There are two main reasons why worms can be found in horse manure. The first is that flies have laid their eggs in fresh manure, which soon hatch into maggots, or little worms. The second is that the horse has been infested with worms that are feeding in its digestive system. Dead worms or worm eggs can often be found in horse manure.
Horses host many internal worms, including bloodworms (also called large strongyles), small strongyles, stomach bots, ascarids (or roundworms), threadworms, pinworms and tapeworms, according to the "Horse Owner's Veterinary Handbook." Horse manure is an effective way to transport eggs of worms to other horse bodies. The manure feeds the newly hatched larvae. They then crawl over the grass where horses feed. Horses eat them along with the grass.
Worms can potentially kill horses. Any adult worms (alive or dead) found in horse manure is a sure sign that the horse is infested or will be shortly. Finding adult worms in horses can be difficult (not to mention unpleasant), according to North Dakota State University. Usually, eggs are present far more often than an adult worm. This is why stool samples should be routinely taken to a vet to check for eggs.
A horse infested with internal parasites and then given de-worming medications will excrete a lot of dead worms with its stool. This may be alarming to the horse's owner but is perfectly normal. Horses cannot completely digest the bodies of dead parasites in their intestines, so the bodies just pass right on out; however, the bodies may be decomposed because of the long length of the horse's gut, according to North Dakota State University.
Horses may look healthy yet many carry many intestinal worms and still need to be on a de-worming schedule. By the time the horse begins to show signs of worm infestation (such as weight loss, hair loss and diarrhea), the horse is heavily infested. The "Horse Owner's Veterinary Handbook" estimates that almost 100 percent of all adult horses are at least infected with large strongyles.
Foals can pick up threadworms through the manure and milk of their mothers, according to the "Horse Owner's Veterinary Handbook." Foals explore the world with their mouths and so are prone to eating any horse manure they find. This causes a watery diarrhea that may include threadworm larvae, according to the pharmaceutical company Merial. Foals grow a natural immunity to threadworms by the time they are 16 weeks old. Until then, all manure should be removed from stalls or pastures where foals live or they will get threadworms.