Q -- I recently read that cattle worms have developed resistance to pour-on dewormers. For years I've only used pour-on dewormers in my operation because they are so easy to apply. Should I stop using pour-on dewormers?
A -- You have asked an important question: Are cattle gastrointestinal (GI) parasites becoming resistant to the pour-on dewormers? Pour-on dewormers belong to a class of chemicals called avermectins. The answer is that although there is some proven evidence of resistance of cattle GI parasites to the avermectins in the United States, it is presently not common. Resistance of sheep and goat GI parasites to dewormers, however, is widespread.
The important GI worms of cattle include the abomasal worms Ostertagia and Haemonchus, and the intestinal worm Cooperia. The abomasal worms cause the most damage to cattle health. Inflammation of the intestine from Cooperia infections, however, added to abomasal damage by Ostertagia or Haemonchus, results in more severe disease than abomasal infections alone.
Avermectin dewormers were never as effective against Cooperia as they were against Ostertagia and Haemonchus. Cooperia has been implicated in most documented cases of parasite resistance to avermectins. Cooperia resistance has not been considered very important, because Cooperia by itself doesn't harm calves much and calves rapidly develop immunity to Cooperia infections.
J Resistance of Haemonchus to dewormers
Tom Craig, DVM, a parasitologist at the Texas A&M College of Veterinary Medicine, has investigated a few severe outbreaks of Haemonchus infection in Texas calves that had received avermectin dewormers. Stocker calves grazing permanent pastures in the summer were affected. Resistance by Haemonchus is of great concern. Haemonchus are vicious bloodsuckers that cause pale mucous membranes due to severe anemia, bottle jaw (swelling between the mandibles), but not much diarrhea.
J Prevention of parasite resistance to dewormers
The following management tip is critical to prevention of resistance to dewormers by parasites -- always use the proper dose of dewormer.
Underdosing exposes worms to less than lethal concentrations of the drug, which kills all non-resistant worms and allows resistant superworms to take over. Underdosing may also result from use of generic ivermectin, much of which is made in China. Although they have an adequate concentration of ivermectin, ineffective carrier chemicals result in poor absorption.
Along with proper dosage, other practices can be used, such as rotating the class of dewormer.
Worms present in the cattle in the spring are the ones that survived the drug used last year, so use a different class of dewormer on them. The two classes commonly used are the avermectins (pour-ons or injectable) and the benzimidazoles (white liquids or pastes given orally).
Or, you can use a product until it fails. This is an excellent approach because it determines if there is parasite resistance in your own cattle. It requires annual fecal egg count tests before and after deworming to ensure that your treatment is effective. Here's how the fecal egg count reduction test (FECRT) works. At spring working, collect 20 fecal samples at random from calves that are over two months of age and have eggs-per-gram tests performed by your veterinarian, the Texas Veterinary Medical Diagnostic Laboratory or by a cooperating pharmaceutical company.
The samples will probably have moderate to high worm egg counts. Collect 20 calf fecal samples off the pasture 14 days later and repeat fecal eggs per gram counts. There is no resistance to your dewormer if the average number of eggs per gram drops by 90 percent or more. Keep using the same class of dewormer as long as there is no resistance.
The only proven cases of resistance of cattle parasites to dewormers in Texas have been severe Haemonchus infections in stocker calves during summer grazing of permanent pastures. Key clinical signs were pale mucous membranes and bottlejaw.
Be meticulous in matching volume of dewormer to body weight of animal treated. Never underdose. Rotate annually when alternating between classes of dewormer or consider using the same dewormer until resistance is found using the FECRT program.
Anyone who wonders if the deworming product they are using is fully effective should contact their veterinarian to help implement a FECRT program.