Deworming Product Options and How to Use Them
Deworming Product Options and How to Use Them
Every region and every farm/ranch is a little different in the risk for internal parasites, and the challenge to control them is usually ongoing. Warm climates and mild winters enable internal parasites to have a longer transmission season, to be picked up by grazing animals. This also means these parasites have more opportunity to become resistant to the chemicals we typically use for controlling them. It’s important to understand the products we use, their mode of action, and how to reduce the incidence of resistant worms.
Internal parasites are often a bigger challenge than external parasites like flies because we can’t see the worms and we don’t know how many are impacting our cattle. Unless cattle are heavily parasitized to the point of clinical signs (weight loss, diarrhea, edema beneath the jaw, anemia) the only clue is reduced performance–such as lower weight gains or more open cows. Worms are an added stress that can also make the host animal more vulnerable to other diseases.
Deworming cattle at the proper times of year, at the most appropriate stage in the life cycle of a certain parasite to eliminate the egg-laying adults–to minimize the re-contamination of pasture with worm eggs–can help keep reinfection to a low level. The problem we face today, however, is increasing resistance to the drugs we use. Worm populations are becoming harder to kill. Stockmen need to rethink their parasite programs or soon our deworming tools will be much less effective.
All cattle on pastures that have been previously grazed by cattle pick up internal parasites. Dr. Ray Kaplan, Professor in the Department of Pathobiology at St. George’s University in Grenada in the West Indies (formerly Professor of Parasitology, University of Georgia), has studied parasites for many years. He warns producers that we need to change our strategies.
In the days before deworming products, preparations derived from plants were the original therapeutic interventions used by stockmen to control parasites. Various herbal treatments were common but the specific plants utilized depended on the local plants in different parts of the world. In more recent years, some farmers also fed small amounts of lye (sodium hydroxide) mixed with feed, or used apple cider vinegar, diatomaceous earth, or tobacco to try to kill worms.
Tannin-rich forages, such as lespedeza, birdsfoot trefoil and chicory, can reduce parasitic worm burdens and fecal egg counts in cattle by inhibiting larval development and egg hatching. Forages containing 2% to 4% condensed tannins in dry matter can act as natural dewormers. Traditional methods were often complemented and aided by rotational grazing (to move the cattle away from the worms and not come back to that area of pasture for a long enough period that the worm larvae that hatch from the eggs passed in manure have died). It also helps to use pasture drag harrows to break up manure and not bring cattle back to that pasture until the manure has all dried out. When vegetation is green and temperatures are warm and moist, avoid feeding on the ground in areas where worm larvae can migrate onto vegetation.
In years past, producers also controlled parasites by rotating different classes of livestock such as sheep, cattle, horses, etc. on various pastures. Most of the worms are host specific and won’t mature in the wrong host. Stockmen monitored condition of the animals and tried not to overstock pastures. But close attention to management went by the wayside when the new wonder drugs became available.
As deworming products got cheaper and easier to use, producers and the people making the recommendations for deworming were lulled into a false sense of security because the drugs are so good, Stockmen began to depend more on commercial deworming products, beginning with phenothiazine in the 1950s. The benzimidazoles where introduced in the 1960s, the imidazothiazole/tetrahydropyrimidines in the 1970s. Avermectin/milbemycins came along in the 1980. For a while, a new class of anthelmintics became available each decade. These were effective and relatively inexpensive drugs, so parasite control became based on frequent or strategic use–to maximize livestock health, productivity, and profitability.
This approach was successful for several decades, but now there are no new deworming drugs and we’re seeing increasing resistance in all drug classes. Kaplan says all the economically-important parasites of all livestock species are now becoming resistant to dewormers.
His studies (and tests performed at his laboratory), on several cow-calf farms in Georgia and on stocker cattle purchased at stockyards, showed that resistance to the avermectin dewormers in cattle is common and widespread. More than 90% of the farms he tested had problems with resistant Cooperia (the most common intestinal worm). “Resistance in Cooperia and Haemonchus (a stomach worm) are common, but we’ve also seen resistance in Ostertagia (the brown stomach worm). As resistance increases, this will pose a serious threat to cattle health and productivity,” he said.
