Horse colic is the leading cause of premature death in horses, which is why preventing colic caused by parasites is so important. Colic causes severe pain that will make the horse roll back and forth in the hay to try to relieve itself.
There are two types of colic that is caused by worms: spasmodic colic and colic that entails surgery. Spasmodic colic has been found to be due to small Strongyles and Tapeworms. In large numbers, Large Redworms have been found to cause blood clots, ulcers and intestinal blockages, resulting in colic that requires surgery.
Many kinds of parasites cause diarrhoea in horses, as do other things. Small Redworm infection frequently is one cause of diarrhoea in horses, especially among young horses in the spring. Veterinarians treating horses with diarrhoea often find it difficult to locate the cause, because it’s impossible to detect the buildup of worm larvae in the intestinal walls. Diarrhoea is dangerous for horses as they can quickly become dehydrated and go into shock. A veterinarian should see a horse with diarrhoea immediately.
Sometimes young horses with large infestations of Small Redworms will not develop diarrhea. Instead they’ll lose weight, have stunted growth and fail to thrive because the parasites are consuming their body's nutrients. To date, no one knows why some horses respond in this way to a larger burden of Small Roundworms.
Another sign of worm infestation is when foals and young horses develop localised body fat around the abdomen while their ribs show through (Potbelly). A rough, dull coat can accompany this condition. Large Roundworms often are the cause.
There are different causes for mouth sores in horses. One cause is bots from the botfly. Female botflies lay their eggs on the horse’s coat and the horse licks up the eggs when it licks its coat. Typically the eggs enter into the stomach and eventually pass out through defecation. However in warm weather and with moisture present, the eggs can hatch on the horse’s coat and migrate to the mouth. There the larvae will cause sores around the teeth and on the tongue.
Internal stomach ulcers are often due to irritation from ingested larvae of botflies. With excessive burden, bots can cause stomach rupture. A horse that goes off its feed because of mouth sores may also have stomach ulcers and should be seen by a veterinarian. Larval cyathostomosis is a disease related to worms in horses that may already be fatal by the time symptoms are observed in a horse. This disease is caused by a sudden eruption in the horse’s gut of thousands of Small Strongyle larvae that have burrowed into the intestinal walls and “cocooned” themselves in mucosal cysts. The damage caused by this disease quickly results in chronic diarrhea, which in turn can lead to dehydration and shock; dramatic weight loss; low-grade colic; and overall weakness. The severity of the illness depends entirely upon the level of parasite burden in the horse. In milder cases, the horse may show only a weight loss for which there is no explanation, or it may simply not perform as expected. In severe cases, death often results.