Good riddance to roundworms on pasture

cattle grazing in green grass

Deworming programs typically target roundworms in the gastrointestinal tract, when in fact it takes a live host and grass for roundworms to complete their life cycle and perpetuate infections.

This makes grazing season an opportune time to deworm because the only way cattle pick up new roundworm infections is by ingesting third-stage larvae (L3) and the only place roundworm eggs can develop through the larval stages is on grass.

Imagine if you can, the ballooning larval population on pasture if each cow in a 500-head herd is infected with 1,000 female worms and each lays 200 eggs every day. That’s 100 million eggs a day that end up in manure pats from the adult animals alone.

Parasitologists are now promoting a deworming treatment five to six weeks into the grazing season as a way of reducing the roundworm load on pastures and curtailing reinfection of cattle during the grazing season.

The idea is to let cattle vacuum up larvae developing on the grass early in the grazing season. The early-stage larvae die inside the gastrointestinal tract. It takes about 20 days in calves and up to 40 days in mature cattle for the infective L3s to become egg-laying adults. So deworming at this time provides a one-two punch, cleaning up the cattle and stopping egg shedding for the next four to six weeks, says Roy Lewis, a private veterinary practitioner from Alberta and a technical services veterinarian with Merck Animal Health.

The company is in the process of summarizing three years of trials at the Western Beef Development Centre in Saskatchewan looking at the economic benefit of deworming cattle on pasture in Western Canada using its oral dewormer, Safe-Guard. (see “Does deworming pay?” at bottom)

Routine deworming in spring before turnout and/or in fall isn’t a guarantee that pastures will be free from roundworm larvae come spring. Although transmission can’t occur in winter, because larvae need grass, moisture and warmth to reach the infective L3 stage, it has become obvious that larvae can survive Canadian winters on pasture.

“We absolutely know that clean cattle going out to pasture pick up lots of larvae, in some cases within six weeks of being on pasture. Larvae seem to have a finite life and if not picked up in spring, by midsummer they are starting to die off from desiccation,” Lewis explains.

Another survival mechanism is inhibited L4 larvae in yearling and adult cattle. L4s are the L3 larvae ingested late the previous season that hibernate instead of progressing to the adult stage. None of the dewormers currently available is effective against inhibited L4 larvae. So if some adult worms were resistant to the product used last fall, the stage would be set for an egg-laying frenzy in spring.

The test for resistance is a simple fecal egg count taken before the treatment and two weeks after. Anything less than a 90 per cent drop in the egg count is considered evidence of resistance building, Lewis explains.

Recent work in Canada by Merck and an independent researcher found egg-count reductions from various pour-ons ranged from nil to close to 90 per cent. Most products reduced egg counts by 30 to 70 per cent.

The company’s 2012 cross-Canada survey of roundworms in cattle showed an overall average of 26 EP3G (eggs per three-gram sample). Cow samples averaged 15.2 EP3G with 76 per cent above the five EP3G economic threshold for deworming mature animals.

The threshold for calves and yearlings is 10 EP3G. The survey samples from calves averaged 24.0 EP3G with 67 per cent above the threshold. Yearlings averaged 32.0 EP3G with 71 per cent above the threshold.

“This clearly shows that in Canada, with our colder climate putting parasite transmission at pastures in suspended animation, internal parasite resistance is still becoming a problem,” says Lewis. Deworming cattle on pasture and alternating and stacking classes of dewormers are strategies that will help achieve effective parasite control and slow the evolution of resistance to current dewormers.

Safe-Guard is a fenbendazole dewormer, whereas all of the pour-on products are from the macrocyclic lactone class. Roundworm populations resistant to pour-on dewormers won’t have built-in resistance to Safe-Guard because the modes of action differ. Fenbendazole is delivered straight to the roundworm in the abomasum, small and large intestines and kills on contact, but it won’t control parasites that live outside of the gastrointestinal tract. The macrocyclic lactones are designed to control parasites that live inside and outside of the gut, so they need time to be absorbed through the hide into the bloodstream and transported to the infection sites.

There is no evidence grubs and lice are developing resistance to the pour-ons.

Likewise, there is no evidence cattle roundworms have developed resistance to fenbendazole over the 25 years it’s been available. As a precaution, Merck has started sampling roundworm populations in cattle to monitor for signs of resistance to its product.

Resistance to pour-ons snuck up on U.S. producers and veterinarians a dozen years ago after years of treating them like the silver bullet for parasite control.

A fecal egg count is a simple way to find out if any treatment is effective. Gather heaping tablespoons from at least 20 fresh manure pats, seal the representative sample in a bag, keep it cool and deliver it to the local veterinary clinic. Most can do worm counts under a microscope right on site.

Deworming with Safe-Guard

It can be used as a chute-side drench but Lewis says the preferred use on pasture is as a powder mixed with minerals or feed. Safe-Guard pellets are more often fed with grain in feedlots and bison operations.

The powder has to be thoroughly mixed in with the mineral or feed because only a small amount is needed so it needs to be evenly distributed for every animal to get a good dose. It’s also light so more likely to blow away when just sprinkled on top.

Intake will be variable, but when thoroughly mixed 80 to 90 per cent of cows and calves will consume an effective dose. Overdosing is not a concern since the toxicity is very low for animals and dung beetles and other insects in the manure.

In Canada Safe-Guard is considered a drug, not a feed additive so you’ll need a prescription from your vet to purchase it.

To figure out the correct mineral-to-Safe-Guard ratio, monitor mineral intake during the weeks leading up to treatment and estimate the total weight of all animals, including calves, at treatment time. The dose is five milligrams per kilogram of body weight. Mix that amount into the mineral the herd will consume over a week to 10 days and place it in a feeder protected from the elements. The withdrawal time is 13 days after the Safe-Guard-mineral mix is consumed.

In feed spread the amount needed to dose the herd over three to six days’ worth of feed.

A second treatment may be needed depending on the roundworm load, weather conditions, and whether lungworms are a problem because their life cycle differs somewhat from the roundworm.

More details are found at, or give Lewis a call at 403-308-3057.

Related articles on the Canadian Cattlemen: Don’t Ignore Drug-Resistant Parasites and Do We Need to Worm Livestock at Turnout?

Does deworming pay?

Cattle have more tolerance to roundworm infections than sheep, bison and horses, so it’s rare to see outward signs of infection in cattle. Inside, though, roundworms take a toll on performance by damaging gastrointestinal tissues. Reduced feed intake is apparent, suggesting that roundworm infections actually depress appetite. Research shows they also suppress the immune system, making animals more susceptible to disease, but weakening their response to vaccination.

It’s difficult to tell if treated calves do put on extra pounds without having a control group for comparison.

At the Western Beef Development Centre in Saskatchewan, cows were dewormed with Safe-Guard drench just before turnout and cows and calves were dewormed on pasture 35 days later by adding Safe-Guard powder to mineral.

In 2012, treated calves averaged 18.2 pounds more than the control calves at weaning for a net return of $15.05 per calf.

In 2013, even though fecal egg counts were much lower than 2012, treated calves had a 26.8-pound advantage, for a net return of $21.25 per calf from the treatment. Treated cows were also 60 pounds heavier at weaning than untreated cows.

Results from the 2014 trial weren’t publicly available by press time. It involved only cows with heifer calves and looked at the economic benefit of deworming on pasture coupled with use of a growth implant.

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