Frightening signs farmers are losing the war on superworms

John Shirley

Perception often prevails over reality. No harm, some would say. Better to be in a fool's paradise than in none. Take a scene of lambs, or calves, or any livestock, frolicking in a grass field on a sunny summer's day. Such a scene is wonderful to behold. As a nation we market our food on such pleasant vistas.

In reality, though, these animals are defecating and urinating on the pasture they are eating. As a consequence, they are supporting and promoting a population of nasty parasites which inhabit the grass carpet.

These nasties, which we usually term worms, are capable of robbing the vitality, even taking the life itself, of the lovely young livestock feeding on the grass.

Across the world, farmers have long waged war on the grass nasties and have generally been able to keep them in check by hitting them with drugs delivered by our pharmaceutical companies.

But there are frightening signs that farmers are losing this war. We are seeing the emergence of superworms on farms which are resistant to the medical compounds that we have had in our armoury.

This trend is very similar to the emergence of the hospital superbugs, which are resistant to antibiotics. I am told that in South Africa and New Zealand, some farms are so infested with the superworms that grazing sheep on them is no longer an option.

In the fight against the worms, the conveyor belt for new drugs delivered nothing for a stretch of more than 20 years. Then, like the man waiting a long time for a bus, along come two.

The first new anti-worm drug for sheep was Monepantel. This is marketed by Novartis Animal Health under the brand Zolvix. Last week, Pfizer Animal Health launched another new sheep product, Derquantel, which will be included in the brand product Startect.

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The value of these products is that they have new modes of action and will kill all of the pasture worms in sheep that have become resistant to the drugs already on the market.

Across the world, scientists have hailed the emergence of the new drugs. They are seen as vital in the war against the parasites which are constantly getting smarter.

We now know that in the past we made mistakes in how wormer drugs were used, mistakes which allowed the emergence of the resistant superworms.

The arrival of the new chemicals is seen as giving farmers and vets a second chance to halt the emergence of the resistant superworms and to put the farmer in long-term control of pasture parasites.

In Ireland, a group of volunteers that includes IFA, vets, regulatory agencies and research institutes have been assembled under the co-ordination of retired Department of Agriculture parasitologist Dr Tom Murphy with the aim of introducing best practice in the use of sheep and cattle wormers.

These 'best practices' of today have turned many of the 'best practices' of yesteryear on their head.

For decades, sheep farmers have been reared on a diet of 'drench your stock at prefixed dates and move them to a clean worm-free pasture'.

Now, the recommendation from Dr Murphy's group is to drench your sheep only if they need it. When you must drench, ideally sheep should be held off pasture in a shed for a couple of days post drenching and then moved onto a contaminated rather than a clean field.

The principle behind the new recommendation is that if drenched sheep are shedding resistant worms, it's preferable that these worms have competition from susceptible worms on a contaminated pasture rather than giving them a free run to multiply on a worm-free pasture.

Dr Murphy said that poor dosing practices, such as under-dosing for the weight of animal or the use of faulty drenching guns, have also contributed to the growth of resistant parasites.

Ideally, he would base dosing on flock faecal worm counts and selectively dose the lambs showing signs of a worm burden rather than blanket treat the whole flock.

How widespread is the wormer resistance problem in Irish flocks?

There are several worm species and, until now, three categories of wormers (white/benzimadazoles, yellow/levamisole, and clear/macrocyclic lactones) to treat them.

Dr Murphy quotes Teagasc surveys in 2007, which showed more than 90pc of flocks had a resistance to the white wormers and more than a third showed resistance to clear wormers. That's serious.

If in Ireland we term the resistant worms as superworms, then in New Zealand and South Africa they are already hit with mega superworms. Their flocks are showing resistance to all three drench categories.

Unless the resistance threat in Ireland is addressed we are heading down the same road as the southern hemisphere producers. Tom Murphy has put together a talk on combating wormer resistance that is suited to sheep farmer meetings.

Waterford vet Charles Chavasse, a member of the Murphy voluntary group, told a meeting of Galway sheep farmers last week that strategic use of the new wormers could retain their efficacy for the next 40 years.

Speaking about Startect, he said that this product should be strategically rather than routinely used against worms. It is very useful as a quarantine dose for new sheep coming onto the farm. It can play a vital role in treating sheep in flocks where resistant worms have been identified. To this end, Pfizer are offering a free test for resistant worms. This will operate through vets.

Startect is a new sheep drench from Pfizer based on the active ingredients Derquantel and Abamectin. It kills all the economically important stomach, gut and lung worms in sheep but is not a fluke treatment. As a 'prescription-only medicine' , it will be available through vets. It is expected to retail at about €1.50 per treatment. It has a 14 day meat withdrawal period.

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