Authored by Natalie Brown, Doctor of Veterinary Medicine Candidate, Virginia-Maryland Regional College of Veterinary Medicine, Class of 2023
What is Anthelmintics Resistance?
Anthelmintics, or dewormers, are a group of antiparasitic drugs that target intestinal worms. Well-known anthelmintic products include those such as Safeguard, Panacur, Prohibit, Ivomec, and Cydectin, amongst others. For decades, these drugs have been heavily relied upon by key stakeholders in production medicine. Under the illusion that mass anthelmintic use was necessary at the ever-slightest hint of parasitism, practitioners, producers, and pet owners have historically been quick to reach for these drugs. Unsurprisingly and consequently, resistance has emerged at an exponential rate. Anthelmintic resistance is defined as the ability of parasites to survive doses of drugs that should kill them under normal circumstances. Although some degree is anticipated as a natural phenomenon, the misuse, or overuse of these drugs has accelerated the process. Today, anthelmintic resistance continues to develop across the globe. In view of the imminent threat to your current and future herds, a basic understanding of resistance and how to use these drugs responsibly is essential.
How Does Anthelminthic Resistance Develop?
Like many species, parasite populations maintain a high degree of genetic diversity. For some individual worms, unique genetics may grant them resilience that exceeds the rest of the population. When exposed to anthelmintics, these robust worms remain unaffected, outsurviving the other, weaker worms. Gradually over time and with repeated drug use, the weak worms die out while the resilient, or resistant, worms continue to thrive and reproduce. The ultimate result is a population comprised of resistant parasites only. The parasitic burden becomes difficult to manage, as treatment options are limited, allowing the worms to establish residency and flourish.
Anthelmintic resistance is a complex process, and this oversimplified explanation fails to address numerous other factors involved. Parasite biology, grazing practices, regional differences in both climate and parasite prevalence, and animal management influence resistance alongside the genetic component. Key risk factors associated with the development of anthelmintic resistance include frequent treatment, under-dosing, mass administration, and using the same drug class repeatedly over an extended period.
How is Resistance Detected and Monitored?
Anthelmintic resistance may be suspected based on clinical signs of parasitism despite treatment; however, this is insufficient as a standalone diagnostic. To best estimate resistance on your farm, fecal examination tests must be performed. The fecal egg count reduction test (FECRT) is the main method of detection currently used in practice. This test involves collecting multiple stool samples and quantifying the parasitic load. Following this initial sampling, deworming is performed, and samples are re-collected two weeks later. If anthelmintics are effective (i.e., there is no detectable resistance), a reduction in the number of eggs should be noted following deworming. If your burden does not decrease by 95%, you have parasite resistance on your farm. Results should however be interpreted critically, as this method does have limitations related to the species being tested and the biology of each parasite. For a definitive diagnosis of resistance, four conditions must be met: (1) the proper drug dosage was used, (2) the drug was within the noted expiration date, (3) fecal samples were stored correctly, and (4) a standardized FECRT was performed. Other methods to estimate anthelmintic resistance include egg hatch tests, larval development assays, and various other molecular techniques, though these are less commonly utilized.
Why Should I Care About Anthelmintic Resistance?
Much like antibiotics, the lack of efficacious anti-parasitic drugs poses major threats to animal health. It is unlikely that novel anthelmintics will be introduced in the immediate future. When current drugs begin to fall short in terms of effectiveness, there is little to no restoration that can be done at that time.
In a post-anthelmintic era, common and previously benign infections may hold the potential to rapidly decimate or severely impact herds. This is already noted on many farms and operations around the world, where practitioners have run out of options to control once-manageable parasites. Likewise, anthelmintic resistance threatens sustainable livestock production and may hold downstream effects on global food supply in the long term. Additionally, parasites not only impact the health of livestock, but humans and companion animals as well. Multiple taxa may therefore be at risk when there are no longer viable treatment options to reach for.
Presently, resistance can threaten the health of your animals and the economics of your production facility. Managing resistant worms is difficult, expensive, and oftentimes unrewarding. Animals suffering from significant parasitic burdens may appear normal or may exhibit severe clinical signs. Once resistant populations are established on your farm, significant time, effort, and financial investment must go into treatment and control. Even then, animals may not survive depending on the parasite involved.
When is it Okay to Use Anthelmintics?
