Many horses that suffer from diarrhoea, particularly young horses in the spring time, do so because of small redworm infection
It is the larval stages of the small redworm (cyathostomins) that cause problems. During the grazing season horses can accumulate tens of thousands of larvae within their gut walls. These larvae go into a state of arrested development or “hibernation.” At a later time, often in the spring, the larvae will re-activate or wake up from hibernation and erupt out of the gut wall.
If tens of thousands of larvae all do this at the same time it causes massive damage to the gut wall leading to diarrhoea, weight loss and sometimes death. This is a particularly difficult condition for veterinary surgeons because:
It is difficult to diagnose
The drugs available are not very effective at treating it
It is difficult to detect the build up of larvae in the gut wall
A significant proportion of horses with this condition will die in spite of intensive veterinary treatment
Some horses with heavy small redworm burdens will suffer from weight loss, poor condition and failure to grow without developing diarrhoea.
These problems can arise without the larvae erupting from the gut wall. The exact pathological mechanism by which this occurs is poorly understood at present.