Worm Control In Horses

Our approach to worming horses has changed considerably over the years and this is driven largely by new research and the emerging problem of resistance to our commonly used wormers. There is a lot of conflicting advice about the best strategy for reducing resistance and traditional advice has centered on the theory of rotating wormers on an annual basis rather than using a different wormer each time. This is based on evidence (derived from sheep and cattle research) that using a prolonged rotation of drug classes ie on an annual basis delays the onset of resistance in comparison with shorter rotations ie per treatment. There are two major problems with this. One is that with horses, we are very limited in terms of what wormers are available. In reality, we only have three distinct classes of wormers at our disposal.  Many wormers whilst appearing to be different in terms of packaging and labeling have either the same or similar active ingredients and therefore there is not a true rotation. In addition to this, there is already good evidence to suggest resistance of certain types of worms to a particular class of drugs for example it is well known that the Small Strongyles (Cyathostomes) have widespread resistance to Benzimidazole (eg Panacur) drenches and worryingly there is now significant evidence to suggest that Ascarids (a type of roundworm in young horses) are developing resistance to the Mectin based drenches. This means that using an annual rotation of these classes of drugs could result in serious problems with the resistant worms.

There is now a general acceptance that Targeted Strategic worming is the way to go and this basically means targeting specific types of worms at specific times of the year and this largely amounts to rotational use of wormers on a seasonal rather than annual basis. New research has also highlighted that we may be over-worming our horses in some instances. It is now well known that not all horses are equally susceptible to worms. This seems obvious in terms of young and very old horses being more susceptible or those with suppressed immune systems as a result of underlying diseases. However, it has been shown that in a given population of healthy adult horses 80% of the worms are harboured by only 20% of the horses. This means that many horses, when using appropriate monitoring and environmental control (see below) can safely receive less worm drenches per year.

We can reduce our reliance on wormers in a number of ways and by using wormers strategically, we are maximising their efficacy and slowing the development of resistance. No single worming protocol will suit every situation but the following is a guide on which worms you should be targeting at what times of the year:

Other ways we can reduce our reliance on wormers is by managing the pasture in a way that minimises re-infestation and maximising the efficacy of each wormer. This can be done in a number of ways:

1. Reducing stocking density or number of horses per paddock
2. Not mixing age groups ie very young horses with older horses
3. Poo picking the pasture at least twice weekly
4. Spelling and harrowing paddocks during hot dry weather conditions
5. Alternate grazing of paddocks with cattle or sheep
6. Quarantine new arrivals by worming with a highly effective broad spectrum wormer (Ie Equest plus Tape) and keeping them separate for 48 hours and disposing of the manure.
7. Accurately estimating the weight of each horse ie using a weight tape
8. Minimising wastage of oral pastes where possible.

A new approach to worming Adult horses where optimum environmental management is possible involving regular testing of faecal samples to monitor worm burdens and reduce the number of wormers given per year. As mentioned previously, 80% of worms are carried in only 20% of horses and by identifying the offenders we can tailor a worming schedule to the individual horse. In reality a large number of horses may only require worming twice a year but this decision needs to be based on close monitoring and testing of faecal samples.

This is an example of a seasonal rotational worming program

Summer	Autumn	Winter	Spring
Pyrantel or Morantel	Oxfendazole plus  Pyrantel	Moxidectin or 5 Day Fenbendazole	Ivermectin or Abamectin plus Praziquantel
Equiban	Strategy T, Farnam Wormer, Equinox	Equest, Panacur 5d	Equimax, Equimec plus tape, Eqvalan

Some seasonal variation in the worm life cycles will exist year to year and this needs to be taken in to account when designing your own worming strategy.