The negative impact of parasites on livestock productivity and welfare is well-documented, and the burden of these infections is not to be undermined. Parasitism causes massive economic loss to agricultural industries, through both the deaths of animals and the cost of prevention and control measures. On small-scale operations, severe parasitic infections can plague herds. Appropriate and responsible use of anthelmintics is encouraged and there are many cases where use is absolutely warranted. It is, however, not okay to use anthelmintics to compensate for poor management practices.
Balancing the benefits of anthelmintic use with the risk of resistance should be approached on a case-by-case basis. Work with your veterinarian to determine the best protocol for parasite control on your farm.
What Can I Do To Prevent Resistance On My Farm?
Although the emergence of resistance cannot be prevented, there are many elements of management that can be used to slow development. Ensure you use integrated parasite control with any strategies elected.
Use Anthelmintic Drugs Correctly
Store products properly, in regard to temperature, light, and oxygen exposure, and check expiration dates before administration. Always follow label indications or veterinary instructions when administering anthelmintics. This includes using the correct drug for the species being treated, administering it via the proper route, and providing an accurate dose. Nonetheless, the limited number of anthelmintics labeled for minor food animal species often necessitates extra-label use. In this situation, be sure to consult with your veterinarian before administration.
Use Combination Protocols
Combining drugs from different classes may help delay the development of resistance. Do not mix drugs in the same syringe and apply one drug immediately after the other. Wellfarm Veterinary Consultants can work with you to determine the best combination protocol. Ensure that you do not use drugs within the same class, as this can facilitate resistance. Rotating the anthelmintic class used is a common deworming strategy, although the true efficacy of this method is not well documented.
Underestimating the weight of your animal will lead to underdosing. Ensure you have accurate weights and recheck these values before deworming. Likewise, calibrate equipment such as automatic syringes and drench guns frequently to confirm that correct volumes are being administered. For oral medications, place drench guns far enough into the oral cavity to prevent liquid from spilling out.
Choose Efficacious Products
Injectable and oral products are more efficacious than pour-ons. Avoid the use of long-acting products when possible. These drugs select for resistance via slow and gradual decreases in drug concentration. Pre-mixed combination products should be used cautiously as well. When used incorrectly, these products hold the potential to confer multi-drug resistance at once.
Use Alternatives Products When Possible
Some natural substances have anthelmintic properties. Likewise, the use of products such as copper oxide wire particles, diatomaceous earth, and Duddingtonia flagrans may also be used as supplements to your deworming protocols and reduce your dependence on anthelmintic agents.
Refugia refers to a subpopulation of parasites that remains unexposed to anthelmintics. By maintaining refugia, you allow susceptible parasites to exist on your farm and infect your animals. The ultimate goal behind this strategy is a proportion of susceptible worms that outnumbers the resistant. As the population of susceptible worms increases over time, the genes that confer resistance are diluted out of the population.
One refugia-based method is targeted selective treatment. Targeted selective treatment or targeted selective non-treatment involves treating specific animals in the herd or leaving a certain portion of the herd untreated, as opposed to mass deworming. Tools such as FAMACHA scoring, fecal egg counts, and animal weights are often used to guide these decisions, although they are not necessarily predictive measures. In general, deworming should be performed when needed and routine use should be avoided unless indicated. Routine deworming may be warranted for animals entering feedlots, cattle from backgrounding systems, and when purchasing animals with unknown medical history or exposure to multiple sources.
Consider the Parasite
Avoid broad-spectrum products, if narrow-spectrum agents provide the coverage needed. Likewise, consider the biology of the parasite in question. Some parasites may maintain lifecycles where seasonal or environmental considerations are essential to incorporate in your control strategy and in planning refugia-based methods.
Consider Pasture Management
Proper pasture management may reduce the need for excessive anthelmintic use and thus delay the development of resistance. Strategies such as rotation grazing, co-grazing with other appropriate species, manure management, and general barn hygiene may be used in conjunction with other methods.
Prevent the Entry of Resistant Parasites Into Your Herd
Purchase livestock from reputable sources when possible and follow proper quarantine and testing protocols when introducing new animals into your herd.
Resistance is a serious, costly, and complex problem, but manageable with proper action. If you have any questions regarding anthelmintic resistance in your herd, reach out to Wellfarm Veterinary Consultants.
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