Livestock owners should develop a plan to treat animals to reduce parasites

As livestock take to spring pastures to graze on bountiful green grass, deadly parasites may be lurking nearby. Producers are facing an increasingly difficult battle against worms in their herds, due to the growing resistance the parasites are acquiring to antiparasitic drugs.

In cattle, calves under 2 years of age are the most susceptible to parasites. In small ruminants (sheep and goats), the problem of anthelminthic resistance impacts animals of all ages and can be difficult to control. “Death is typically the first sign in small ruminants that you have a worm problem,” Dr. Eva Wray, with the University of Arkansas Animal Science Parasitology, said. 

The Problem

Through the years, worm species have started to develop a resistance to the drugs used to wipe them out. Producers typically use anthelmintics (dewormers) to treat animals for parasitic worms. However, anthelmintics have begun to fail, in some cases on a worldwide-scale. 

After animals are treated with anthelmintics many of the parasitic worms die, but the surviving parasites pass resistant genes to their offspring. “So, you are always going to have their progeny that they are putting out on pasture that will have some kind of resistance to that chemical they were exposed to,” Wray stated. 

The Solution

Unfortunately, no plan will completely eradicate worms and eliminate parasite resistance. However, there are strategies producers can implement to minimize the problem. 

A Fecal Egg Count Reduction Test (FECRT) is a test used to determine if an anthelmintic is working. In order to get a fecal egg count, a fecal sample needs to be collected and analyzed. “Ideally, a fecal egg count reduction test (FECRT) would be used to monitor for resistance and determine which anthelmintics will be effective on a particular operation,” Dr. Craig Payne, Extension veterinarian with the University of Missouri, advised. 

Sample collection can be taken as livestock are moved through a chute during vaccinations or ear tagging. They can also be collected in the field. “You don’t have to sample 100 percent of your population,” Wray explained. “Twenty to 30 percent of your animals will give you a good idea of what is going on inside your animals as a whole.”

Ideally, producers collect fecal samples to be tested from specific animals. Experts say 20 to 30 percent of a herd carries 70 to 80 percent of the parasites. “If you can fecal sample regularly, you can find that 20 percent that has your worms and if you cull them, you have taken care of 80 percent of your worm problem,” Wray added. 

Proper dosing of anthelmintics helps combat resistance. Veterinarians advise producers to administer anthelmintics according to label dose and individual animal weight. “Deworming based on an average weight leads to underdosing in a proportion of animals which decreases efficacy and increases the risk of resistance,” Payne explained. 

When using pour-on anthelmintics, apply as a narrow strip extending from the withers to the tailhead. Improper application can reduce absorption and result in underdosing. 

If possible, identify the animal or animals with worms through a fecal egg count. “In the past, blanket treating, treating all the animals in the herd, was standard,” Wray explained. “But, because of anthelmintic resistance, that’s not really the case anymore. We want you to identify who needs a treatment based on a fecal egg count and just treat that one. You need to protect the drugs that still work by limiting exposure.”

Through an FECRT the efficacy of a dewormer can be determined. If a fecal sample is collected prior to treatment and then again 14 days after treatment, producers will know whether the drug is working. 

Changing the brands or types of dewormers fails to impact resistance. “Rotation of anthelmintic classes does not appear to slow the emergence of resistance,” Payne stated.

Other strategies include feeding condensed tannins, found in bale or pellet form, as a natural remedy. Rotational grazing is another helpful approach. Keeping cattle on pasture that is 3-inches or taller will help them avoid ingesting worms while they graze. Parasite larvae are typically located on the bottom 3 inches of plants and grass. 

“Current strategies being discussed to slow the development of resistance include use of refugia, the concept of leaving a proportion of the parasite population unexposed to anthelmintics, and administering multiple effective anthelmintics simultaneously,” Payne explained. This approach should only be implemented under the direction of a veterinarian.


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