Overcoming Parasite Resistance Requires New Strategies

“We have to get over the idea that we want to have zero worms in these animals. That is never, ever going to happen.”


As it turns out, everything you were told about controlling parasites in livestock was wrong.

The old practice of regularly deworming the whole herd and then putting them out onto fresh pastures has led to new strains of anthelmintic-resistant parasites, and threatens to render some very valuable management tools useless.

Worse still, rotating medications, long thought to be the best way to kill resistant worms, eventually erodes the effectiveness of the entire arsenal of dewormers.

“We have to stop thinking that we’re going to use the best wormer three or four times, kill all the worms, and then everything is going to be fine,” said Dr. Wayne Tomlinson, a MAFRI extension veterinarian, at a Manitoba Sheep and Goat Association Symposium in Eriksdale last week.

“We have to get over the idea that we want to have zero worms in these animals. That is never, ever going to happen. I think the worms are always going to win. We’ll never totally beat them, so we might as well learn how to live with them.”


Internal parasites evolved along with livestock for thousands of years, and have developed survival strategies such as winter latency periods where they hide out in the lining of the intestines for months on end. Worms that survive deworming drugs pass their resistant genes onto their offspring and eventually become the dominant population.

A better strategy is to deworm only the skinny animals, and in the fat animals, maintain a population of “refugia,” the term used to describe less aggressive parasites.

Animals that need to be repeatedly dewormed should be culled in order to develop a herd that can thrive despite persistent, low-level parasite infection.

“It’s kind of a strange concept, but we actually want to maintain a population of worms that we can kill easily,” he said. “Then we use those worms to compete against the other worms so we’ll always be able to keep them under control.”

A new approach is needed, said MAFRI sheep and goat specialist Mamoon Rashid, because parasite resistance is becoming a global problem. Apart from the three existing classes of dewormers, no new modes of action have been developed since 1991.


There is talk of new drugs on the way, but nobody knows when they’ll be ready or how much they will cost.

In the southern United States, where heavy moisture conditions create an ideal environment for parasites to cycle through sheep and goats, studies have shown that 90 per cent of the worst parasites are already resistant to ivermectin due to traditional – and ultimately flawed – control strategies. Even in cold environments such as New Zealand, resistance rates are pushing 25 per cent.

Small ruminant producers in the hardest-hit areas are now effectively operating without dewormers, and face annual death losses as high as 20 per cent a year, mainly due to the ravages of the barber pole worm, known by its latin name haemonchus contortus, which causes bottle jaw.

A new strategy for keeping parasites at bay, called “Smart Drenching,” takes a multi-pronged approach.

First, sheep and goat owners identify which dewormers are still working via fecal egg counts before and after treatment. Anything less than a 95 per cent drop in fecal egg counts after deworming indicates resistant bugs.


Then, each animal is weighed in order to determine the correct dosage. For sheep, producers are advised to follow label recommendations to the letter. For goats, doubling the dose is a good rule of thumb.

If resistant worms are present, dewormers from two different classes are used, and animals – except ewes in late gestation – are kept off feed for 12 to 24 hours before treatment. A followup dose 12 hours later increases the effectiveness of treatment with benzimidazoles.

A low-cost tool for selectively controlling parasites by weeding out the “Typhoid Mary” animals in the flock is the FAMACHA card system developed in South Africa.

Available from MAFRI for about $10, the laminated plastic card uses a red colour chart that is held up close to the open lower eyelid of sheep or goats and used as a screening device for anemia, the best indicator of parasite infection. Barber pole worms can drain as much as half a cup or more of blood per day from heavily infected sheep, said Rashid.

If the eyelid matches a dark shade on the chart, that means the animal is in good health. If it is lighter, or nearly white, that indicates severe anemia and the animal should be separated from the flock and dewormed.

Animals that require repeated deworming treatments should be culled. To prevent the introduction of resistant worms to the rest of the flock, new animals that are brought in should be quarantined and dewormed on a drylot until their fecal egg counts are negative.

Non-drug management techniques also go a long way towards controlling parasites, Rashid added.

Because larvae are picked up via water droplets on forages, animals should not be allowed to graze pastures that are wet or lower than three inches. Also, long pasture rest periods allow larvae more time to dry up and die off. [email protected]

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