Cattle producers and veterinarians have enjoyed the benefits of highly effective internal parasite control products for many years. Strategic use of these products has allowed producers to maintain high levels of production from their animals even in the face of significant parasitism challenges.
However, traditional models of parasite control in cattle may not be sustainable into the future. Frequent, long-term use of anthelmintic (deworming) products can lead to resistant parasite populations, meaning those parasites are no longer killed by the deworming product. Sheep and goat producers are well aware of the challenges caused by the development of resistance in parasite populations.
Anthelmintic resistance in cattle parasites has been slower to develop and receives less attention than the issues in small ruminant production. One reason that resistance in cattle parasites is less recognized is related to the biology of the most important parasites. In small ruminants, the most important parasites are blood-feeding parasites that can cause life-threatening anemia. Resistance is easy to recognize when animals continue to become anemic and even die after being dewormed. The most important parasites of cattle are not blood feeders. The primary effect of these parasites is decreased production, which is usually sub-clinical and difficult to recognize.
Anthelmintic resistance in cattle parasites has been documented in cattle from most of the United States. Resistance to all classes of anthelmintics has also been documented. That doesn’t mean every operation is going to have resistant parasites but the potential is there, particularly when the same products have been used for long periods of time. The types of products used can also influence the development of resistance. As the dose of anthelmintic reaching the parasite becomes reduced or more variable, the likelihood of resistance increases. Pour-on products are convenient and easy to use but have been shown to result in lower and more variable doses of the product reaching the parasites. Some studies have shown that resistance is more common when pour-on products are used.
The only way to recognize the presence of resistance in parasites is to test for it. Testing for resistance presents some challenges but can be done effectively with a little extra effort. Currently, the most effective way to detect resistance is called the Fecal Egg Count Reduction Test (FECRT). A fecal sample is collected prior to an animal being treated with an anthelmintic, and another sample is collected approximately two weeks later. If the parasitic egg count in the post-treatment sample is reduced by less than 90 percent compared to the pre-treatment sample, evidence of resistance exists. Most veterinarians can perform the fecal egg counts or the samples can be sent to the Oklahoma Animal Disease Diagnostic Lab. A general recommendation is to test at least 20 randomly selected animals (or the entire group if it’s fewer than 20 animals).
One important concept in the fight against resistance is the idea of refugia in parasite populations. Refugia means that some parasites are not exposed to anthelmintic treatment and, therefore, are not resistant. Refugia helps keep the resistant genes diluted in the overall parasite population, resulting in reduced, or at least slower, development of resistance across the parasite population. Refugia is commonly maintained in sheep and goats using the FAMACHA© System, in which only animals showing signs of significant anemia are dewormed. Unfortunately, maintaining refugia in cattle parasite populations is less straightforward, and research studies identifying the most effective methods to maintain refugia in U.S. cattle production systems are lacking.
If resistance is detected, a variety of steps can be taken to provide adequate parasite control and slow the development of further resistance. Using combinations of anthelmintic products, altering grazing management strategies, and alternate grazing by another species are all possible tools in the fight against resistance. Producers are encouraged to discuss testing for resistance with their veterinarian and to work with their veterinarian to develop sustainable parasite control strategies that will remain effective well into the future.