The most common worm in horses are small strongyles - over 95% of horses will be infected with only small strongyles. As such, small strongyles are the main worm that needs to be managed in horses.
One aspect of the small strongyle lifecycle is that after infective larvae are ingested off the pasture, they can ‘hibernate’ in the lining of the intestines, rather than developing into adults. This process is known as hypobiosis, and the larvae are considered “encysted”. Many people worry about encysted larvae as most dewormers have no impact on them.
However: encysted larvae are only a problem when they leave hypobiosis, and it is usually only a problem is they if do it on a large scale. This is where the adult population of worms that live in the lumen of the intestine play an important role.
There is a theory that the adult worms will regulate the development of larvae. That is, the presence of adult worms within a horse’s intestines will prevent both mass development of ingested larvae into adults and the mass emergence of encysted larvae.
Parasites do not want to kill their hosts, and if anything, parasites benefit if the host is healthy. Worms and horses have co-evolved for millennia, so it should not be surprising that worms have evolved a strategy to maintain a balance between parasite and host. Therefore, the presence of adult worms can be thought of as having a protective effect against the potential harm caused by larvae.
When a horse is dewormed, and the adult population of worms is removed, it leaves an open playing field for larvae to infect, which can lead to health issues. In fact, one study found that horses that had been treated in the previous 6 months were in fact at a HIGHER risk for colic, than those that hadn’t been treated.
Most adult horses, that are in good health and condition, will tolerate a burden of small strongyles without any ill-effects. Often, issues only occur when we interfere – there are often horror stories from people about their horse colicking after deworming. However, it’s not the dewormer’s fault (despite what people will blame), but rather the fact that the worms were removed in the first place, disrupting the natural balance between parasite and host. If your horse is ‘due’ to be wormed, maybe think twice. How high was their EPG following a faecal egg count? Is the horse in good condition otherwise? What is the horses’ age, and will treatment of that horse benefit any others in the paddock? (e.g. super-shedders, broodmares). Because it may be that for the health of the horse, you are better off not treating.
If you’d like to read into this more, Stancampiano et al (2017) wrote a really nice review article (which I’ve based this piece on) – reference is below.
Stancampiano, Laura, Federica Usai, Andrea Marigo, and Riccardo Rinnovati. "Are small strongyles (Cyathostominae) involved in horse colic occurrence?." Veterinary Parasitology 247 (2017): 33-36.
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