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Reduce animal stress by deworming during drought

Drought plus internal parasites stress cattle and can put your animals, particularly calves and stockers, at greater risk of illness.

Submitted by Boehringer Ingelheim

Stress on your herd comes in many forms, and the added nutritional stress created during drought conditions is one more factor that can impact the health and productivity of your animals.

Drought creates nutritional stress

Lower quality and reduced amounts of forage during drought limit an animal’s energy, protein, vitamin and mineral status. Parasite infections heighten nutritional stress by decreasing the animal’s appetite while irritating its gastrointestinal tract and reducing the ability to absorb the nutrients it does consume. As a result, cattle with poor nutritional status may be less able to respond to a disease challenge. The poor nutritional status can also limit the animal’s ability to respond adequately to vaccines and develop immunity.

Immune systems weakened by parasites put your animals, particularly calves and stockers, at greater risk of illness both during the grazing season and later in life.

“For optimum gain and ongoing health, cattle need to be able to fully utilize available nutrition,” said DL Step, DVM, with Boehringer Ingelheim. “Deworming helps reduce the negative effects of nutritional stress caused by drought-limited forage.” 

Increased opportunity for parasite infestation

Most internal parasites share a similar and straightforward life cycle. Adult worms, living in the animal’s gastrointestinal tract, produce eggs that pass out through the manure. The eggs hatch in the fecal pats, and the larvae migrate to nearby blades of grass, where they’re ingested by an animal. Within the animal’s GI tract, the larvae grow to adulthood and begin the cycle again.

During dry conditions, parasite development may be limited. However, because grass is grazed closer to the ground, animals may ingest greater numbers of the larvae present in the lower 2 to 3 inches of grass and forage.1 And, though cattle don’t typically graze near fecal pats, when forage is limited and grass is greener near fecal pats, cattle will eat nearly everything.

Dr. Step points out that there also is potential for cattle to ingest more parasite larvae when grazing in low-lying areas, around water tanks or near streams where moisture is available, and grass is greener. Because cattle like the green grass in these areas, manure tends to be more concentrated, and, in turn, parasite larvae can be more prevalent.

Drought stress impacts parasite behavior

While many living things struggle to survive in drought, internal parasites don’t take a break. In fact, they can behave differently when encountering drought stress, and be even more problematic. Research has shown that the survival of infective larvae within dry dung pats is enhanced by drought conditions, rather than reduced.2

Ostertagia ostertagi, commonly known as the brown stomach worm, is one of the most economically devastating internal parasite species affecting cattle. An infection can reduce weight gain by up to 20 pounds per calf, and is estimated to cost the U.S. cattle industry $2 billion per year due to lost productivity and increased operating expenses.3

And, Ostertagia ostertagi has a survival mode for drought.

Adult worms feed on the animal’s stomach lining, causing irritation and fluid loss, interfering with the digestive function of the stomach. Larvae or immature worms invade the gastric glands in the lining of the stomach, disrupting the glands’ normal structure and function. At certain times, such as during drought, these larvae have the capacity to “arrest development” or become “inhibited” and “encysted” in the stomach lining for months. When conditions are favorable to continue their life cycle, the inhibited larvae rapidly increase in size and emerge in large numbers, damaging the gastric glands and limiting normal digestive function of the stomach.

Best time to deworm?

When is deworming most effective in minimizing animal stress during drought?

Dr. Step encourages cattle producers to work closely with their veterinarian to design a deworming strategy appropriate for their herd and individual situation.

“Deworming should be part of your preventive health program,” said Dr. Step. “Your veterinarian knows your operation and the parasites prevalent in your region. He or she can assist with diagnostics to advise you on the best product, dose and timing for your operation, whether you’re turning cattle out in the spring, preparing calves for weaning, receiving incoming cattle, or managing cows to withstand winter weather.”

Effective solutions for encysted parasites

Working with your veterinarian also is the best way to determine if encysted O. ostertagi may be an issue.

With inhibited larvae present, a dewormer with adulticidal and larvicidal effects needs to be administered. Dewormers with this property include macrocyclic lactones or “pour-on” dewormers and some of the benzimidazoles or “white” dewormers.

“No matter the weather, controlling parasites is an important aspect of optimizing the health, well-being and productivity of your herd,” Dr. Step concluded. “During drought, deworming is another risk-management tool to help reduce animal stress in an already stressful situation.”


1 Paras KL, Kaplan RM. Assessment, management and control of internal parasites in beef cattle production systems, in Proceedings. Annu Am Assoc Bovine Pract Conf 2019;52(1):55–59.

2 Barger IA, Lewis RJ, Brown GF. Survival of infective larvae of nematode parasites of cattle during drought. Vet Parasitol 1984;14(2):143–152. 

3 Stromberg BE, Gasbarre LC. Gastrointestinal nematode control programs with an emphasis on cattle. Vet Clin North Am Food Anim Pract (2006);22(3):543–565.


©2022, Boehringer Ingelheim Animal Health USA Inc., Duluth, GA. All Rights Reserved. US-BOV-0434-2022-A




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