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Resistance of gastrointestinal worms to anthelmintics

Robert Fears Published on 24 April 2012
Cattle in pasture

Often at cattleman seminars, a speaker will sound the alarm on gastrointestinal worm resistance to anthelmintics (dewormers).

Is the alarm justified? Is there a resistance problem?

Thomas Craig of the department of veterinary pathology at Texas A&M University says that internal parasite resistance is occurring.

Craig feels resistance could become a bigger problem for dairies and perhaps in young stocker cattle because, in both cases, animals are often concentrated in small areas. Craig does not see an immediate problem in cow/calf operations.

Resistance confirmations

Craig cites two pharmacological company-conducted research studies as resistance confirmation. One of the studies was conducted in April 2011 on a stocker operation in central Texas.

Table 1: Average number of adult worms found in harvested animals

A group of calves on pasture were treated with an injectable macrolide and compared to an untreated control group.

Examples of a macrolide are Ivomec, Dectomax and Cydectin. Animals in the study were harvested and necropsies conducted for adult worm counts. Count averages are presented in Table 1.

More barber pole worms were counted in the macrolide-treated calves than in the controls, which definitely shows a resistance problem.

Only 32 percent reduction of Cooperia was obtained, leaving 68 percent of the population to further build resistance to macrolides.

A second study was conducted in northeast Texas on a dairy farm during October 2010. Three groups of calves on pasture were used in the study.

One group was treated with a macrolide, the second group with a macrolide and benzimidazole combination and the third group was an untreated control.

Table 2: Average number of gastrointestinal worm eggs per gram of fecal materialExamples of benzimidazole-containing wormers are Valbazen, Panacur and Synanthic.

Fecal samples were collected for egg counts on the day of treatment, two weeks following treatment and one month post-treatment. Averages of these counts are shown in Table 2.

In the untreated controls there was a significant increase in egg counts with each sampling, showing that worm populations were actively reproducing.

The macrolide treatment was only providing 42 percent reduction two weeks post-treatment and, by one month post-treatment, there were more worms than at treatment.

When benzimidazole was added to the macrolide, 84 percent reduction was obtained two weeks post-treatment and 71 percent one month post-treatment. These results indicate there is parasite resistance to macrolide.

Table 3: Average number of adult worms found in harvested animals.Calves in the northeast Texas study were harvested and adult worms were counted. The average numbers are shown in Table 3.

Significant differences from the post-mortem adult counts are percent reductions of barber pole and cooper’s worms between the two treatments.

Macrolide provided 73 percent reduction of Haemonchus and 60 percent reduction of Cooperia.

The combination of macrolide and benzimidazole provided 83 and 99.9 percent reduction. These data further show parasite resistance to macrolides.

Resistance prevention

There are ways to fight anthelmintic resistance through herd management. Avoid anthelmintic use unless it is absolutely necessary.

Check fecal pats for worm egg presence to measure population fluctuation. Worm egg counts help determine application timings and are an aid in monitoring treatment effectiveness. If eggs are not found, there is no reason to treat older cattle.

“In the southern U.S., there is strong evidence that cows and suckling calves more than two months old will benefit from a macrolide treatment,” says Craig. “Treatment can be done when cows are pregnancy checked, the calves vaccinated or during other activities.

When calves are weaned and moved to another pasture, treatment with a benzimidazole will help them get a better start. In the high rainfall areas of the southern U.S., cows and especially first-calf heifers benefit from worming as they enter winter.

“Stocker calves, especially those on permanent pastures, are at high risk for all parasites,” continues Craig. “Therefore it is essential to treat when they are put on pasture and then repeat treatment one to three months later depending on the anthelmintic used.

Because these calves are at risk and pastures may be laden with resistant worms, it may be advisable to have your veterinarian check fecal samples two to four weeks after treatment to aid in determining if the anthelmintic actually worked.”

If anthelmintic use is necessary, rotate treatments among active ingredients when acceptable parasite control can be obtained with more than one type of chemical.

All currently marketed anthelmintics are labeled for brown and barber pole stomach worm control. An exception is the arrested larval stage L4 of the brown stomach worm, on which efficacy has not been demonstrated with morantel (Rumate) or levamisole (Prohibit).

Regardless, several options exist for stomach worm control that allows rotation between active ingredients.

“Cooperia small intestinal worms infect young cattle up to 18 months old,” says Craig. “They may be controlled by fenbenzimidazol (Safe Guard and Panacur) or other benzimidazoles, levamisole or morantel.

Regardless of the treatment, Cooperia are hard to control, partly due to the age of animals that require treatment. Fortunately they generally have less long-term effects on cattle production. ”

Increase effectiveness with correct timing

“An anthelmintic must be administered at the right time during the parasite’s life cycle for effectiveness,” says Floron Faries, Jr., doctor of veterinary medicine at Texas A&M.

“The correct time is when cattle have become infected, the parasite is beginning to develop and cause damage and conditions are best for transmission.”

“Life cycles of the more common cattle internal parasites are similar,” explains Steve Blezinger, a nutritional consultant in Sulphur Springs, Texas. “Infected cattle pass eggs in the manure and, with favorable weather conditions, the eggs hatch and develop into third-stage larvae in about 14 days.

These larvae move from the protective manure up moist grass blades and are eaten as the cattle graze. They then enter the gut lining and mature into egg-laying adults three to four weeks after they’re eaten.

“Fourth stage in the life cycle of the brown stomach worm (Ostertagia ostertagi) is different,” Blezinger continues. “Fourth-stage larvae are able to stay in the stomach glands for up to six months.

These are called inhibited or arrested larvae. The ability to inhabit and then leave the stomach glands seem to be triggered by weather or nutritional factors.”

“Worms are usually transmitted when soil temperatures are 55 to 85 degrees in rainy periods,” says Faries. “In most areas, these warm wet periods occur in spring (April through June) and in fall (October).

Pasture larvae slow their activity and hibernate in winter (November through March) and die from heat, sunlight, drying and nutrient depletion in summer (July through September).”

“This year we have evidence of some parasite transmission all winter because of the wet and mostly warm weather,” says Craig. “Ostertagia and one species of Cooperia are normally transmitted during the winter in Texas. Generally, Haemonchus and other Cooperia species are transmitted during the summer.”

“Administering a drug at the right time breaks the worms’ life cycles and prevents them from increasing their numbers,” says Faries. “The right time normally depends on the parasite and development of optimal environmental conditions, which include moderate temperatures, rainfall and wet grass.

For stomach worms, administer drugs three to six weeks after optimal environmental conditions develop.”

Worm resistance to anthelmintics is real. To avoid increasing the problem, use anthelmintics only when necessary, rotate among the active ingredients and treat during the right time of year.  end_mark


Cattle ingest worm larvae from blades of grass as they graze. Photo courtesy of Robert Fears.

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