National Cattlemen June 2024 | Page 9

By Dan Cummings , DVM , DABVP Beef Cattle Practice Heritage Vet Partners

Since the introduction of anthelmintics ( dewormers ) in the 1950s , the U . S . cattle industry has been able to remain a step ahead of costly internal parasites ( worms ). In fact , through the 1980s , producers and veterinarians experienced the introduction of a new anthelmintic class each decade . These timely innovations were heavily relied upon to protect livestock against the negative impacts caused by parasite infections . More recently , data suggests anthelmintics remain the cornerstone for controlling parasites . The 2017 National Animal Health Monitoring System Beef survey suggests approximately 90 % of all types of cattle are dewormed at least occasionally . This provides some evidence that the cattle industry continues to invest in dewormers as a mainstay to mitigate production losses in beef cattle operations , and rightfully so . Investing in dewormers has consistently provided strong returns to the producer , reinforcing the practice of deworming cattle . However , there is growing concern in the scientific community that current deworming practices , which have paid dividends in the past , may need further evaluation . Regardless of opinion , my colleagues and I support the position to preserve dependable animal health technologies such as dewormers . These products provide exceptional value to our industry and improve the health and well-being of our cattle ; therefore , it is our collective responsibility to preserve their utility .

Clinical research and shared experience provide evidence to support the decision to deworm growing calves raised on pasture when transmission of internal parasites may occur . Unfortunately , the practice of deworming may lead to unintended consequences as a result of placing selection pressure on the parasites ( Figure 1 ). The specific unintended consequence is termed “ antiparasitic resistance .” Antiparasitic resistance occurs when a parasite can survive proper anthelmintic treatment with a drug which was once effective against that species . Proper treatment implies the drug is administered according to the manufacturer ’ s label ( i . e . correct dose , route of administration , etc .) based on the individual animal ’ s body weight at the time of treatment . In an effort to mitigate antiparasitic resistance and preserve the value of dewormers , researchers and veterinarians are exploring two tactics , combination treatment and refugia-based parasite control .

Combination treatment is the practice of administering two or more dewormers from different drug classes concurrently . Three classes of dewormers are approved for use in cattle : benzimidazoles , imidazothiazoles , and avermectin / milbemycins . Based on what we know currently , a population of parasites may develop resistance to a single active ingredient in one drug class over time when exposed to the drug , but this does not confer resistance to a second or third drug class . Administration of two or more dewormers from separate drug classes achieves higher treatment efficacy due to an additive effect of the drugs ( Table 1 ). Furthermore , this treatment regimen offers a return to broad spectrum efficacy by targeting additional stages of parasites and leaves fewer resistant survivors .

The establishment of refugia may be a new concept for cattle producers and veterinarians , but it has become a viable

Figure 1 . Selection of resistant parasites observed with drug exposure over time .

approach for caretakers raising small ruminants and other domestic species . Refugia is defined as the portion of parasites not exposed to selection pressure from a dewormer . In essence , by implementing refugia , a portion of the parasite population remains in “ refuge ” from the dewormer . The refugia population is then able to pass susceptible genes to the next generation of parasites . There are a few different methods available for establishing refugia-based parasite control programs . An example is selectively non-treating a group or portion of the herd based on level of infection and / or productivity . Careful consideration must be given to the animals , environment , parasite species , climate and much more before selecting the appropriate method to establish refugia . Producers are encouraged to contact their veterinarian who is well-positioned to offer insights on the preferred approach for a specific operation .

Combination treatment and refugia-based programs are only two of the tools available to mitigate antiparasitic resistance and preserve the efficacy of our dewormers . Accounting for other variables in the system , such as extent of parasite burden , forage management , nutrition , genetics , personnel , etc ., is critical when desiring to better understand your parasite control program . In the end , as stakeholders in this dynamic and complex industry , it is incumbent upon all of us to preserve the efficacy of available animal health technologies . Whether these technologies are inherited from past discoveries or yet to be developed from future research , we must do our part to ensure they remain viable for future farmers and ranchers .

85 85 97.75

90 90 99 99 99 99.99

Table 1 . Example of higher treatment efficacy (%) achieved when using different anthelmintic classes in combination . www . NCBA . org NATIONAL CATTLEMEN 9