Internal parasites, which include more than 150 different species are silent killers to the horse's digestive system. Some of the most common are large strongyles (bloodworms or redworms), small strongyles, tapeworms, roundworms (ascarids), lungworms, pinworms, threadworms, and bots.

The strong association between equine parasitism and risk of colic has been widely known for several types of worms that commonly infect horses.