HABRONEMIASIS and the Horse Eye

from Elite Equestrian magazine July August 2023 summer issue

For the equine enthusiasts,

The same parasitic disease that causes summer sores, Habronemiasis, can also affect the horse eye. Although it may be described as “fly eggs” causing ocular irritation, this is a misnomer since it is a larval stage of the parasite that migrates into eyelid skin and conjunctival tissue. The presence and death of the larvae cause an immune response, which can lead to inflammation of the eyelids, third eyelid, bulbar and palpebral conjunctiva, and the nasolacrimal system. Secondary effects on the cornea can be even more serious.

Understanding the parasite, its life cycle and treatment is important in managing the infection. Habronema spp. (Habronema muscae, Habronema microstoma and Draschia megastoma) are stomach worms that adhere to the stomach wall of a horse and release eggs that pass in the feces. The normal life cycle is completed when larvae that develop from the eggs are ingested by horses and develop into adult stomach worms. Alternatively, the larvae may be ingested by an intermediate host – stable or house ﬂy - that transports and deposits larvae onto the face. The larvae begin an aberrant migration into the tissues around the eyes and eventually die during their migration or exposure to parasiticide treatment.

Prevention of stomach worms, managing manure properly and controlling flies are important steps in minimizing summer sores and eye infections. Treatment with ivermectin and moxidectin dewormers will kill larvae and adult stomach worms. Disposing of manure in a timely and proper manner will help prevent the development of larvae in the manure. A multifaceted approach to fly control will help to decrease fly populations. Fly spray and fly masks are simple husbandry methods in compliant horses. A possible dilemma, however, is that nearby farms may also contribute to problems if Habronema spp., manure and fly populations are not properly managed.

Treatment of eye infection depends on the location and severity of the inflammation. It is important to note that if one horse on a farm is affected, then all horses on the farm have the potential for exposure and infection. The degree of inflammation can be highly variable between horses. This may reflect differences in individual immune responses and the fact that certain individuals will develop a more serious hypersensitivity reaction to the larvae. A common presentation is swelling or puffiness of the corner of the lower eyelids toward the nose, but some individuals will develop large open sores on the eyelids and hard, pebblelike granulomas. The white-yellow granulomatous lesions, mistakenly described as fly eggs, represent the immune reaction to the larvae and can be very rough and irritating. Granulomas in the palpebral conjunctiva and posterior aspect of the third eyelid can create corneal ulcers.

It is often beneficial to remove superficial granulomas and especially if they are causing or may cause corneal irritation. Removal of the granulomas may also decrease inflammation. Larvae developing or dying at different stages could cause new granulomas to develop even after initial removal of granulomas. If corneal ulcers are not present, topical corticosteroids are used to control inflammation and subconjunctival injections may be administered. For large eyelid lesions, intralesional corticosteroids can be very effective. Systemic anti-inflammatories, including corticosteroids or Banamine may also be used. Treatment with an ivermectin or moxidectin dewormer is also initiated and repeated in 2-4 weeks.

Deworming programs to target the stomach worms and larvae are one part of the control of ocular Habronemiasis. Management of horse manure will help reduce the populations of infective larvae. Stable and house flies are the intermediate host that result in deposition of larvae and aberrant migration into the ocular tissues. Therefore, seasonal increases in fly activity will naturally cause an increase in ocular Habronemiasis. Fly control is critical in reducing the transmission of larvae. Each case of ocular Habronemiasis will require different types of treatment depending on the severity, location, and degree of inflammation.

Dirk works to ac�vate and enhance the body’s own way of healing itself. Manual techniques are applied to help restore op�mal func�on of the musculoskeletal system. By the use of manipula�on and adjustments, stretches and releases, and mobiliza�on techniques, it is an extremely beneﬁcial non-invasive therapy for horses.

• Reduce time spent cleaning pastures or paddocks