Internal parasites can significantly impact cattle health. Parasites result in reduced feed intake and gains and may contribute to poor reproductive performance.

Dr. Sarah Potts, University of Maryland Extension dairy and beef specialist, explained that beef cattle will never be 100% parasite free. “Cattle will naturally carry a parasite load,” she said. “They can cope with the presence of some parasites but the goal is to reduce the parasite load to a point where animals do well and make sure the parasite load isn’t out of control.”

Haemonchus is a parasite that adult cattle can become immune to while calves remain susceptible. The main clinical sign is anemia. Cooperia infects the lining of the upper small intestine. While small parasite burdens due to Cooperia are usually tolerable, large infestations cause serious problems. Clinical signs include weight loss and poor production but no anemia.

The biggest problem is brown stomach worm, or Ostertagia, which is most harmful to calves less than 15 months old. “This worm affects the acid-producing cells of the abomasum, which can cause protein malnutrition,” said Potts. “They’ll have a rough hair coat, diarrhea and weight loss and significantly reduced feed intake.”

Brown stomach worm is one of the most economically impactful parasites for cattle. Cattle graze grass and ingest the infective third stage larvae. Immature worms migrate to the gut mucosa, where they mature. Adult worms then lay eggs in the digestive tract; eggs pass in feces and are deposited with manure on pasture. The first and second stage larvae live in manure, then the infective third stage larvae develop and remain infective for weeks or months in manure pats or vegetation where larvae migrate following rainfall.

Potts said it’s important to note the several weeks prior to larvae becoming infective because it’s an opportunity to make pasture management and treatment decisions. High numbers of eggs can remain dormant for a long time when weather conditions are unfavorable for survival.

One of the challenges in treating cattle for internal parasites is resistance. “Dewormers have been around since the 1950s,” said Potts. “No new classes of dewormers have been introduced for 40 years, so resistance is becoming a problem.”

Resistance is lower than expected dewormer effectiveness when other causes (such as user error or underdosing) are ruled out.

The traditional approach for deworming has been to treat every animal several times each year. This has resulted in a population of resistant parasites that survive each deworming application. Resistant genes are passed to the next generation of parasites, resulting in a greater proportion of resistant parasites. This approach has led to selecting for parasite resistance.

When a dewormer is effective, the majority of parasites are eliminated by treatment. Dewormers become less effective when there are more resistant parasites in the animals’ manure. Grazing cattle pick up these resistant parasites on pasture, and the parasites mate with non-resistant parasites. The genetics of the stronger, more resistant parasites overcome the non-resistant genetics and are now present in the herd.

“Overuse and misuse of dewormers has contributed to the development of parasite resistance,” said Potts. “This has implications for animal health and farm economics.” Potts noted that the small ruminant industry has seen resistance issues for quite a while, which has resulted in alternative worm management strategies.

While resistance is present in cattle, it’s much less noticeable because parasites that have developed resistance haven’t significantly affected performance yet. “There have been reports of nematode species being resistant in Europe where Cooperia and Ostertagia are resistant to dewormers,” said Potts. “In the U.S., Cooperia and some Haemonchus species are resistant.” A 2020 study in Maryland suggested Cooperia, Haemonchus and possibly Ostertagia were developing resistance.

The best approach to overcome (or at least slow down) resistance is through integrated parasite management, which involves maintaining good animal nutrition and health, pasture management and judicious dewormer use – including assessment of parasite load, targeted deworming, and proper dewormer selection, application and evaluation.

Parasite loads vary from animal to animal, season to season, year to year and farm to farm. If a parasite burden of a group or herd is high, consider selective deworming. Potts emphasizes that what might be a “high” parasite burden for one herd may not be high for another, so it’s useful to establish a baseline for your farm. She suggested sending manure samples to a lab several times a year to establish a baseline for tracking changes.

“Analyze a composite fecal sample for a group of [15 to 20] animals,” said Potts. “If there’s a substantial increase in worm count relative to the baseline, some animals may require deworming.”

Targeted deworming means not every animal in a group is dewormed. Older cattle develop a tolerance while younger cattle (under 12 months) are most affected by high parasite loads. The 80/20 rule states that 20% of animals carry 80% of the worms – the challenge is to accurately and economically identify the problematic 20%.

Fecal egg counts for each animal are the only way to determine which animals carry high parasite burdens. Individual samples can determine which animals have the highest burdens. Potts said this can become expensive, and one alternative is to select several animals to not treat.

“The premise for non-treatment is to maintain refugia,” said Potts. “Refugia is a population of worms that remain unexposed to a dewormer. Their purpose is to help dilute the gene pool that are susceptible. It reduces the selection pressure for worms that possess resistance traits.”

While refugia doesn’t stop resistance, it can help retain efficacy for available dewormers.

Non-treatment strategies include categorizing animals according to risk and deworming only high-risk animals. High-risk animals include young calves (under nine months), unthrifty animals and newly purchased animals. Moderate-risk animals include yearlings (under 16 months) and cows in late pregnancy. Low-risk animals include adult cattle. While this method is easy, it isn’t the most effective way to quell resistance.

Potts suggested deworming all animals in the high-risk group and leaving a random subset of animals untreated. One method might be to deworm calves but leave every 10th calf untreated.

Another method is to rank all animals in the high-risk group based on performance and leave the top 10% to 20% untreated because these animals are not likely to benefit from deworming. This method is more labor intensive and may not be practical, but it is more effective.

The most effective way to determine which animals should be dewormed is to do a fecal egg count for each animal in the high-risk group and only deworm those with a high parasite load.

New animals to the farm can introduce resistant parasites. Any new animals should be quarantined for two to three weeks, dewormed and kept on a dirt lot to prevent pasture contamination with potentially resistant parasites.

by Sally Colby