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Courtesy of Ceva.

Historically it has been recommended to worm adult horses approximately every 6-8 weeks with a broad spectrum wormer.
It is now recommended to worm based on faecal egg counts (FEC) to help reduce resistance development. Using the same product repeatedly encourages resistance to develop.
Rotating between different classes of wormers will help reduce resistance. There are 3 main classes available for use in horses:

  • Macrocuclic Lactones (ML) – ivermectin, abamectin
    and moxidectin
  • Benzimidazoles (BZ) – oxfendazole and fenbendazole
  • Pyrimidines (THP) – pyrantel and morantel

What’s the difference between drugs in the same class?

Fenbendazole and oxfendazole: Older drugs with efficacy against adult cyathostomes and some roundworms. Suitable for rotation.
Ivermectin, abamectin and moxidectin (mectins): Broad-spectrum, effective against all worms except tapeworm, often combined with morantel or praziquantel for tapeworm. Abamectin is effective against
some ivermectin-resistant strains!.
Pyrantel and Morantel: Effective against tapeworms, roundworms and some cyathostomes, often used in combinations. Morantel is a newer variant of pyrantel. Studies indicate morantel has a higher safety
margin than pyrantel. Morantel has been demonstrated to have a zero milk withhold time in cows (compared to pyrantel) so it may be safer to use in mares with foals.

When planning your rotational program, it is important to choose products from a different class. For example, if you are currently using moxidectin and praziquantel, you should consider using something like oxfendazole.

Do long-acting wormers really last as long as they claim?

When the ivermectin products were first launched, the egg reappearance period (ERP) after deworming horses was eight weeks and for moxidectin-based products it was claimed to be around 14 weeks.
During a recent Australian study, horse owners submitted follow-up faecal samples in the weeks after an initial faecal egg count reduction test (FECRT) which suggested that cyathostome egg shedding is starting faster than expected after worming with moxidectin and was below 12 weeks in some areas*.
Horse owners in the USA have also recently reported shortening ERP times following both ivermectin and moxidectin treatments (just 4 weeks in some cases).


  1. AMMO® ALLWORMER is a combination product consisting of the newer, safer molecule morantel and abamectin. The combination of abamectin and morantel with similar efficacy will help reduce resistance development.
  2. AMMO® ALLWORMER comes in a new peppermint flavour one of the top flavours chosen by horses.
  3. Recent studies conducted on properties in the HunterValley, Riverina and Sydney demonstrate that AMMO® Allwormer had greater efficacy against resistant ascarids than Equimax and maintained a consistent ERP of 8 weeks under Australian field conditions.
  4. AMMO Rotational contains oxfendazole, an easy-to-use rotational partner.

For more information visit


Wilkes BJA, et al. Aust Vet (2017) J 95(3):85-88

McConaghy FF, et al. Aust Eq Vet (2015) 34:39

Veterinary Pharmacology and Therapeutics, 7th Edit. (1995) 908

Beasley AM, et al. Veter Parasitol (2017) 8:127-132

Lyons ET, et al. Parasitol Res (2011) 108:355-360

Goodwin D, et al. App Animal Behav Sci (2005) 95:223-232