Without an effective worming regime, horses are more likely to suffer from colic, weight loss and diarrhoea.
Haemorrhage and damage to the blood supply of the intestines can also occur, and some of these conditions can be fatal. Regular appropriate worming is of paramount importance in helping horses to stay healthy. Other measures to reduce horses’ access to worms include regularly collecting faeces from the pasture; feeding hay in the field from racks, rather than the ground to avoid contamination; and not overcrowding pastures.
Regular resting and reseeding of meadows can also be helpful, and rotating fields between different species can cut down on infective levels of larvae on the pasture.
There are many different ways to worm horses effectively and the plethora of products on the market and advice available can make planning a worming regime extremely confusing. Some owners have regular faecal tests done and only worm when evidence of worm eggs or larvae is found in their horses’ dung; however, worms can be present and causing gut damage even when they are not passing detectable eggs in the horse’s faeces.
Others use a worming plan that involves worming all the horses on a premises against the same types of worm with the same product at the time of year when that type of worm is most susceptible. This system can work well, as long as the worms on a particular premises are not resistant to the wormer that is being used; occasional pre – and post -worming faecal worm egg counts can be employed in order to check the efficacy of wormers.
Finally, daily in-feed deworming is available in the USA and can be used with some success. Whichever system is in use, any new horse on to a premises should be isolated, wormed with a broad spectrum wormer, and kept in for 24-48 hours to avoid seeding the pasture with large numbers of worm eggs. Thereafter all horses on the same premises should be wormed together with the same product for the best results (though this can be difficult in multi-owner livery yards).
For horses kept in the UK worming in December/January with a wormer containing a macrocyclic lactone (such as moxidectin) to kill bots and also encysted larval redworms (a five-day course of Panacur Guard can also be used to treat the latter). In March and September a double dose of a tetrahydropyrimidine wormer (for example, Strongid P) should be used to kill tapeworms.
Then from May to September wormers should be used at the intervals recommended by the manufacturers for the treatment of roundworms/redworms picked up from the pasture.
To reduce the potential for the worms on a premises developing resistance to a wormer type, the wormers used can be rotated on a three-year cycle between a benzimidazole, a tetrahydropyrimidine and a macrocyclic lactone; however, since resistance to both of the former groups of wormers can occur, some yards use only the latter group.
In the southern USA, the different weather patterns mean that during the summer, worm eggs and larvae on the pastures are usually killed by the heat, whilst the pastures remain infective for roundworms and redworms well into the winter (when the cold in the north results in the worms hibernating). This means that the above programme needs to be adapted to ensure regular deworming through autumn and Winter in the southern states, but that worming in the summer months is less important in this particular region.