Updates on cattle parasite resistance
During the National Cattlemen’s Beef Association CattleCon this month in New Orleans, one of the most popular features is the Cattlemen’s College education sessions. Speaking on parasite resistance this year was Dr. Ashley Steuer, DVM, Ph.D., Zoetis.
She began by noting, “One-size-fits-all does not apply to parasitology.” This is true because livestock have to contend with both external and internal parasites, and she dove deep into three specific internal pests for cattle.
First, though, she had to clarify some things. “Resistance is not to be confused with under-dosing or ineffectiveness,” Steuer said. She also noted factors that affect resistance, such as overtreating animals. “We want to make sure we’re dosing appropriately based on body weight.”
She also defined refugia, which she said may be becoming more critical. Refugia are organisms not exposed to a drug that still remain susceptible; that can be not exposed in an animal in a different life stage; that can be in the environment (larvae on pasture, for example); and can be those in untreated animals. Ensuring there are some susceptible worms inside an animal to reproduce means creating a “refuge” for worms so those that are non-resistant (susceptible) still remain in the population base.
Steuer spoke first about flukes (Fasciola hepatica), which cause fascioliasis. These worms live in the livers of cattle, sheep and deer, which obtain infection from the ingestion of snails on pasture. While in cattle, flukes will migrate to the liver where the adults will mate and produce eggs, which then exit through the bile ducts into the small intestine. It can take up to two months to form a patient infection.
Flukes tend to be a bigger issue in the South, as they’re found where it’s warm, humid and muggy – Steuer pointed out reports from North Carolina, from pockets in Tennessee, but also from eastern Canada). She said the fluke problem is spreading with climate change.
Fluke infections can cause fibrosis and cirrhosis of the liver, which leads to condemnation of livers at slaughter. Severe cases can lead to acute anemia, hemorrhage and death; chronic cases can cause fibrosis, stenosis and calcification of bile ducts. It can predispose cattle to secondary clostridial infections.
Evidence of flukes can be seen in a reduction in feed conversion, decreased average daily gain (ADG) and reduced milk production. Producer can also see a decrease in body condition score (BCS), weakness or bottle-jaw.
“How do you know flukes are your problem?” Steuer asked. A necropsy can be done afterward, and you can set up a fecal exam with your vet. She also recommended looking at areas similar to yours in climate and landscape, and examining ponds and puddles that attract snails.
Flukicides in the U.S. target adults but not juveniles, so cattle may require more than one treatment if infestation occurs. Clorsulon features an eight-day withdrawal period, and Albendazole has a 27-day withdrawal period. But, she said, the best control is to break the fluke lifecycle. Use molluscicides, drain pastures and control cattle access to wet areas.
Cattle owners can control flukes through strategic deworming. Animals should be treated at the end of the transmission (wettest) season, to ensure the flukicides kill the adults.
Steuer added that there is no evidence at this point that flukes are resistant to treatment in the U.S. but there are some reports of resistance worldwide.
The other internal parasites Steuer presented updates on are familiar gastrointestinal nematodes (GINs): Ostertagia ostertagi, or the brown stomach worm; Cooperia spp., or the bankrupt worm; and, perhaps surprising to cattle producers, Haemonchus spp., or the barber pole worm.
Ostertagia lives in a cow’s abomasum. It obtains the infection from ingestion of larvae on pastures. There is no intermediate host for it (as with flukes), and adults are not the pathogenic stage for brown stomach worm – the juveniles are. They can cause serious destruction as they grow (type 1 or summer ostertagiosis) or as they accumulate (type 2 or winter ostertagiosis). They destroy the abomasal lining, leading to indigestion, diarrhea, anemia, weight loss and bottle-jaw; in chronic or subacute cases, producers will see a decrease in ADG.
The Cooperia worms live in the small intestine. Researchers have found that if it takes over, or meets with concurrent infections, Cooperia poses a significant economic loss in the cattle industry, as it can lead to diarrhea, decreased BCS and decreased ADG. “This is a bigger player today,” Steuer said. “It stops conversion of your feed, which can lead to a big economic loss.”
Barber pole worm (Haemonchus) is usually more of a problem in small ruminants, but Steuer said it’s being seen more and more in cattle, although disease is typically subclinical or due to other diseases. Haemonchus leads to economic losses through lower ADG; it can also lead to anemia and bottle-jaw in some cases.
There have been no recent reports of resistance for Ostertagia. Cooperia is the most common GIN to have resistance reported. Haemonchus resistance is widespread in sheep and goats, but unknown (and suspected) in cattle. Not many reports have been made on it in the past five years, according to Steuer.
“What’s important is to take the tools available and add them to your toolbox,” Steuer said, but only use the tools that make sense for you and are financially feasible for your operation.
Consider using fecal egg counts – but remember they do not indicate how “wormy” an animal is, as they don’t count juveniles or adults. You can also utilize larval cultures and testing. She said these are useful in determining the populations of parasites and types – but they are labor intensive and time consuming, and therefore can be costly. The benefit is they can also be used for determining resistance.
Steuer emphasized the importance of deworming schedules. “Deworm only those that need it (young stock, heifers, yearling stock, and those with high stocking densities),” she said. “Deworm before and after the grazing season. If we can’t target deworm, then try to decrease your frequency. And do not rotate dewormers – stay with one class.”
As for pasture management, the basics are still the best: use rotational grazing, and if possible, let parasites sit on empty pasture for three to six months. Don’t overgraze either. Steuer added that it’s better to move animals, then treat them, due to refugia.
What else can producers do? Steuer listed the options: End rotational deworming; use combination products, if they’re available and they work for your herd; decrease the frequency of deworming; increase refugia; use good pasture management wherever possible; and use diagnostic testing whenever possible.
“Remember, where there are cattle, there are parasites,” she said. And many of them love that 60º – 80º window and 40% – 70% humidity.
She ended by reminding producers to let their veterinarian and Extension agent know if they think they have parasite resistance in their herds – researchers need that information.
by Courtney Llewellyn
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