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Gastrointestinal roundworms ‘steal’ profits

by Wyoming Livestock Roundup

“It’s challenging because gastrointestinal worms are invisible stealers of profit,” said University of Calgary Faculty of Veterinary Medicine Associate Research Dean John Gilleard.

During a March 28 webinar for the Beef Cattle Research Council, Gilleard discussed the transmission cycle and impact of gastrointestinal roundworms in beef cattle operations.

“From the producer’s point of view, these worms are invisible because we see very little sign of them quite often, but the impact is a loss of production,” he continued.


To understand gastrointestinal roundworm infection, Gilleard commented that it is imperative to understand their lifecycle.

“The adults in the gastrointestinal tract produce these eggs, which pass into the feces,” he said.

The eggs then hatch in the feces to release microscopic larvae, which then undergo several developmental stages before they reach infective larval stages.

“These infective larvae eventually pass into the soil and onto the pasture,” continued Gilleard. “Some of these pass up onto the blades of grass, where they’re grazed and ingested. Then they grow into adult worms to complete the cycle.”

He stressed, “Because a lot of this lifecycle is in the environment outside the host, these parasites are very dependent and affected by climate, like temperature and humidity.”

Normal flora

“A general point we can make is the fact that it’s actually normal for livestock to have parasites,” stressed Gilleard.

He explained that gastrointestinal parasites are part of the normal gut flora, and while animals are grazing, they will have some level of infection.

“It’s actually quite healthy for animals to have a low level of infection, but the modern husbandry systems that we’re using drives infection levels to get above a certain level,” he commented.

According to Gilleard, the goal in parasite resistance is not to eliminate all parasites.

“We in parasite control are not trying to eliminate parasites – which is frankly impossible – but we’re trying to keep them down, so we don’t see any production impact,” said Gilleard.


According to Gilleard, the real time that infection occurs when looking at production systems is when animals are in a pasture setting.

“In the cow/calf system and stocker calves at pasture, we get transmission,” he said, “and that’s where we’re really concerned about selecting dewormers to combat parasite resistance.”

However, when animals enter feedlots, the transmission cycle is broken because larvae don’t survive in the environment.

“Of course, cattle enter into feedlot situations with parasite loads from pasture, which is why we treat them with anthelmintics, but there’s no transmission in the feedlot,” commented Gilleard.

He continued, “It’s a dead-end for the parasite, so we’re less concerned about selecting for parasite resistance.”


“Another thing that’s important to understand is how infection builds up over the grazing season,” said Gilleard.

He explained that the grazing time period from April through October is when the environment is conducive for parasite eggs to produce infective larvae.

“If eggs are produced out of season, like in November and December, they’ll just die. They won’t do anything,” noted Gilleard.

Using the example of grazing stocker cattle, Gilleard commented that there will be a low level of parasite eggs in feces at the beginning of the year, and larvae will begin surviving on the pasture.

“They will graze and ingest these larvae, which will develop into adult parasites. Then those adult parasites will produce eggs, so the egg output begins to gradually increase throughout that grazing season from April to October,” commented Gilleard.

As the eggs produced by the increased number of adult parasites develop into larvae, a large increase in larvae load is observed at the end of September and into October.

“That’s when we get the peak levels of that parasite and peak levels of impact,” he continued, noting that production number is extremely variable from year-to-year and location-to-location, depending on climate.


In addition to impacting digestion and absorption of nutrients, Gilleard noted that one of the largest consequences of a high parasite load is appetite suppression.

“One of the biggest impacts is actually the appetite suppression that goes on. This will have an impact on grazing, and we can imagine the difference in weight gains between animals doing more or less grazing,” he said.

In a U.S. study looking across several northwestern states, researchers found an increase in average daily gain by 0.12 to 0.54 pounds per day between treated and untreated animals, depending on the regional location.

“How does this parasite impact translate to money? It’s estimated in the U.S. that it’s about $2 billion per annum as of a 2006 study,” continued Gilleard.

He concluded, “I go back to the point that it’s subclinical disease. Animals don’t look sick. They don’t look like they’re in terrible condition, but they’re just not gaining weight at the same rate.”

Emilee Gibb is editor of Wyoming Livestock Roundup and can be reached at

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