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The development of worm resistance

What is resistance?

Resistance is a term used when worms within an animal have the capability of surviving treatment with a particular type or family of wormer. When worms develop resistance to one type of wormer they are normally found to be resistant to other wormers of the same type or family. For example if worms became resistant to moxidectin there is a very high likelihood that they would also be resistant to ivermectin and abamectin.



A horse is dewormed


If some worms survive the treatment they may be resistant to the chosen wormer.


Those worms reproduce resulting in pasture contamination with resistant worms.


The horse eats a mixture of resistant and susceptible worms.


With continual treatment with the same family of wormer the numbers of resistant worms will increase.


The wormer fails as resistance levels increase.


Anthelmintic Families

1. Macrocylic Lactones (‘Mectins’): - These are effective against a wide range of roundworms but have no activity against tapeworms. They are also active against the larval stages of bot flies.

  • Abamectin (EQUIMAX®)
  • Moxidectin

2. Benzimidazoles (BZ’sor ‘azoles’): - Single doses are effective against larval and adult roundworms. They are not effective against tapeworms or bots. Resistance to this class of drug has been documented.

  • Oxfendazole/Pyrantel (STRATEGY-T®)
  • Oxibendazole (OXIMINTH®, OXIMINTH® PLUS)
  • Fenbendazole

3. Praziquantel (active against tapeworms only): - It is currently the wormer of choice against tapeworms in horse.


4. Tetrahydraprimadines (THPs): - These chemicals are active against larval and adult roundworms and tapeworms.

  • Pyrantel (STRATEGY-T®)
  • Morantel 

What are the options for the best use of these families of drug?

rotation wormersIt is important to “rotate” wormers by alternating from one compound to another. This should not be done every time you worm, but rather on a seasonal basis. In Australia, seasonal conditions are ideal for parasite growth during most of the year, particularly the warm wet months of spring and autumn, however, the lifecycle of different parasites mean that they may be more active at different times of the year.

When considering a rotational program, you must also ensure that you are actually changing the compounds that you are using. Many of the wormers on the market contain the same or similar active ingredients and switching the brand name or colour of the box may not mean you have actually changed the active you are using. There are two main families of active: the “mectin” or “ML” class those with names ending in “-ectin”, and the “BZ” class of active those ending in “-azole”. There are other classes of active ingredient, but they are less commonly used and are generally used in combination with other actives to target specific parasites.

To ensure the best seasonal worm treatment, it is recommended that a BZ-based wormer like Strategy-T is used in spring and summer, and a mectin-based wormer like Equimax is used in autumn and winter. You may also want to include a special treatment in early spring for encysted cyathostomes if these are a problem for your horse. Either fenbendazole* or moxidectin can be used under veterinary supervision.

What else should be done to ensure effective worming?

1. Use effective wormers.
It is important to use an effective wormer. Using wormers that already have established resistance on your property will not protect your horse from worm related disease.

2. Rotate class or family of wormer on a seasonal basis.
By using a different mixture of drugs from a different family we can slow the emergence of resistance to wormers. Rotating on a per wormer basis allows the worms to “remember” the last wormer and thus resistance to two classes or families of wormer may develop concurrently. Strategy-T paste contains oxfendazole and pyrantel. These are two different drugs, (from different families to abamectin and praziquantel), that work together synergistically killing roundworms and tapeworms. This synergy allows Strategy-Tto work well against benzimidazole resistant worms showing little or no cross-resistance with other members of its family. (A wormer containing both abmectin and any type of BZ is not a rotational choice to Equimax, Equimax LV or any other ‘mectin’ based wormer as it itself is a ‘mectin’ based wormer.)

3. Use the minimum number of treatments in a year.
The more times worms are exposed to a chemical the more likely they are to develop resistance. Make use of faecal egg counts to establish the minimum number of worming treatments required for your property and do not over worm. Your vet will be able to assist you with faecal egg counts.

4. Give the correct dose.
Modern wormers are very safe and it is better to err on the side of slight overdose than to under dose. Under dosing will accelerate selection for resistance. If owners are using a liquid wormer it only takes a small amount of product to be spat out to run serious risk of underdose. The Virbac range of wormers are now treat up to 700kg.

5. Avoid introducing resistant worms to your yard.
Quarantine worm all new horses prior to turn out and keep them confined to a box for 48 hrs after worming to prevent pasture contamination. Ideally have faecal worm egg counts performed after worming and prior to turn out to establish if a problem exists with newly introduced horses.

6. Practice good pasture hygiene and “poo pick” at least twice a week.
Worm eggs and larvae are contained in the horse’s faeces and by removing the faeces you are preventing reinfestation. Stables should be cleaned out at least once daily. Clients should be advised not to spread the manure back onto the pasture.

For more information on rotation and resistance go to 3D Worming.

Worming young horses

Young horses need to be treated differently to older horses because they are at risk of mectin resistant ascarids. Ascarids appear to be the worm developing resistance to the mectins. Generally ascarids are not seen in horses over 2yrs of age as horses seem to develop a natural immunity to these worms as they mature. Horses under two should be wormed every six weeks with Equimax Elevation and then put on the Equimax/Strategy-T rotation program.