Although not as prevalent as strongyles, roundworms or tapeworms, there are other equine parasites that are important to recognize and combat. The lesser known intruders listed on this page cannot be identified via fecal flotation but may equally impact your horse’s health.
Dictyocaulus arnfieldi (Equine lungworm)
Donkeys are the common host of D.arnfieldi and are considered to be the primary source of pasture contamination for horses. The adult female worm produces eggs that already contain first-stage larvae, which hatch before they’re passed out in the donkey’s manure. The larvae become infective in pasture within about five days. When the eggs are ingested they migrate by way of the lymphatic system to arrive in the lungs in another five days. In a patent infection, egg laying begins about 28 days after initial infection in the lungs, and the larvae travel up the trachea via coughing. Once in the throat, they’re swallowed and make their exit via the intestinal tract. It is rare for lungworms to successfully reproduce in horses, however, larval lungworms living in the lumen of the bronchial tree will cause serious lung damage in horses. Interestingly, donkeys harboring the lungworms usually do so without any outward sign of disease. They serve as “quiet contaminators”.
Diagnosis is based on clinical signs, epidemiology, and presence of first-stage larvae in rectal collected feces. First-stage larvae or larvated eggs can be recovered by performing a Baermann on fresh feces. Bronchial lavage can also be used to reveal D.arnfieldi infections in horses without a patent infection.
Clinical signs of D. arnfieldi infections include: chronic bronchitis, chronic cough and eosinophilia. Since a horse with clinical signs is unlikely to have a patent infection a good approach is to check the donkey and treat the horse. At this time there are no reports documenting drug-resistance in D. arnfieldi and it can be treated with ivermectin and moxidectin. Advising clients to avoid donkey-horse cohabitation is a good way to prevent this disease in the horse.
Gasterophilus spp. (Bots)
Female bot flies are often observed on warm, sunny days hovering near horses in an attempt to attach their sticky “glue-like” eggs to various areas of the horse. Bot flies are common virtually everywhere horses are kept, with two major species found in the United States: Gasterophilus nasalis and Gasterophilus intestinalis.
G. nasalis lays its eggs on the hairs under the jaw. The eggs hatch spontaneously five to six days after being deposited, and the larvae crawl downward to the chin until they pass between the lips and enter the mouth. G. intestinalis lay their eggs on the hairs of the forelegs and shoulders of the horse where the larvae are stimulated to emerge by the horse licking or biting the attached eggs.
Although the migration patterns of G. nasalis have not been researched, it is most likely similar to G. intestinalis in which the larvae spend about a month in the oral cavity where they burrow into the mucosa of the tongue or pockets in the spaces between the upper molar teeth. Molting occurs into second-stage larvae and the parasites migrate to the stomach. G. intestinalis larvae attach in clusters to the non-glandular part of the stomach whereas G. nasalis larvae are usually found in a small dilatation in the first few inches of the duodenum.
Bot larvae use their mouth hooks to attach to the wall of the stomach or duodenum for up to 12 months. When late spring arrives the larvae release their grip on the mucosa and pass out with the manure to pupate (mature into adult flies) in the soil. Adult bot flies emerge from the pupae in three to nine weeks depending on the temperature and the cycle repeats itself.
There is very little evidence associating Gasterophilus infections with clinical illness. Most horses can support substantial populations of these parasites without apparent disease. This is not to say that bots don’t have an impact on the horse’s health; their presence can cause disease too subtle for current detection methods. Particularly heavy Gasterophilus infections have been associated with abscesses and gastric ulceration. Some researchers have implicated them in cases of stomach rupture and peritonitis, but others argue that the presence of bots might be coincidental, not causative.
Fall is the best time to treat for bots (about 30 days after the first hard frost). Additionally, those tiny yellow bot eggs glued onto your horse’s legs, chest, shoulders, and maybe under his chin can be removed with a sharp blade or bot block to prevent initial ingestion.
Habronema and Draschia “Stomach Worms”
Habronema muscae, H. microstoma, and Draschia megastoma inhabit the glandular portion of the equine stomach, with a special predilection for the margo plicatus (a sharp demarcation that separates the glandular and non-glandular stomach). H. microstoma and D. megastoma deposit larvae, but H. muscae lay eggs containing larvae. The larvae are ingested by house and stable fly maggots which are also developing in manure. The larval worms develop inside the maggot, becoming infective third-stage larvae at about the time that the adult fly emerges from its pupa. Larvae are deposited on the muzzle, eyes, sheath, or open wound as the flies feed. Larvae that are licked and swallowed by the horse will eventually make their way to the stomach where they will mature.
With the exception of Draschia megastoma which was once associated with the formation of fibrous nodules (but have become relatively rare due to the advent of modern dewormers), Habronema species are of little consequence in the stomach. However, their larvae are responsible for a skin condition known as cutaneous habronemiasis, a.k.a. summer sores. These granulomas develop when the larvae are deposited in minor wounds and moist areas of skin, such as the conjunctiva of the eye. Cutaneous habronemiasis is characterized by very rapid production of granulation tissue that refuses to heal during the fly season, the granulomas are also extremely itchy, and secondary trauma often occurs as the horse attempts everything possible to find relief.
Larvae can be difficult to find, a Baermann technique should be used to isolate the parasite. Fly control is a great way to prevent transmission.