Cattle management programs usually include one to two anthelmintic treatments per year, so an effort should be made to avoid development of resistant internal parasites. Anthelmintics are medicines that eject or kill intestinal worms. They are commonly known as dewormers.
There are basically two ways to fight anthelmintic resistance. The first is to rotate treatments among active ingredients when acceptable parasite control is obtained with more than one type of chemical.
All currently marketed anthelmintics control brown and barberpole stomach worms. An exception is the L-4 strain of the brown stomach worm on which efficacy has not been demonstrated with morantel. Regardless of this fact, several options exist for stomach worm control that allows rotation between active ingredients.
Cooperia small intestinal worms infest young cattle up to 11 months of age. They are controlled by fenbenzimidazoles, levamisole, and morantel that allows treatments to be rotated among the three active ingredients. Regardless of the treatment, Cooperia are hard to control partly due to the age of animals being treated.
There are fewer choices for liver fluke control. Albendazole is effective against deer and common liver fluke. The combination of ivermectin and clorsulon controls common liver fluke and has limited effectiveness against the deer fluke. So, there is not much of an opportunity to rotate flukicides.
The second way of combating resistance is to avoid using anthelmintics unless it is absolutely necessary. Checking fecal pats for presence of worm eggs and a count of the number of eggs per gram of feces provides a measure of population fluctuation. Worm counts help determine application timings and are an aid in monitoring treatment effectiveness. If eggs are not found, there is no reason to treat older cattle for stomach worms.
“Liver flukes can be difficult to diagnose, but there are methods and indicators that help determine if liver flukes are present in a herd,” says Christine Navarre, Extension Veterinarian,Louisiana State University Agricultural Center. “One of the best ways to spot a liver fluke problem is to examine the livers of cattle that die or request information on liver conditions of cattle slaughtered from your herd. You can have feces inspected, but flukes are hard to spot. You also can use the process of elimination. If your cattle aren’t gaining or breeding back properly, and you have tried treating for other parasites or conditions, try treating for liver flukes. If animal condition improves, there is a good chance your herd is infected and you should set up a routine treatment program.”
“In the southern United States there is strong evidence that cows and suckling calves, more than two months old, will benefit from treatment with a macrolide such as ivermectins, eprinomectin, doramectin or moxidectin as they enter summer,” says Tom Craig, Veterinary Medicine at Texas A&M University. “Treatment can be done when cows are pregnancy checked, the calves vaccinated, or during other activities. When calves are weaned and moved to another pasture, treatment with a benzimidazole will help them get a better start. Cows, and especially first-calf heifers benefit from deworming as they enter winter in the high rainfall areas of the southern United States.
“Stocker calves, especially on permanent pastures, are at high-risk for all parasites,” Craig says. “Therefore, it is essential to treat when they are put on pasture and then repeat treatment one to three months later, depending on the anthelmintic used. Because these calves are at risk and pastures may be laden with resistant worms, it may be advisable to have your veterinarian check fecal samples two to four weeks after treatment to aid in deciding if the anthelmintic actually worked.
Deworming is an important practice for maintaining herd health and obtaining weight gains. The cattle industry will suffer greatly if internal parasites become resistant to treatment. So let’s do our part in avoiding such a catastrophe.