Deworming relationships, resistance and refugia

Gloria Hafemeister


Resistance to parasite treatments is becoming a big concern and is presenting challenges to graziers.

“If you’ve been using the same treatment for years, I’d suggest you switch to something else," said Sandy Stuttgen, a veterinarian and Taylor County UW-Extension educator. "If you want to break a worm cycle, stop grazing or learn how to manage the grass.”

Speaking to graziers at the recent Beef Grazing Field Day at the Arlington Research Farm, Stuttgen explained the relationship between a cow as a host and the parasite.

She defined a parasite as an organism that derives nourishment and protection from other living organisms. A grazier’s concern is the cow that serves as a host.

“There are two worm populations,” she said, “those within the cow and those on the grass.”

During hot, dry times of the year, few parasitic stages can survive on pasture; therefore, they thrive inside animals. She also pointed out that there are more worms this year because there is more growth without the usual summer slump.

Within a pasture, she said the grass growing along fence lines or feed bunks can harbor immature parasite stages and serve as a source of cattle infection.

Temperature, moisture and grass management have profound influences on the available infected larvae.

“New eggs deposited will hatch and larvae will molt to the infective stage, to crawl away from the fecal pat and up on the blades of grass as far as 4 inches," Stuttgen said. "Larvae movement is facilitated by cool, moist conditions."

During spring and early summer, there are more parasites on the grass than there are in the animal. As grass hits the summer slump in a normal year, larvae are less as well. In the hot part of summer, grass is brittle, but as it cools down in the fall and there is an increase in grass growth, there will also be an increase in larvae consumed.

“An overgrazed pasture will result in more problems so it’s best to consider making hay off of the paddocks to break up cycles,” she said. “The life cycle of larvae is interrupted by prolonged absence of grass.”

If it is not possible to till a pasture and rotate to another field and grazing is necessary, she suggested putting older animals out to the grass first so they vacuum up the larvae present. Older animals have an age dependent tolerance to worms.

When deworming is done, do it three to six weeks after turnout following green up in the spring.

“As long as you have temperatures and weather conditions that support green grass, then know there will be larvae present,” she said. “Intensive grazing and overstocking results in more larvae in a pasture.”

“The bottom line is in order to effectively manage the larvae, you have to effectively manage the entire relationship — the host, parasite and environment.”

Testing important

Stuttgen strongly recommends using a fecal egg count to determine the parasite burden.

When sampling, she said, “Pay attention to the age cattle you are sampling. It is not recommended to sample nursing calves. Twenty percent of the population shed 80 percent of the eggs.”

She recommended sampling from 20 percent of the herd or 20 animals. For some that may be the entire herd. Use the results as a guide as to which treatment will be most effective.

Once the sampling is done and animals are treated, it is important to sample again to determine if the treatment was effective.

Dealing with resistance

As for resistance, she said adult larvae can survive because of either inappropriate dosing (under-dosing, incorrect application or not enough product) or by having resistance genes.

“Worms guarantee their survival by producing numerous eggs coupled with their short life cycles that allow for genetic diversity,” she said.

The best practices to reduce the development of resistance include the following

  • test using a fecal egg count;
  • apply, inject or orally administer the correct dose of dewormer per animal
  • retest to verify effectiveness
  • deworming more than 2-3 times a year increases dewormer resistance

“Parasites killed by the treatment are no longer capable of breeding," she added. "It is important to think about the population of remaining live parasites who will be capable of breeding for future generations.”

Stuttgen recommends thinking about where resistant eggs are deposited.

“Don’t move cattle immediately after treatment; allow cattle time to ingest susceptible eggs from that current paddock,” she said. “Treated cattle will be ‘seeding’ the next pasture with resistant eggs, from those internal worms which survived the treatment.”

She recommends practicing surveillance of cattle and pastures by collecting and recording data. When buying cattle, ask about the history of the source herd’s deworming practices and do fecal tests on the new animals before co-mingling onto pastures.

When cattle graze there is an increased opportunity to pick up internal parasites. Resistance to common treatments can be an issue.