Dual action designed to avoid resistance

A new drench is designed to provide effective treatment of sheep without allowing parasites to develop resistance

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Parasite resistance to products approved for use on sheep is causing major problems for producers who find current options don’t do an adequate job of deworming.

A new product from Zoetis, coupled with new recommendations on treatment protocols, is designed to provide effective treatment without allowing parasites to develop resistance.

Dr. Melanie Wowk, large animal technical services veterinarian with Zoetis, said Startect, a drench introduced in Canada last fall, has a dual action intended to circumvent parasites’ ability to adapt and resist treatment.

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Why it matters: One of the largest challenges for sheep farmers is controlling parasites and managing resistance to dewormers.

The product is a combination of two anthelmintics: abamectin and derquantel. The former is in the same class as ivermectin, which until now was the only on-label parasitic treatment for sheep. Derquantel is a new anthelmintic never used before in Canada so no parasites here will have resistance to it, said Wowk.

The dual action of the product is a further hedge against development of resistance because each action works on a different aspect of the parasite. It also meets producers’ need for more treatment options, she added.

“The main reason for this is because of the vast amount of resistance that we are seeing in sheep flocks across the country and what we’re really trying to do is prevent that from happening with this product so that we’ll have it to use for many years to come.”

She said parasites in some flocks are completely resistant to ivermectin.

“It’s a huge cost to the Canadian sheep industry. We’re not only affecting performance of these animals but we’re affecting overall general health.”

Startect is effective against common sheep nematodes, lungworms, nasal bots and itch mites. It has a 14-day withdrawal time for meat animals and comes as a ready-to-use oral drench in five-litre and one-litre packs. It should not be used on ewes producing milk for human consumption.

Wowk said the product has been used successfully in Australia for about five years and has been subjected to numerous on-farm trials to prove safety and efficacy.

She cautioned producers to use the appropriate dose based on animal weight, at one millilitre per every five kilograms.

In a webinar presentation, Wowk outlined a deworming protocol designed to limit parasites’ ability to build resistance to Startect. It is a “five star” plan:

  1. Manage the level of pasture contamination.
  2. Use anthelmintics appropriately.
  3. Monitor and treat animals selectively.
  4. Quarantine and treat new introductions.
  5. Investigate treatment failures.

The first point relates to refugia, a method of retaining some level of parasite infestation so the entire population cannot develop resistance. That requires leaving some sheep untreated.

“We have to keep in mind that it’s OK for these animals to carry parasites. They have evolved with parasites and both the sheep and the parasite have developed their own ways to deal with each other,” said Wowk.

“Sheep can lead a very productive and healthy life and still have parasites and I think that’s one thing that we have forgotten when we come up with these deworming protocols.”

Ewes are the heaviest carriers

Ewes tend to have the highest parasitic loads in spring, when their bodies are giving priority to growing lambs and making milk rather than fighting parasites. At that time they expel more parasite eggs onto grass, which are later ingested by new lambs.

As parasite loads build in lambs, they decrease in ewes as their immune system kicks in, Wowk explained.

Dosing the flock and then moving it to a clean pasture is not recommended and in fact leads to parasitic resistance.

“What happens is, all they have left is resistant worms in their system… It just becomes loaded with those resistant worms.”

The better option is to treat and then wait a few weeks for the animals to regain susceptible parasites. Then deworming will be effective.

However, Wowk also recommended monitoring and treating animals selectively. Treat older animals and young stock as separate groups and treat only when necessary. Only about 30 in every 100 sheep are high shedders of parasites, she said. The rest are probably able to control the number of parasites through their immune systems.

However, once a high shedder, always a high shedder, so those animals should be culled.

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