals in the mob that are thriving.
• These will be the refuge for worms susceptible to drench chemicals – so will hopefully delay the time when all worms are resistant to chemicals.

Will refugia work?

• Some sheep trials have been done in sheep and showed that drench resistance was slowed up.
• But other sheep work in Australia showed that refugia will only delay the evolution of drench resistance where a large proportion of the worm population escapes exposure to drench chemicals.
• The key thing is that the idea of having a population of worms in a flock susceptible to drench chemicals is a concept which seems to be common sense too.
• It’s simply to avoid as much ‘chemotherapy’ to kill worms as much as possible to delay the day when all worms are genetically resistant.

What’s good for refugia?

• Drench on the basis of a Faecal Egg Count (FEC) and veterinary advice, and not by the calendar.
• Reduce the number of drenches given.
• Extend the intervals between drenches.
• Leave a proportion (5-15%) of the top animals undrenched.
• Concentrate on drenching the poorer-performing animals in the mob.
• Don’t drench mature animals, as their immune system should protect them from worms.

What’s bad for refugia?

• Drenching thinking that this will kill the worm larvae on the pasture. This is a myth.
• Using only one class of stock on an area – you need to mix the species grazing to act as vacuum cleaners for the worms that affect other classes of stock.
• Note sheep and goats share the same species of worms.
• Drenching young animals and then putting them on to clean pasture. This will definitely allow the drench resistant larvae to multiply and when they get back inside the host, to meet other drench resistant worms to keep on multiplying and increasing drench resistance in the flock or herd.

How can you find out if there are drench resistant worms on your farm?

1. Realise that there are many different species of worms, and nto all of them may be resistant to the drenches being used at any one time.
2. Also remember that over 90% of them are in the form of larvae on the paddock and not inside the animal.
3. The test for drench reistance is called a 'Faecal Egg Count Reduction Test' or FECRT/
4. You have to identify a sample of individual animals, then do a FEC to find out their worm burden before drenching them with the particular drench family to be tested.
5. The you do an FEC again, and if the product under test was fully effective, then it should have killed at least 95% of the worms.
6. See your veterinarian for full details of the protocol needed and the costs.

The new drenches

• The two new families of drench chemicals which have appeared on the market work in a different way, and it is stressed that they will kill any worms that have developed resistance to the previous chemicals.
• But parasitologists are warning that care needs to be taken with their use, otherwise we will go down the same track and produce the same resistance problems that has happened before.