These bright red parasites are small strongyles, which have fast become one of the most troublesome parasitic worms in horses. Only two classes of chemicals are effective in killing them, so a swift change in your worming program may be required.
In previous decades, worming primarily targeted large strongyles. When it was first proposed, rotational worming worked, but today, these outdated practices are proving no match against the problematic small strongyle.
Encysted Small Strongyles
Only moxidectin or fenbendazole-based wormers are successful in combating the encysted small strongyle. Encysted means the worm is enclosed within a cyst in the horse’s intestine, which is the normal third phase of this parasite’s lifecycle.
If a standard wormer is used – one that isn’t moxidectin or fenbendazole-based – only the non-encysted small strongyles are killed, leaving the encysted small strongyles in place. If your horse has just a small number present, they will be fine, but a large number can be fatal.
If many non-encysted small strongyles are killed at once, the wormer will, in effect, signal the encysted worms to emerge simultaneously – replacing the ones that have been killed and producing a large amount of toxins in the intestine.
This potentially deadly situation is called larval cyathostominosis, which can lead to colic, weight loss, diarrhoea, oedema and death. However, if a moxidectin or fenbendazole-based wormer is used, all encysted small strongyles are also killed without cyathostominosis occurring.
Treating Small Strongyles
The additional difficulty with small strongyles is that even a horse in apparent good health may have a high worm burden. In fact, any horse that has had colic or been simply ‘off’ after worming may need to be treated for encysted small strongyles.