Six Problems That Can Be Due To Worms

By John Byrd, DVM
Problem #4:  Ulcers

Bots, Draschia megastoma and Habronema musccae are 3 classes of worms that can be responsible for ulcers in the stomach.  Bot worms are the larval stage in the cycle of a Bot fly.  They look like a small honeybee and they fly around horses’ legs, gluing eggs to the hair on the horses’ front lower legs.  When licked by the horse these eggs hatch and the larvae attach to the horse’s tongue.  Then they burrow into the horses tongue and gums and find their way to the stomach where they attach to the mucosa.  When large numbers of larvae attach, they do it in close proximity to each other and when they all release their attachment at one time a fairly superficial ulcer is revealed.  These ulcers are generally not very severe.
Draschia megastoma (largemouth stomach worm) and Habronema musccae live in the stomach.  Draschia megastoma can cause nodules in stomach that can be like an abscess that can rupture into the stomach or into the abdomen.  Fortunately this is very rare.  Habronema cause some irritation to the lining of the stomach.  Stable and houseflies carry the larval stages of these 2 worms.  When the flies feed on the horse’s skin, the larvae leave the flies and get into the wound and can cause severe skin ulcers that are commonly known as summer sores.   Summer sores most often are seen in horses in the southern states and California.  The ulcers are frequently seen in the corners of the eyes and lips, and in geldings and stallions, they form on the penis and sheath.
Tapeworms sometimes appear to be responsible for ulcers in the area of the intestine where the small intestine connects to the large intestine.  These ulcers seem to be due to tissue damage caused by the attachment of the worm’s mouthparts, especially when there are large numbers present.  These ulcers have been observed during colic surgery and during necropsies.
Most ulcers in the digestive tract of horses are not caused by worms, but by stress or some other disorder that affect the bacterial flora or cause chronic irritation to the gut.