When it comes to treating internal parasites, there’s no one-size-fits-all approach. Understanding the environment and knowing what parasites are active at different times of the year are important when creating a deworming plan, says Wayne Ayers, DVM, technical consultant veterinarian with Elanco.
“Cattle on an irrigated pasture are completely different than cattle coming off the desert,” he says. “We need to consider how parasites’ life cycles relate to the season to determine how we want to approach control.”
Why Deworm?
There are 15 different internal parasites, four or five of which cause the most economic loss. The brown stomach worm tops the list, specifically in young stock.
“When the brown stomach worm is present, it’s important to choose an anthelmintic dewormer because their larvae can inhibit and overwinter leading to a secondary disease called Type 2 Ostertagiasis,” Ayers explains.
Internal parasites cause digestive issues in the abomasum, which can lead to permanent damage.
“Young stock that have a heavy infection of inhibited larvae and go through a Type 2 disease can become less efficient because of damage to the abomasum that develops scar tissue,” he says. “We want to kill them before they come out because that’s when they cause damage.”
White wormers (benzimidazoles) work well at killing adults in the GI tract, he adds. The two subclasses of the macrocyclic lactone group, avermectins and milbemycins, do a good job of killing both adults and larvae.
“The youngest cattle are at the most risk because they get everything that’s out on the pasture and possibly in relatively high numbers. Their immune system has not been exposed to these parasites to develop any kind of immunity to the larval stages or even the adult stages,” Ayers explains.
“The older an animal gets, the more times they’ve seen it, and their immune system will actually suppress the infections and decrease the egg laying capacity of the females that reside in the GI tract,” he adds.
Resistance is on the Rise
At one time, the industry was convinced cattle should be dewormed every time they ran through the chute.
That approach might have worked in the 1980s when ivermectin came on the market, Ayers says, but it’s not a good practice today because resistance is developing.
When parasites are exposed to the same chemical over and over again, or the dosing isn’t correct, the last parasite remaining will have a high tolerance for the drug or be resistant all together. An infection sets in because of the tolerant or resistant parasite, leading to poor treatment response and a poor return on investment.
“I have a saying: ‘What’s in my cow is what’s on my pasture, what’s on my pasture is what’s in my calf, and what’s in my calf is what causes me economic loss. The losses aren’t from the cows.’”
While there are situations when cows must be treated at certain times of the a year because of heavy parasites, it’s not common as the cow’s immune system usually limits parasite infection.
“It’s certainly not common across the northern part of the U.S. where we have the benefits of cold winters or in the Southwest that experiences hot, dry summers. These weather extremes don’t favor larvae survival,” Ayers says. “Unfortunately, in the Southeast, the long warm and humid summers favor the larvae.”
In addition, he says the larval load can vary from pasture to pasture. For instance, a large Bureau of Land Management or Forrest Service-type allotment will generally have a lower larval load than an irrigated pasture on the same ranch.
