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Test Your Knowledge of Management Practices That Affect Dewormer Resistance

Thanks to Michelle Arnold for this excellent article!

Barberpole worm (Haemonchus contortus) from

There are many important diseases in sheep and goats but none are as common or threaten the herd health as much as internal parasites (worms). Although there are many types of worms that may contribute to a parasite problem, the blood-sucking barber pole worm (Haemonchus contortus) is the most prevalent and most important, especially in the southern US. This parasite causes the highest death loss in young animals during their first year on pasture, especially around weaning time.

Traditionally, parasites have been controlled by frequent, regular administration of chemical dewormers. However, with the emergence of multiple drug resistant parasites, new approaches are required. Resistance is a genetic change in a population of worms that allows some individual worms to survive and multiply despite administering the proper dose of dewormer at the right time.

It is now essential to view all dewormers as extremely valuable and limited resources that must be used intelligently with prevention of resistance as a goal. It is a well-known fact that 20% of animals harbor 80% of the worms in a typical herd. After these 20% are identified for treatment (usually by FAMACHA©), then a dewormer must be selected and administered with the goal of maximizing the kill of adult worms.

Take the following 10 question True/False quiz to test your knowledge of how common worming practices may help or hurt the resistance problem.

1. Sheep and goat producers often underestimate the weight of their animals and do not give enough dewormer so many more worms survive treatment, increasing drug resistance.

Animals should be weighed individually or dosed according to the heaviest animal in the group and dosing equipment should be checked frequently for accuracy. The only exception to the rule of dosing according to the heaviest animal is when using levamisole, which can be toxic at high levels.

2. Goats metabolize drugs much more rapidly than other animals and require a higher dose than sheep or cattle to achieve effectiveness and slow development of resistance.


Lamb with parasites

It is generally recommended that goats be given a dose 2X higher than the sheep or cattle dose on the label for all dewormers except levamisole; levamisole should be administered at 1.5X the label dose and individual animals should be weighed due to the risk of toxicity. By law, it is necessary to consult a veterinarian before using any drug in a manner not specifically listed on the label.All  are most effective when administered by mouth to small ruminants. Pour-on formulations are poorly absorbed and should not be used.

3. When giving a dewormer by mouth (drenching), it is important to deliver it over the back of the tongue so the full dose is swallowed and lodges in the rumen.

If the drench is not given over the back of the tongue, the esophageal groove may close and most of the dewormer will bypass the rumen. This will shorten the duration of action and effectiveness will be reduced.

4. Witholding feed for 24 hours before worming increases the rate of digesta transit and decreases drug availability and effectiveness.

Witholding feed actually slows the passage of digesta, allowing more contact time between the dewormer and the parasites which increases effectiveness. This is especially true when using the benzamidazoles (Safeguard®, Valbazen®) and ivermectin. However, bear in mind feed should never be withheld from a ewe or doe in the final weeks of pregnancy due to the risk of pregnancy toxemia.

5. Giving two doses of the same dewormer, 12 to 24 hours apart, is dangerous and costly.

Repeat dosing is a safe and effective way to increase contact time between parasites and the dewormer. If using levamisole, repeat the dose at 24 hours (rather than 12) for safety. Repeat dosing can be used in addition to withholding feed, especially with the benzamidazoles, to enhance the contact time between parasites and the drug.

6. Rotating or changing dewormers to a different class with each treatment causes resistance to develop simultaneously to more than one drug.

It is recommended that one drug be used until it is no longer effective and then the drug should be switched. If a rotation is used, resistance develops slowly but surely to all of the products used but the producer is often unaware until multiple drug resistance is severe.

7. Using 2 or more different dewormers at the same time offers a significant advantage in delaying resistance because it reduces the number of worms that survive treatment.

In order for a parasite to survive after administration of two dewormers from two different classes, it has to have resistant genes to both drugs (which rarely occurs). In Australia and New Zealand, the use of combination dewormers is routinely practiced and has been for many years.

8. A good practice is to worm all of the animals in the herd then move them immediately to a new, clean, safe pasture.

It is very important not to move treated animals immediately to clean pasture unless a portion of the herd was not treated with a chemical dewormer. Although this does not seem to make sense, if only the resistant (bad) worms survive treatment, they are also the only ones to multiply and contaminate the pastures. The last thing any producer wants is a pasture contaminated with only the larvae of resistant parasites.

9. Dewormer resistant worms can only come from 2 sources; either home-grown from poor worming practices or they are purchased when new animals enter the herd.

In a closed flock or herd, resistance within a worm population is typically a slow, gradual process requiring many years for the genetic changes to take place. Alternatively, resistant worms can be purchased, and bypass the many years of genetic change necessary to reach high levels. Depending on how many animals with resistant worms are purchased, treatment failures can occur practically instantly in a herd that never had previous problems.

10. Once dewormer resistance is diagnosed as a clinical problem, reversion to susceptibility will likely never occur.


Haemonchus contortos eggs.

Unfortunately, unless the effectiveness of a drug is monitored over time with a fecal egg count reduction test (FECRT), resistance will not be noticed until there is failure, resulting in sick or dead animals despite correct use of that dewormer. Once resistance to a certain class of drug is noticed, it is a permanent change in the genetic makeup of the parasite.

The development of resistance to dewormers will continue because of the parasites’ short life cycle, high reproductive rate, rapid rate of evolution and extremely large population size that give parasitic worms an exceptionally high chance to change their genetic makeup quickly. Specific strategies can be utilized to maximize the effectiveness of treatments.

Next week we’ll cover “FAMACHA scoring” as a way of figuring out which animals to treat as part of a program to prevent dewormer resistance.

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