Worms and worming - new thinking

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Should horses be wormed?

The answer for most of us, having been told many times that parasitic worms are bad for horses, causing ill-thrift, dull coats, weight loss and in the worst cases colic, is a resounding YES. But new research is showing that trying to completely eradicate worms may not be the best course of action for maintaining horse health.

The development and promotion of effective chemical wormers for horses has lead to a culture among horseowners of blanket prophylactic use of these chemicals – owners worm indiscriminately without knowing what worm burden their horses are carrying. This type of usage is slowly declining however for many years the strong message was ‘worm every 6 – 8 weeks, and change your wormer each time’, and the industry is taking time to adjust to more subtle methods of parasite control.
The strategy of worming frequently has had a positive effect on arguably the most dangerous species of equine parasite, the large redworm. This type of worm has been virtually eradicated – a good thing, as its more severe effects were to block blood vessels supplying the gut, causing the most serious form of colic.
The strategy has, however, had a less beneficial effect on numbers of the most common of the horse parasites, the small redworm.
In any population of animals, there is variation – think about humans who all look, speak, act and function differently. Parasitic worms are no exception – although distinguishing George from Fred is slightly more difficult that in humans, aspects of how each worm functions are slightly different. The differences are due to the genetic ‘shuffling’ that takes place each time a sperm and an egg join to produce a new baby worm. In a large population of worms, there will be many many different combinations of genes, some of which will confer the ability to survive particular environmental circumstances. This means that in a large group of small redworms, a small number will be able to survive a dose of chemical wormer.
If after a dose of wormer, a lady worm and a gentleman worm survive, they are likely to pass on their ability to survive the wormer to the baby worms they produce. This phenomenon is known as ‘resistance’ – the worms are resistant to the wormer. Resistance is already common to some types of wormer – those containing benzimidazole or related chemicals have the highest associated resistance, and resistance to all active ingredients will eventually happen.

The message to ‘change your wormer each time’ was intended to lessen problems with resistance. The idea was that at each worming, a tiny number of worms that were able to withstand that particular wormer would survive. These worms would then multiply into a whole population that were resistant to that wormer. Changing the active ingredient of the wormer for the next round of worming would then wipe out the worms resistant to the initial wormer. Unfortunately, in a large population of worms that are resistant to, for example, benzimidazole, there will be a tiny number that are also resistant to pyrantel or ivermectin. By rotating wormers frequently, it is possible to select for those worms that are resistant to more than one chemical, and produce a population of superworms that have multiple resistance.


The best way to avoid the development of resistance is to reduce the use of chemical wormers and make use of other worm control techniques.
The most effective of these is removal of dung from pasture. Most parasitic worms of horses, and certainly the redworm, lay egg in the horses gut which are then excreted with faeces. Removing faeces removes worm eggs and so prevents re-infection of horses on the pasture.
Additional control measures include rotation of pasture and mixed grazing with ruminant animals.
Faecal worm egg counts should be used to assess the efficacy of the worm reduction programme that is used. Egg counts give an estimate of the worm burden of the horse. The exception to this is tapeworm. Tapeworm eggs are not reliably detected by the standard worm egg count test. Tapeworm burden can be accurately determined by a blood test, however this must be done by a vet!

It has always been accepted that the best result from a worm egg count was for the horse to have no worms at all. This, however, is also under debate.
There is evidence other animals that a parasite burden can be beneficial to the animal. There is a small amount of evidence of this in horses also – there appears to be a greater tendancy for horses that carry no worms to develop RAO (COPD).

There is also an argument for maintaining a population of non-resistant worms in any horse herd. This is termed ‘refugia’ – a refuge is provided for worms. Horses are maintained at around 500 eggs per gram (many horses will never carry this level of worm burden due to their own innate resistance to worms). This population of worms provides competition to any resistant worms that may be produced – if resistant worms appear, they do not have an empty environment to colonise – they have competition for resources and so do not multiply exponentially. The population can be maintained by using chemical wormers and not collecting faeces, especially just after worming, or by using pasture management and minimising use of chemical wormers.

Visit www.soloequestrian.com to read about Solo Harness, the loading innovation