Resistance to dewormers can be managed

Parasite pressure may not be out of control, but producers could see lower reproduction rates and weaning weights


PHOENIX, Ariz. — The last new deworming product came out in 1998 and efficacy was nearly 100 percent.

That is not the case today as resistance to parasites is being reported around the world.

“We have currently no new molecules coming to the market,” said veterinarian Douglas Ensley with pharmaceutical company Boehringer Ingelheim.

“I see some resistance in every product we have on the market today,” he said at the National Cattlemen’s Beef Association convention held in Phoenix earlier this year.

He said most people may not see the effects of serious parasitism but they may notice lower reproduction rates and weaning weights in their cattle.

They probably do not know how well the current dewormer is working.

“Something you need to think about with your veterinarian is, what is happening in my herd,” he said.

The parasite cycle starts when a cow eats the larvae found in a dew drop on a blade of grass. It enters the cow’s system and becomes an adult that lays eggs.

When the brown stomach worm gets into the abomasum, it burrows into the stomach wall and its activity causes an irritation and affects digestion.

Intestinal worms can cause diarrhea and nutrients are not absorbed.

This situation can affect fertility because the cattle are not eating or growing well.

“The most impacted is the young calf. He is growing and he needs to eat,” he said.

Replacement heifers as well as first- and second-calf cows are also affected. They are still growing, producing milk and getting pregnant again. They struggle if they are full of worms.

It is commonly said 80 percent of worms are in 20 percent of animals.

When people deworm, a number of things can go wrong.

People think they know their animals’ weight but a scale is needed to get an accurate number so the correct dosage is given.

Too many are under dosed so he recommends dosing to the heaviest animal in the herd.

“I would rather over dose than under dose. I want the maximum product to get to as many parasites as possible,” Ensley said.

Fecal egg counts should be considered to discern what parasites are present and assess how well a deworming treatment worked.

Fecal samples must be collected according to the product post-treatment and collected from the same animals that were dewormed.

“If I don’t get a 90 percent reduction, I start getting concerned,” he said.

A lower level of kill may indicate resistance to the product has occurred.

“We say we have resistance when it gets below 90 percent,” he said.

Fecal egg counts should be collected at the right time. If cows were treated with ivermectin or benzimadazole, producers should collect fecal samples 14 to 17 days after treatment. If using moxidectin, collect samples 21 to 28 days later.

Anthelmintic resistance commonly occurs through selection rather than mutation. Certain worms carry a resistance gene and when the dewormer is used they can survive. This trait among the survivors is passed on to the next generation of worms.

One way to get ahead of the resistant worms is the concept of refugia.

Ensley recommends deworming 90 percent of the herd and leaving 10 percent untreated.

If 100 percent are treated the worms released will be resistant.

However, the untreated cattle can shed susceptible types that can mate with the resistant pests and dilute the problem.

Adult animals shed a lot of parasite eggs on a pasture. Over time the parasites accumulate.

“I don’t have to treat every animal to have the impact on the parasite load in the pasture. I just have to deworm at the right time and cut back the cows’ egg passage and allow fewer eggs to be shed on pasture,” he said.

He suggests deworming bulls, cattle younger than three and mature cows.

“If the cow is thin and looks rough, go ahead and treat her,” he said.

Worm species in cattle

Cattle may have 14 different species of gastro-intestinal roundworms. Different species live in different locations in the intestine. As there are usually just a few of these roundworms present, the harm they cause is not always apparent and can be difficult to assess.

Four species live in the abomasum:

  • barber pole worm (Haemonchus placei)
  • brown stomach worms (Ostertagia ostertagi and O. bisonis)
  • threadworm (Trichostrongylus axei)

Six species live in the small intestine:

  • thread-necked worm (Nematodirus helvetianus)
  • four species of bankrupt worms (Cooperia spp.)
  • cattle hookworms (Bunostomum phlebotomum)

Four species live in the large intestine:

  • nodular worm (Oesophagostomum radiatum)
  • (whipworm (Trichuris discolor)
  • (large-mouthed boll worm (Chabertia ovina)
  • (hairworm (Capillaria bovis)

Here are a few additional tips:

  • Get the veterinarian involved.
  • Determine the parasite population in the herd.
  • Choose a dewormer with great efficacy.
  • Give the proper dose based on weight.
  • If using a pour-on dewormer, it must be distributed evenly over the body because it is absorbed in the hair follicles.
  • Periodically evaluate product efficacy.
  • Deworm before breeding and calving.

Why treatments fail

  • Wrong dose. Using a half dose can lead to resistance because only the most susceptible parasites are killed.
  • Treating at the wrong time.
  • Too many treatments.
  • Inaccurate treatments.
  • Product failure.
  • Animal variance with more fat deposits, diet, sex and GI tract physiology.

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