Five Myths of Internal Parasite Control
Identifying the misunderstandings of parasite control
25 June 2026
3 minute read
By: It’s estimated that the cattle industry loses about $3 billion each year in lost weight gains, poor feed conversion and increased disease because of internal parasites.1 With the financial impact and animal welfare concerns on cattle operations, it is important for producers to understand parasite control, as well as the misconceptions about parasite control.
Here are five parasite control myths that might be putting a producer’s management program at risk.
Myth No. 1: All active ingredients in parasite control products have the same efficacy.
There are different active ingredients and different classes of dewormers, which should be used strategically on an operation for effective parasite control, advises Mark Alley, DVM, managing veterinarian with Zoetis.
Products such as Dectomax® Pour-On, Dectomax® Injectable or Valcor® (doramectin and levamisole injection) provide both extended therapy and good efficacy against both adult and inhibited Ostertagia, the brown stomach worm. However, in populations of cattle where Cooperia, Nematodirus or Haemonchus are an issue, white dewormers such as Valbazen® Suspension may be a better selection. It is important that a producer has a discussion with his or her veterinarian or animal health provider to determine which is most appropriate.
Myth No. 2: My animals look fine, so I don’t have a parasite resistance problem.
“Parasitologists agree that no dewormer provides 100% effectiveness against parasites,” said Dr. Alley. “We make the assumption that all parasite control products are 100% effective, but even with 50% kill of parasites, producers will see a clinical improvement in the animals.”
Dr. Alley says producers can’t tell visually if there is a resistant parasite problem in the herd. They need to work closely with their veterinarian to diagnose resistant parasites and establish a comprehensive deworming program.
Myth No. 3: Parasites cannot withstand winter’s cold temperatures.
“It is a mistake to think it gets cold enough to kill parasites over the winter,” Dr. Alley said.
Parasites can simply overwinter in cattle or pastures. While winter may take its toll on many things, studies demonstrate that infective larvae were able to survive on pastures during winter months.2,3
Myth No. 4: Antiparasitics can be administered to work at a producer’s convenience.
Timing is critical for administering antiparasitic products. Often, producers deworm when it’s most convenient for them, rather than when it’s most effective to control parasites or most beneficial to the animal. Dr. Alley recommends deworming prior to turnout on fresh pasture, typically in the spring, and later when pastures become dormant. Geography and weather conditions may alter this timing.
Myth No. 5: Dosing to the average weight of the group is adequate.
It’s important for producers to not only match the dewormer to the type of parasite challenge but also to administer each dose per the animal’s calculated weight. Incorrect dosing has been identified as a major contributor to the development of resistant parasites. A common practice is to dose products to the average weight of the herd, rather than to the individual weight of the animal. In this case, half the herd could be underdosed.
For more information about parasite management, please visit with your veterinarian or Zoetis representative.
IMPORTANT SAFETY INFORMATION: Do not treat cattle with Valcor within 15 days of slaughter. Not for use in female dairy cattle 20 months of age or older, including dry dairy cows; not for use in beef calves less than 2 months of age, dairy calves, and veal calves. Safety has not been evaluated in breeding bulls. Use with caution in cattle treated with cholinesterase inhibitors. This product is likely to cause injection site swelling; tissue damage (including granulomas and necrosis) may occur. These reactions have resolved without treatment. See full prescribing information at ValcorTough.com/pi.
Dectomax Injectable has a 35-day pre-slaughter withdrawal period. Dectomax Pour-On has a 45-day pre-slaughter withdrawal period. Do not use in female dairy cattle 20 months of age or older. Do not use in calves to be processed for veal. Dectomax has been developed specifically for cattle and swine. Use in dogs may result in fatalities.
Cattle must not be slaughtered within 27 days after the last treatment with Vabazen. Do not use in female dairy cattle of breeding age. Do not administer to female cattle during the first 45 days of pregnancy or for 45 days after removal of bulls.
Consult your veterinarian for assistance in the diagnosis, treatment, and control of parasitism.
