In treating internal parasites in cattle, efficacy less than 100% eventually leads to at least some drug resistance. But while even the best treatments are unlikely to achieve 100% efficacy, strategies such as combined treatments, refugia and objective monitoring for efficacy can improve the sustainability of currently available anthelmintics. That message came through in several presentations at the recent American Association of Bovine Practitioners (AABP) conference in St. Louis.
During the beef-cattle sessions, Jenifer Edmonds, a veterinary parasitologist at Johnson Research, LLC in Idaho, outlined parasite resistance trends and practical implementation of fecal egg counts.
Edmonds says “drugs don’t make parasites resistant, lack of 100% efficacy does,” adding that parasitologists generally have used 95% efficacy as a goal for minimizing resistance, but that might not be adequate over time. Also, she says, efficacy of a product can vary widely between locations depending on the types of nematodes present and their past exposure to anthelmintics.
Parasite treatments at lower-than-recommended dosage favors emergence of resistance, Edmonds notes. This frequently occurs when producers use pour-on products at dosage levels intended for external parasites, or when dosage varies due to inconsistencies in delivery or when cattle lick the compound from the backs of herdmates.
Resistance to dewormers first appeared in sheep back in 1964, when Haemonchus contortus, or the “barber's pole worm,” began to resist common treatments. By the early 2000s, cattle parasites from the genera Haemonchus, Cooperia and Ostertagia began showing multi-drug resistance in some herds. In 2018, the FDA’s Center for Veterinary Medicine (CVM) asked drug sponsors to voluntarily modify their labels to include information on resistance.
When an antiparasitic drug is used alone, resistance can develop fairly quickly. Edmonds cites the example of Monepantel, a novel class of anthelmintic for sheep. The product was introduced in 2009, and the first case of resistance was documented in 2011.
Using a combination of two or more products, from different anthelmintic classes, can boost efficacy well above that for either drug alone, significantly delaying the emergence of resistant worms. This is especially true if concurrent treatments are used along with a planned program of refugia, the practice of leaving some animals untreated to maintain a population of worms fully susceptible to treatments.
In a statistical model based on using concurrent treatments with Monepantel and Ivermectin, Edmonds says, resistance was delayed by 10 years with a low level of refugia, and 40 years with a high level of refugia.
A sustainable control strategy also should include a monitoring component to measure efficacy over time. Fecal egg count reduction tests, which involve collecting and testing manure samples from around 20 cattle before and after treatment, provide an incomplete, but useful measure of efficacy trends in a herd. For analysis, she says the Modified Wisconsin Sugar Flotation Method is more sensitive than the McMasters test.
A newer test, called Mini FLOTAC, provides even greater sensitivity, and now is available through Dr. Ray Kaplan’s parasitology laboratory at the University of Georgia. The Mini FLOTAC is recommended for research and fecal egg count reduction tests while other fecal egg counting methods are still appropriate for use by clinical veterinarians for routine fecal testing. For more information and current pricing, please contact the lab at (706) 542-0742 or .
BMPs for Sustainable Parasite Control
For many years, the biggest questions producers faced in controlling internal parasites related to timing. Available anthelmintics, in delivery forms including injections, pour-ons, oral pastes and free-choice minerals offered high levels of efficacy, and depending on the locations and production systems, producers planned deworming schedules and chose products accordingly.
Today, however, with most important nematode parasite species affecting cattle becoming increasingly resistant to at least one class of anthelmintic drug, veterinarians and producers need to think about more than timing to maintain sustainable programs that protect drug efficacy.
During the AABP conference, Christine Navarre, DVM, MS, from Louisiana State University, outlined the evolution of best management practices (BMPs) in parasite control, as the industry struggles to adapt to the realities of drug-resistant parasites. “We don’t have good solutions,” she says, “but we’ll have big problems if we don’t do something.”
Navarre reminded veterinarians that viable parasite-control programs need to minimize economic losses and health problems including:
- Decreased calf weaning weights.
- Sub-standard fertility and reproduction.
- Reduced milk production.
- Compromised immunity.
- Clinical disease from parasitism.
Impacts of parasitism and drug resistance on individual ranches can vary widely depending on location, management, weather, other diseases, nutrition and other variables, making diagnostics and strategic planning important for tailoring programs at the herd level. In areas affected by drought for example, parasite eggs can build up in manure while remaining dormant. When wet conditions return, the larvae all emerge, migrate onto plants and infect cattle.
Producers also should monitor the effects of their management systems. Intensive rotational grazing programs for example, can increase the level of parasite infection in the short term because cattle graze closer to the ground. Over time though, the long rest periods for individual paddocks might reduce parasite populations as the worm larvae die off without the presence of cattle to serve as hosts.
Parasite infection and tolerance also can vary significantly between individual animals on the same pastures, suggesting genetic selection could provide a non-drug tool for minimizing parasite-related losses. That selection pressure has not occurred in most herds though. “We’ve selected cattle under the influence of effective drugs,” Navarre says.
Navarre favors use of refugia strategies along with fecal egg counts and fecal egg count reduction (FECR) tests for sustainable and effective parasite-control programs. Refugia refers to the practice of leaving some animals untreated to maintain a population of worms fully susceptible to treatments, as a means of slowing emergence of drug-resistant worm populations. Fecal egg count tests can help determine when to treat and which animals would benefit most, while FECR tests help assess and monitor efficacy of treatment programs.
In a refugia program, producers might, for example, treat all calves, replacement heifers and bulls while leaving mature cows untreated. Stocker operators could base treatment decisions on fecal egg counts, or use weight as a decision tool, leaving the heaviest 10% of a group untreated.
Navarre recommends veterinarians and producers work together to incorporate a variety of practices to limit parasite pressure while slowing development of drug resistance.
- Use non-drug management practices such as good nutrition, stress reduction, colostrum management and vaccines to enhance immunity to other diseases.
- Don’t buy resistant worms. Deworm new arrivals with more than one class of anthelmintic and hold imported cattle in a drylot for at least two to four days.
- Use refugia in pastures, animals or both. Don’t deworm all animals before turnout onto clean pastures.
- If using generic drugs, select those whose sponsors have published data to back up their efficacy.
- Use pour-on products sparingly, particularly for control of external parasites.
- Use the full label dose for maximum efficacy.
- Consider using concurrent treatments with at least two different classes of anthelmintics along with refugia.
- Monitor fecal egg counts.
- Monitor performance measures such as weight gains as indicators of overall health.
Looking ahead, Navarre says the industry could develop better diagnostic methods to guide treatment decisions, biological or other non-drug controls and even EPDs for selecting parasite resistance or tolerance. But for now, the multi-faceted issue of parasite control, particularly in the face of drug-resistant worms, requires a multi-pronged approach.
Edmonds adds that producers should work with their veterinarians to develop those multi-pronged control strategies, while monitoring to ensure at least 95% efficacy, to ensure long-term success in preventing performance and reproduction losses associated with internal parasites.
For more on parasite-control strategies, including concurrent treatments and refugia, see these articles from BovineVetOnline: