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Advising horse owners on worming...

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01 July 2011, at 1:00am

BEN GASKELL Pfizer equine veterinary adviser, provides some pointers on helping clients make informed decisions about best worming practice for their horses

WORMING protocols are continuing to evolve as scientific knowledge, understanding of the lifecycles of parasites and the threat of wormer resistance increases. 

Horse owners are getting to grips with the benefits of faecal worm egg counts (FWECs) and are also becoming more aware of the need to select appropriate wormers by looking at the chemical ingredients on the pack rather than simply selecting by brand name, but it can still be a potentially confusing process. 

Your clients rely on you to be up to speed with what’s what when it comes to making an informed choice on the best anthelmintic for their requirements.

Hard to resist 

Resistance to wormers is now widely regarded as a serious issue in the field. The use of FWECs has been established as a practical means of addressing the unnecessary dosing which can drive the development of resistant worms, but it’s important to recognise that FWECs have limitations and must not be regarded as a complete solution for worm control. 

An FWEC simply enables a targeted approach to be taken to tackle a specific worm burden, rather than the old-fashioned method of routine worming, which was designed for the worst-case scenario and led to over-worming in many cases.

Macrocyclic lactones 

Given the basis for resistance development, logic suggests that an ideal anthelmintic would be a potent molecule for which there is no existing resistance and which can be given as infrequently as possible. 

Dosing at less frequent intervals appears to be an important factor in delaying the onset of resistance as fewer generations of parasite are exposed to the drug and there is less chance of resistant traits being passed on to their offspring. 

The macrocyclic lactones (MLs) group of wormers, such as ivermectin and moxidectin, are widely used for the control of roundworm, encysted small redworm and bots, but many owners are unaware that there are fundamental differences between the products available within this group. Moxidectin is a second generation ML from the milbemycin group while other MLs belong to the avermectin group. 

Moxidectin has a different chemical structure to the other MLs giving it some unique properties. It is 100 times more lipophillic than other MLs and after absorption it is slowly released from body fat – one explanation for the unique persistency of moxidectin.

Potency and persistency 

Moxidectin kills the parasite by inducing an irreversible flaccid paralysis and eventual death. It is the only product currently licensed to treat encysted small redworm (cyathostomin) in a single dose and has been shown to be more potent against ML-susceptible and resistant strains of cyathostomin than other MLs. It is also effective against benzimidazole-resistant worms – an important consideration when treating small redworm. 

Moxidectin has the longest return to eggs present in FWECs of all the anthelmintics licensed for horses. A licensed 13-week dosing interval allows moxidectin to be used less frequently than other MLs – effectively reducing the selection pressure as fewer generations of cyathostomin are exposed to moxidectin. 

Being familiar with the product differences within the macrocyclic lactone group should enable you to help your clients make informed, objective decisions about best worming practice for the best health of their horses, whatever their preferences or circumstances. 


  1. Sangster (1999) Veterinary Parasitology 85: 189-204. 
  2. Robinson et al (2008) Use of larval migration inhibition assay to investigate suspected macrocyclic lactone-resistant cyathostomin populations. In Proceedings: Equine Parasite Drug Resistance Workshop, 31st July and 1st August, Copenhagen Denmark; p24.