While there is the potential for a horse to be infected by more than 150 different species of internal parasites, there are four main types: bloodworms, small strongyles, roundworms and tapeworms.
Large Strongyles (Bloodworms)
Bloodworms, such as the large strongyles (Strongylus vulgaris), wreak damage as they migrate through the interior of the equine bowel and attach to the lining of the cecum or colon. They cause hemorrhage, irritation, ulcers of the intestinal lining, and/or anemia.
Invasion of the cranial mesenteric artery (the primary blood supply to the intestinal tract) by Strongylus vulgaris larvae is termed verminous arteritis. Blood flow can be reduced by 50% from inflammation at the root of the artery and thrombi can form, potentially dislodging to block smaller blood vessels supplying the intestines. Even mild interference with intestinal blood flow leads to abnormal motility problems and the potential for diarrhea, weight loss, and/or colic.
Two other forms of large strongyles exist: Strongylus edentatus migrates through the lining of the intestinal tract (peritoneum) while Strongylus equinus journeys through the liver and pancreas. Inflammation in these organs can lead to unthrifty condition or colic.
Small strongyle (cyathostomins) larvae encyst within the large intestinal lining for a period of arrested development, sometimes as long as two years. Once mature, the larvae emerge from their cystic sacs, creating physical damage to the bowel along with release of debris and byproducts that elicit focal inflammation of the bowel. Synchronous emergence of encysted larvae (thousands at a time) can seriously compromise a horse’s health from diarrhea, weight loss, poor growth, low blood protein, anemia, and/or colic.
Parasite-related problems from small strongyles are more prevalent in horses stressed by poor management, inadequate nutrition, illness, crowding, transport, training and competition or those with poor immune systems or Cushing’s disease. Subclinical issues can occur in more robust individuals, such as decreased feed efficiency and/or diminished performance quality.
Ascarids are roundworms that particularly affect young horses less than 18 months of age until the youngster can mount an effective, native immune response. A heavy infestation causes a distinctive unthrifty appearance—pot-bellied and a rough hair coat—along with weight loss, poor growth, lack of energy and colic. Ascarids can also trigger respiratory problems—fever, cough, nasal discharge, and possible pneumonia —as they migrate through the lungs on their journey to the intestinal tract.
The most concerning ascarid problem is small intestinal blockage by a tangled mass of 10-inch long adult worms, particularly following deworming treatment in a horse that has not been on a regular parasite control program.
Tapeworm infections require an intermediate host, the oribatid mite, to perpetuate their life cycle. A horse on pasture or eating contaminated hay might ingest mites infected with tapeworms. This initiates development into a mature tapeworm in 6-10 weeks. Its sucking mouthparts attach to the intestinal lining, creating localized Inflammation near the junction of the small intestine (ileum) and cecum. Potential ulceration, scar tissue and thickening of the intestinal wall can occur.
Tapeworm-infected horses are eight times more likely to develop spasmodic (gas) colic than uninfected horses. Other serious repercussions of tapeworm infection include ileal impaction, small intestinal rupture, or an intussusception (the bowel telescopes within itself). These conditions occur due to motility derangements created by inflammation and disruptions to nerve conduction in the bowel.
Botflies lay their eggs on the hairs of a horse’s legs, chest, jaw and face. Once bot eggs collect on a horse’s muzzle through scratching or direct fly deposits, they mature into larvae that crawl toward the mouth. Development occurs in stages within the horse’s tongue or between the teeth in the gums and interdental spaces. When swallowed, bot larvae colonize the lining of the stomach, residing there for nearly a year. Upon release, they quickly transit through the bowel, pass through the manure, and burrow into the ground to pupate and hatch into flies in late spring.
Damage caused by bot larvae is mostly restricted to erosions in the mouth and stomach. Most horses are relatively unaffected by their presence.
Other Worm Types
Other internal parasites that can create problems for the horse include:
- Pinworms elicit tail rubbing.
- Threadworms cause diarrhea in foals.
- Lungworms, obtained through contact with infected donkeys, cause respiratory problems.
- Migration of Oncocherca cervicalis filaria beneath the skin causes intense itching.
- Stomach worms (Habronema and Draschia) hatch into flies that cause summer sores.