Managing to Minimize Internal Parasites
Internal parasites spend most of their life inside the host animal. It is important to deworm cattle at the proper times to eliminate egg-laying adult worms before they have a chance to contaminate the pasture with a fresh supply of worm eggs; strategic deworming can help keep reinfection to a low level. Cattle producers today have several tools for controlling parasites, but we have to use them properly or the parasites tend to develop resistance to our control methods.
PARASITE INFESTATIONS–Dr. Joe Gillespie, Technical Services Veterinarian at Merck Animal Health, says cattle producers need to put a lot of thought into their parasite control program to get the best results. “When I talk with producers, however, they are mainly worried about external parasites because those (like horn flies, lice, etc.) are easier to see, but the real benefit and opportunity to help our cattle is to address internal parasite burdens,” he says. Lush green pastures with a lot of cattle per acre tend to accumulate more larvae on the forage plants than dry, arid ranges where widely spaced cattle reduce larval buildup.
“Even on dry rangeland, however, you can find internal parasites in the livestock that graze there,” says Gillespie. “Some producers may feel they don’t have as much risk for worms in those cattle, but that’s not the case. Internal parasites exist anywhere there are cattle.”
After the larvae develop into the infective stage, they take advantage of morning dew and move onto nearby vegetation. They can readily be found on the lower portion of plants, and if the grass is growing swiftly, they move up with the growth. Cattle are most likely to pick up large numbers of worm larvae when pastures are overgrazed and short, grazing fairly close to the ground. The cattle are eating grass at the level where those larvae are attached to the grass. When the ingested forage goes to the rumen, the larvae move farther into the digestive tract. Where they end up (whether in the abomasum, small intestine, or large intestine) depends on the species of worm.
“In the South, we see more parasite risk through fall and winter, with winter grazing. In the North and Northwest we typically see more parasite risk during summer grazing,” says Gillespie. During winter in these regions, the worms stay inside the animals rather than sending eggs out into an environment where the larvae can’t survive.
In just about any situation when the weather is warm, about 90% of the parasite population is in the pasture and only about 10% are in the animals. “Those parasites are not evenly distributed; one animal may have a lot more worms than another. The re-infestation risk is always highest when cattle are grazing. This is why we typically treat them when they are coming off pasture and going into dry lots,” Gilespie says.
PARASITE CONTROL PLAN–“We need to think about cattle coming off pasture, weaning the calves, and how to eliminate the worm burden from whatever exposure they had on pasture. It’s important to reduce that burden before they go into whatever their winter feeding program is,” says Gillespie.
“We want to eliminate as many adult parasites as possible (since they are a drain on the animal), and when we do that we also remove the inflammation they’ve been causing in the digestive tract. This allows for better utilization of forage. When cattle come off pasture we want to make sure that whatever parasite load those animals have are eliminated effectively with the anti-parasitic product we select after consulting our veterinarian,” he says.
You don’t want those cows shortchanged on nutrients by a heavy parasite load, especially if they lost weight during lactation and need to regain body condition again before calving. They also need to be able to generate body heat during cold weather. “Fall deworming can effectively eliminate whatever worms they picked up during the summer grazing period. Then they will be in better condition for their next calving and rebreeding,” says Gillespie.
It is important to consult your local veterinarian, who will know the parasite risks in your geographic area and for your own situation. “Another thing that is important is to figure out what the actual parasite load is, and which parasites you should target. What I suggest is that producers have their veterinarian do a diagnostic test, and this is typically a fecal egg count,” he says.
“We have a limited number of products available, so it helps to know which cattle are at risk and which parasites are the main problem. This helps us make a better decision in terms of what might be the most effective treatment. We want to choose the product that has the best efficacy against that particular population of worms.”
There are two major classes of deworming drugs—the macrocyclic lactones which include ivermectin, doramectin, eprinomectin, and moxidectin (drug names Ivermectin, Dectomax, Cydectin, etc.) Use these as pour-on products or injectables. The other class is the benzimidazoles, typically called the white dewormers. “Both of those classes of products are more than 50 years old (the benzimidazoles came into use only about a decade before ivermectin).It is important to choose correctly. Once we decide which product to use, we want to make sure that we apply it at the labeled dose and labeled application (whether injected, given orally, or pour-on along the back) and apply it correctly,” says Gillespie.
