Merial-Sanofi_Logo_CMYKQ&A on Cattle Dewormer Resistance

Knowing how resistance develops will help curb its development in the future.

When it comes to deworming cattle, a common topic of discussion is resistance. With a variety of dewormers available on the market today, it is important to know how these products work and how they should be used to delay the onset of resistance, which is the responsibility of every herd owner. Understanding the unique attributes of prescription LONGRANGE® (eprinomectin) will help guide your decision-making process when it comes to deworming and avoiding resistance.

The following are some common questions regarding resistance and LONGRANGE:

Merial veterinarian, Dr. Tony Moravec, shares insights about resistance.

Q: What is anthelmintic resistance?

A: Resistance occurs when a parasite population begins to survive treatment from a dewormer, or anthelmintic, that at one time was effective.1 With each successive treatment, a growing percentage of parasites may survive, passing on their resistant genes to the next generation as they breed and multiply. As treatments continue, the newly resistant parasites begin to outnumber those still vulnerable to the dewormer, rendering the treatments ineffective.2

Q: How does resistance develop?

A: Every time we treat an animal with ANY kind of dewormer, there are two opportunities to select for resistance: head selection and tail selection.3

Q: What is "head selection" for resistance?

A: Head selection occurs at the time a dose of a dewormer is given. If a complete kill of parasites is not achieved, some resident parasites have survived and head selection has occurred.3 The most common cause of this is underdosing based on underestimating the weight of the animal. Another common cause is dosing to the average weight of the pen; half of the pen will be adequately treated, while the other half will be underdosed.

Q: What is "tail selection" for resistance?

A: Tail selection occurs during the time when the drug is exiting the animal's body and as it drops below therapeutic or lethal levels. Small amounts of the drug can linger in the animal's body, especially fat tissue, for a long period of time as seen commonly with moxidectin.4 Immature stages of parasites that would normally die if given a therapeutic dose are exposed to this very low dose of the drug and they survive, while very sensitive or susceptible parasites continue to die, and tail selection has occurred.3 This leaves a population of hearty parasites and makes the next generation of worms tougher to kill, or resistant.

Q: How might conventional dewormers cause resistance?

A: Most conventional pour-ons and injections last for only a few days to a few weeks before pasture reinfection begins.1,5,6 Multiple treatments will be needed with these products to ensure low parasite burdens to sustain improved productivity. Each application is an opportunity for head and tail selection for resistance and can accelerate its development.

Q: How is LONGRANGE a different kind of dewormer?

A: LONGRANGE is the first injectable parasiticide product that offers persistent activity for up to 150 days.1,5 This extended duration of activity is provided by the unique THERAPHASE™ technology which allows a gradual release of eprinomectin within two peaks.1,5 LONGRANGE is the only parasiticide that provides effective plasma concentrations with two peaks.1,5 To get the same level of control with conventional products, you'd need three or more applications, each one timed to start working before the last one wears off.6 When you add up those product costs, labor, handling stress and shrink, one dose of LONGRANGE makes a lot of sense. It's all the body condition, reproduction and weaning weight benefits of a strategic deworming program in a single treatment.

Q: What is THERAPHASETM technology?

A: THERAPHASE technology delivers a rapid, very high initial peak and an extended duration of about 70 days. Then a second pharmacokinetic peak begins and peaks at around 90 to 100 days before tapering off, although still remaining above the minimum effective level for up to 150 days, depending on parasite species.1

LONGRANGE is no more likely to select for resistance than a single treatment with a conventional dewormer.7,8

Q: What can be done to combat resistance?

A: This answer is complex because every situation is different; however, there are some common rules to abide by to combat resistance:

  1. Dose to the individual animal weight and not the pen average. This is critical for the success of all deworming products and will decrease head selection for resistance.
  2. Use a potent dewormer labeled for the parasites you would like to treat that ensures a very high initial peak, which can thwart the head selection for resistance. Also avoid "rotating" dewormers as this can cause head and tail selection to multiple classes of dewormer on the premises.
  3. Avoid multiple treatments within the grazing season. Because LONGRANGE offers up to 150 days of parasite control with just one subcutaneous injection,5 multiple treatment frequencies are not an issue, helping to minimize head and tail selection for resistance.
  4. Use a dewormer that allows a long contact kill time that does not use fat stores to deliver the drug for extended periods of time, which can drive tail resistance. LONGRANGE uses a polymer matrix in THERAPHASE that predictably biodegrades into water and carbon dioxide after it delivers the active ingredient, eprinomectin, which quickly exits the body once it is used.5
  5. Timing is everything. Use a dewormer when parasite pressure is highest to ensure that the genetics of the tough-to-kill parasites are diluted out with larger populations of susceptible parasites, delaying resistance.
  6. Leverage refugia — the population of parasites that are not exposed to the drug. When treating with an extended release product like LONGRANGE, the life cycle of parasites is broken and pasture contamination is reduced. Simply leave 10 percent of your cattle untreated, generally those that are in the best condition. These untreated cattle help dilute any resistant parasite population but may still benefit from the lower pasture contamination created by cattle treated with LONGRANGE.

IMPORTANT SAFETY INFORMATION: Do not treat within 48 days of slaughter. Not for use in female dairy cattle 20 months of age or older, including dry dairy cows, or in veal calves. Post-injection site damage (e.g., granulomas, necrosis) can occur. These reactions have disappeared without treatment.

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®LONGRANGE is a registered trademark, and ™THERAPHASE is a trademark, of Merial. ©2015 Merial, Inc., Duluth, GA. All rights reserved.



1 Dependent upon parasite species, as referenced in FOI summary and LONGRANGE product label.

2 FDA.gov. Antiparasitic Resistance in Cattle and Small Ruminants in the United States: How to Detect It and What to Do About It. Helpful Information for Veterinarians. Available at: http://www.fda.gov/downloads/AnimalVeterinary/ResourcesforYou/UCM347442.pdf. Accessed December 11, 2014.

3 LeJambre LF, Dobson RJ, Lenane IJ, Barnes EH. Selection for anthelmintic resistance by macrocyclic lactones in Haemonchus contortus. Int J Parasitol. 1999;29: 1101-1111.

4 Lanusse C, Lifschitz A, Virkel G, Alvarez L, Sanchez S, Sutra JF, Galtier P, Alvinerie M. Comparative plasma disposition kinetics of ivermectin, moxidectin and doramectin in cattle. J Vet Pharmacol Ther. 1997;20:91-99.

5 LONGRANGE product label.

6 CYDECTIN Injectable, DECTOMAX and SAFE-GUARD product labels.

7 Dobson RJ, Lejambre L, Gill J. Management of anthelmintic resistance: inheritance of resistance and selection with persistent drugs. Int J Parasitol. 1996;26(8/9):993-1000.

8 Toutain PL, Upson DW, Terhune TN, McKenzie ME. Comparative pharmacokinetics of doramectin and ivermectin in cattle. Vet Parasitol. 1997;72:3-8.