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Horses, Worms and their Environment

As mentioned previously, parasites have been around for millions of years and have the ability to evolve rapidly to ensure that future generations survive. Parasites are specialised organisms that have learned biological tricks to help them evade the hosts (in this case the horses) immune system. An animal’s immune system also adapts to changes in its environment and the types of infections it is exposed to, just as the worms do but with large mammals the process is much slower. In effect, this means that the worms can stay just a little ahead of the dangers posed to it and continue to re-produce. The main aim of all biological creatures is to produce offspring and each worm does this by successfully laying thousands of eggs in its lifetime.

As ‘masters’ of the horse we have altered the natural environment for equines by grazing them in fenced off, limited areas. Horses are often kept in areas that would be too small to sustain the same numbers if they were not having their nutritional requirements met by the added feed stuffs we provide (hay, cereals etc.). Placing large numbers of horses together on a limited amount of grazing makes the re-production and continuance of generations of parasitic worms easier. The horse is the captive audience; he has no choice about whether he moves to another area and so grazes on the same pastures, whereas in the wild he may leave an area of pasture for a while. It is an observation made by most horse owners that horses tend to have a ‘toilet area’, an area they deposit dung and avoid grazing. I doubt this is because equines have the same aversion to their faeces that people do, I believe it is much more likely that it is a combination of factors that cause this behaviour. The area where the dung is deposited is often an area where there is less good quality grass, probably because the dung itself alters the pH of the soil thereby affecting the type of plants that grow. Those plants that do grow on a more dung-laden area are probably less palatable for the horse. I am also inclined to believe that animals tend to instinctively know ‘what is good for them’, at least some of the time, so perhaps the behaviour of depositing dung and grazing other parts of the pasture is a protective mechanism that helps avoid the ingestion of worm eggs.

Before the advent of the paste type worming treatments it is probable that pasture management played a more crucial role in managing worm burdens in horses. The amount of pasture available to each animal may have been greater than it is today and it is likely that many horses probably shared their grazing pastures with other types of animals. The more available good quality pastureland is, for fewer animals, would logically suggest that the horses would ingest fewer worm eggs, from off their grazing land.

Another important factor is the mixture of types of grazing animals. Parasites are host specific*, this means that horse worms are specially adapted to complete their lifecycle in horses, worms that affect other animals are also specific to them. Where there is a mixture of animals grazing, for example sheep and horses, these animals will accidentally, ingest the eggs specific for the other. Horse worm eggs are unable to develop into fully mature, egg producing adult worms if a sheep has swallowed them. The same is true if horses ingest worm eggs specific to the sheep, the eggs are unable to develop into fully mature adult worms. Mixing types of grazing animals is therefore thought to be beneficial to the animals because it helps to interrupt the lifecycle of the worms. *There is an exception to this rule, Trichostrongylus axei, if present in sheep or cattle can cause problems in horses.

Good pasture management is crucial to the management of worm burdens. Veterinarians and scientists worldwide are recommending a combination of good pasture management and a reduced worming strategy, in order to combat the foreseeable problems of drug resistant worms in horses.