“Where we get into trouble is when we under-dose or use the wrong application. For example, if we are under-dosing we may not get an effective kill. And if we don’t practice correct application—such as we don’t apply a pour-on product all the way down the back—we don’t get adequate distribution of that product. This may change absorption and decrease the amount/dosage that the animal actually receives,” he explains.
The two classes of products are utilized differently and work differently. “The benzimidazoles are what we call a purge dewormer. They kill all the worms today. There’s not much residual drug left in the animal’s body (and it is only in the digestive tract and doesn’t go systemic) so there is not much risk of worms developing resistance to that product; it passes through the animal and kills all the adults and L4larvae (the stage before the adults).It cleans out all the worms right now and is done.”
The macrocyclic lactones used as a pour-on or injectable might have a little longer residual effect. “They may still kill some worms during the following 14 to 21days.Typically these products don’t stay in the system very long, so timing of application is very important. If you are doing this in mid-summer, when the animals still have the opportunity to pick up more larvae on grass they are grazing 30 days later, they may become re-infected and carry worms into the winter,” says Gillespie.
A new problem today, however, is the fact that cattle move around the country much more than they did 50 years ago, and the parasites go with them. Worms commonly found in the South are now showing up in the North, and vise versa. “A perfect example of one parasite—that’s not a worm—spreading all over the country is the liver fluke. In years past these were mainly in the Northwest and Southeast, but now we find liver flukes in Kansas! As we move livestock around, we also move the parasites around and they readily adapt to the new environment.”
PARASITE RESISTANCE ISSUES-The advice Gillespie gives producers regarding resistance issues—whether it’s resistance to antibiotics, internal parasites, or external parasites (flies, lice, etc.)–entails three important things. “No matter what product you use, always use it according to label directions. Secondly, apply that product at the appropriate dose, since under-dosing (and overdosing in some cases) can be detrimental and increase the risk for parasite resistance. Third, apply at the appropriate time to best manage that parasite,” he says.
“When we think about resistance, we need to understand that if we use the same tool over and over, ultimately there will be some parasites that survive. They will be the main ones reproducing, building a population of resistant worms. Maybe they are not completely resistant, but they are more tolerant of the control method we are using, and we can’t kill them all,” says Gillespie.
Overuse, under-dosing, and incorrect application of those products may cause some level of resistance. If you treat the cattle in a certain pasture every year with the same molecule (which is often the same in various product brands—they are essentially the same drug, just with different names), the worms in those pastures will develop some level of resistance.
“Another thing we’ve learned from the small ruminant model is what we call combination therapy, and we are seeing more and more of this. For a product to have a label for a certain application it must have a certain amount of efficacy. If we have two products that each have successful effectiveness and use them in combination, we have multiplied that level of effectiveness; we have a much lower number of parasites that survive and become resistant,” he explains.Farmers and veterinarians widely use this method to control internal parasites, especially in small ruminants, because these animals face significant issues with resistance.
Many people think that if they use X product this year and Y next year and keep alternating them back and forth, they will thwart resistance. “But what often happens in this situation is that instead of getting resistance in 5 to 10 years to one product, we now have resistance to two products.” This is what happened early on with the use of fly tags.
“However, with a combination of products—which the use of is more widely in dealing with internal parasites than with flies—we can delay that resistance much longer, especially when using the correct dose and application. We may eliminate the opportunity for long-term development of resistance.”
“It always comes back to making a plan, with advice from your veterinarian to determine which product would be most effective at a particular time, for your herd, plus the parasite that’s the greatest risk. If you are having a problem with Cooperia (an intestinal parasite that’s most detrimental in young animals), you’d probably want to treat with a benzimidazole (white dewormer) because those products still seem to have effectiveness for that particular worm,” he says.
“It’s important to understand your level of risk and get some help from your veterinarian to make these decisions. Maybe in your area, it may be more effective to deworm the cattle before they go out to pasture. Or maybe it’s more effective to deworm them as they come off the pasture. This may depend on your geography and the level of risk. In some cases in the South where grazing can be done year-round and the weather is not as cold, it might require deworming more than once a year. Your animal health consultant or veterinarian can help you make the best decisions for your herd.
By Heather Smith Thomas
August 2025
Here is Another Article Focusing Around improving your overall long term success
Click here for more information:
Here is the Dairy Side of things
Or if you enjoy the Outdoors, that could be hunting, fishing, or just recreational outdoor activities the Iowa Sportsman is a great option
