Managing Internal Parasites
Cattle producers have several tools for controlling parasites.
July 2, 2025
Internal parasites in cattle include various types of roundworms, tapeworms and liver flukes that can negatively affect health and productivity. Cattle producers have several tools for controlling parasites; however, they have to be used properly, or the parasites tend to develop resistance to the control methods.
Joe Gillespie, technical services veterinarian at Merck Animal Health, says many producers worry about external parasites (e.g., horn flies, lice) because they are easier to see, but the real opportunity to help cattle is to address internal parasite burdens.
In northern climates, when cattle come off summer and fall pasture — with the calves going into feedlots and the cows into a winter feeding program — is a good time to deworm, Gillespie says. “We need to eliminate the worm burden from whatever exposure they had on pasture before they go into whatever their winter feeding program is.”
You don’t want cows robbed of nutrients by a heavy parasite load, especially if they lost weight during lactation and need to regain body condition before their next calving. They also need to be able to generate body heat during cold weather.
“Fall deworming can effectively eliminate the worms they picked up during summer grazing,” Gillespie says.
It is important to consult your local veterinarian, who will know the parasite risks in your geographic area and your own situation, and who will be able to help you figure out what the actual parasite load is and which parasites you should target.
“Where we get into trouble is when we underdose or use the wrong application.” — Joe Gillespie
“I suggest having the veterinarian do a diagnostic test, and this is typically a fecal egg count,” he says.
“I recommend taking fecal samples from calves rather than cows. The adult cow has come into contact with parasites for a longer time and may have developed a greater tolerance level,” he explains. By contrast, the young naïve calf has no immunity from previous exposure and is a better representation of the parasite burden on the pasture.
“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, so we can make a better decision regarding the most effective treatment for that particular population of worms,” he says.
There are two major classes of deworming drugs. One is the macrocyclic lactones, which include ivermectin, doramectin, eprinomectin, and moxidectin (e.g., brand names Ivermec®, Dectomax®, Cydectin®), which can be used as pour-on products or injectable. The other class is the benzimidazoles, typically called white dewormers.
“Both classes of products are more than 50 years old (benzimidazoles came into use only about a decade before ivermectin). It is important to choose correctly. Once we decide which product to use, we must make sure we apply it at the labeled dose and application (whether injected, given orally or as a pour-on along the back),” says Gillespie.
“Where we get into trouble is when we underdose or use the wrong application,” he says. “If we underdose we may not get an effective kill. If we don’t do correct application — if 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.”
The two classes of products are utilized differently and work differently.
“The benzimidazoles are 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’s not much risk of worms developing resistance to that product if we give an adequate dose. It passes through and kills all the adults and L4 larvae (the stage before 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 14-21 days. Typically, these products don’t stay in the system long, so timing of application is important,” says Gillespie. “If you are doing this in midsummer, when cattle still have opportunity to pick up more larvae on grass they are grazing 30 days later, they may become reinfected and carry worms into the winter.”
Editor’s note: Heather Smith Thomas is a freelance writer and cattlewoman from Salmon, Idaho. [Lead photo by DVilfer from Getty Images.]
Angus Beef Bulletin EXTRA, Vol. 17, No. 7-A
Topics: Health , Pasture and Forage
Publication: Angus Beef Bulletin
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