Cooperia does not impact health and productivity to the degree that Ostertagia does, but a more recent study showed that Cooperia infections have a significant negative effect on young, growing cattle. Clinical disease in cattle (with obvious symptoms) due to Cooperia may be uncommon, but significant production losses can result from high levels of infection.
Other major parasites are also becoming resistant to dewormers. In 2018 Kaplan found resistant Ostertagia on one of 8 farms he tested. In another study, he demonstrated that there was almost no efficacy of avermectin drugs against the dormant stages of Ostertagia, even though early studies with these drugs reported 95-99% efficacy against this stage. This loss of efficacy is worrisome, since Ostertagia is a highly pathogenic species; these worms are responsible not only for production loss, but also severe clinical disease and occasional deaths.
No new classes of dewormers have been introduced for cattle since ivermectin in 1981. Second-generation avermectin drugs have helped, but they are the same class of drugs; resistance to any one of them tends to create resistance to all of them.
Reliance on drugs for parasite control is no longer recommended, and will not be sustainable into the future. “There are numerous products currently available for cattle, and convenient to administer, making treatment for worms easier than ever. However, many of these drugs are fairly expensive and may not be working at levels they did in the past; money can be lost to unnecessary or ineffective drug treatment just as easily as to parasites,” said Kaplan. There is risk of losing our ability to effectively control parasites if more resistance develops due to heavy use of these drugs.
Calves are most vulnerable to the damaging effect of worms, since most adult cattle have been exposed for more of their life and have developed some tolerance to worms. Healthy cows older than 3 years, with good body condition and on a good nutritional plane tend to have fairly strong immunity and we rarely see clinical disease due to these parasites, though there may be subclinical reductions in production. “By contrast, young recently-weaned calves in their first year on grass are highly susceptible to parasitism; this group of animals is at greatest risk for production loss and clinical disease,” Kaplan said.
CONTROL STRATEGIES VARY
Favorable conditions for hatching eggs and larvae development include moisture and moderate temperatures (40 to 85 degrees). Hot weather above 85 can be fatal for worms, especially if the weather is dry. Temperatures below 40 degrees inhibit them.
Once the infective larval stage is reached, they can persist longest in moist, cool conditions and are rapidly killed by high temperatures, freeze-thaw cycles, and drying, so optimal periods of transmission vary in different geographic regions. This affects the ,pst appropriate times for deworming; there is no one-size-fits-all parasite control program.
Some general recommendations apply in most situations, however. “Cows in poor body condition due to poor winter nutrition should be treated in late winter, just before calving, but optimal timing will vary depending on season of calving,” said Kaplan. With new grass growth and improved nutrition, the cow’s immune system becomes stronger and she’s more able to limit her own worm burden.
“Yearlings and replacement heifers are at greatest risk for production loss from parasites. Deworming will be important to their health and production, but optimal strategies for applying those treatments will vary greatly, depending on the management and grazing system,” Kaplan said.
According to Dr. Thomas Craig (professor emeritus, Texas A& M University,) the first drug-resistant cattle worms were discovered in Wisconsin, but those particular calves were born and raised in the Gulf Coast and sent to Wisconsin as stockers. “Each drug company will tell you their product is best, and sometimes they are correct. The biggest problem I see is that cows and calves do not have the same populations of parasites, and the drugs that are best for cows miss the important ones in calves,” he said.
“Producers also need to know if their ranch has flukes. This is extremely variable in different locations. Timing of treatment for flukes is important as our drugs are only effective after the flukes have done their damage. Treatment is only to protect the snails and can help the cattle next year.” Regarding parasite control, he suggests talking to your herd-health veterinarian for advice on what to treat for, and when, because it will be different from ranch to ranch even in the same geographic region.
Nancy Hinkle, PhD (Professor, Department of Entomology at the University of Georgia) has done a lot of research and work with fly problems in horses and livestock and says that some of the treatments for flies that work systemically (to kill any blood-sucking flies) will also kill internal parasites. This is not a good thing to use very often, however, because internal parasites are already developing resistance to ivermectin.
