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Superworm fears real: vets

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Published: February 17, 2011

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DENVER, Colo. – Repeated use of the same dewormers in livestock is creating resistance to common chemical treatments.

“We are potentially creating super-worms,” said Jerry Woodruff, a veterinarian and parasite specialist with Boehringer Ingelheim.

Two percent of the parasites could survive, multiply and spread resistance even if two dewormers are used in one treatment, he told a media session at the recent National Cattlemen’s Beef Association convention in Denver.

“They have the genetics to resist removal by both chemicals, and those are the ones that are surviving and producing the offspring for the livestock grazing a pasture to pick up, ” he said.

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Veterinarians and producers have probably not done a good job of diagnosing parasite species correctly and consequently did not use the appropriate treatments, he added.

A fecal egg count should be done to assess the seriousness of the problem.

Treatment may not be necessary if the count is low.

Producers should also conduct a second test, after eggs have hatched, to identify the larvae and ensure the right treatment is used.

“We need to have more logical thought processes on our deworming program on grazing calves,” Woodruff said.

Generic products for internal and external use may have worsened the problem because they are cheap enough for multiple treatments. They may also be administered at the wrong time.

Producers should check and treat for parasites six weeks into the grazing season, but often they treat livestock earlier because it is more convenient.

The earlier treatment may not get the worms at the most significant stage.

Eggs die off in warm, dry weather, which is why semi-arid regions have fewer problems.

However, they can survive a cold winter.

The primary parasites are gastrointestinal worms, including the ostertagia, also called brown stomach worm, cooperia, or small intestinal worm, and nematodirus helvetianus, the thread-necked or thin-necked worm.

An Alberta Agriculture survey found cattle on pasture south of Calgary harbour slightly more ostertagia, while cooperia is the most common type in central and northern Alberta.

Adult worms live and reproduce in the animal’s gastrointestinal tract, where they lay eggs that are passed out in the manure.

These hatch into larvae that migrate up blades of grass and are consumed by cattle as they graze. The larvae continue their maturation in the gut, thus completing the life cycle.

Brown stomach worms are the most economically significant and widespread parasite. They suck nutrients from the inside of the abomasum in the adult stage, while the larval stage hibernates in the mucosa of the abomasum.

“As near as we know, it will stay in hiding there until there is some signal, and we think that is from the animal physiology … that it knows it will survive in Calgary in June,” Woodruff said.

The lining of the stomach is normally smooth but looks like cobblestones when larvae are present.

Stomach glands are important for secreting digestive juices, but the worms make this more difficult.

Three chemical classes are available for grazing animals:

• Group 1, the benzimidazoles;

• Group 2, the imidazothiazoles (levamisole) and

hydropyrimidines (pyrantel/ morantel);

• Group 3, the macrocyclic lactones (avermectins).

No new products are on the horizon.

About the author

Barbara Duckworth

Barbara Duckworth

Barbara Duckworth has covered many livestock shows and conferences across the continent since 1988. Duckworth had graduated from Lethbridge College’s journalism program in 1974, later earning a degree in communications from the University of Calgary. Duckworth won many awards from the Canadian Farm Writers Association, American Agricultural Editors Association, the North American Agricultural Journalists and the International Agriculture Journalists Association.

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