“We tell people to please not use those products for horn fly control because we are trying to conserve them to control the internal parasites,” says Hinkle. Some people feel it’s great to be able to control both the internal and external parasites at the same time, but in the long run it’s not a good idea. If you use these too often for fly control you will soon have some resistant internal parasites.
Some people ask about alternatives to chemicals, such as for organic operations. Some organic producers say that pasture rotation helps keeps fly and worm numbers down. The cattle are moved away from the fresh manure. This can help, especially if they are moved daily and get far enough away by the time the new flies or worm larvae hatch out. If cattle don’t come back to that area for a long enough time, this reduces internal parasite transmission. The worm eggs that hatch into larvae, moving onto forage plants, may dry out and die before they can be ingested by cattle again.
CLASSES OF DRUGS
Brand name and generic dewormers available in the U.S. can be divided into three different classes, based on their chemical structure and mechanism of action. These three classes are macrocyclic lactones (endectocides that can kill both internal and external parasites), benzimidazoles (white dewormers) and imidazothiazoles. Each class has a distinct molecular structure that harms parasites a different way, controls a distinct spectrum of parasites, and for a different amount of time. Awareness of these differences helps producers prevent parasites from developing resistance to the drug class, not just to a specific product.
Avermectins/macrocyclic lactones
Ivermectin was the first of this class, followed by derivatives such as eprinomectin, doramectin and moxidectin. Active ingredients ending in “mectin” or “ectin” or labeled as “avermectin” belong to this class. Moxidectin is a chemically modified macrocyclic lactone; although not technically an avermectin, it functions similarly. Macrocyclic lactones are noteffective against liver flukes.
Avermectins are the most widely used anthelmintics due to the variety of brands, generics and formulations (oral, injectable and pour‑on). Their popularity has contributed to widespread resistance. Switching to another product in the avermectin/macrocyclic lactone class, for example, from ivermectin to doramectin, will not overcome resistance. Switching to a different class, such as the benzimidazoles, may help thwart resistance.
Benzimidazoles
Fenbendazole, albendazole and oxibendazole all contain the same central molecular shape, known as a benzimidazole ring. Active ingredients ending in “dazole” belong to this class. These products are administered orally, which has limited their use compared to pour‑on and injectable products which are easier to do. Because of this, parasites have been less exposed to them and resistance is less common. Any of the benzimidazoles are a good alternative to macrocyclic lactones or can be used in combination. Albendazole is also effective against adult liver flukes.
Imidazothiazoles
This class is identified by active ingredients ending in “misole,” such as levamisole and tetramisole. These are administered as oral drenches and are not effective against liver flukes. Only a few products in this class are available.
There is also another class of drugs that is used for liver flukes. A sulfonamide derivative (clorsulon) is not a dewormer, but can be utilized by cattlemen to control flukes. Clorsulon targets adult liver flukes by blocking their energy production. It is not effective against worms on its own but is often combined with macrocyclic lactones, so one treatment addresses both flukes and worms. Resistance to clorsulon has not been widely reported, but because liver flukes can develop resistance to other products, monitoring treatment results is important. In the U.S., clorsulon is available only as an injectable formulation. There is no pour‑on version, even in combination products.
Understanding which drug class a dewormer belongs to and whether it still works on your farm is essential for optimizing cattle performance and to avoid wasting money. Strategic fecal egg counts and fecal egg count reduction tests are the only way to know if a product is doing its job. Clues on the label (the name of the drug itself) can help you identify the drug class and choose the right tool for the parasites you’re dealing with.
Some classes of dewormer do a better job controlling certain parasites than others. Endectocides provide coverage against both internal and external parasites, while white dewormers work well against only internal parasites. Most macrocyclic lactones persist within the animal for about 28 to 30 days, with the exception of the extended-release formulation that may last up to 150 days. By contrast, the benzimidazoles, also known as purge dewormers, clean all the worms out of the digestive tract and in two days are gone.
April 2026
By Heather Smith Thomas
