THE MERCK VETERINARY MANUAL
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Eyeworms of Large Animals

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Eyeworms (Thelazia spp) are common parasites of horses and cattle in many countries, including those of North America. Horses are infected primarily by T lacrymalis, whereas cattle are mainly infected by T gulosa, T skrjabini, and T rhodesii. The latter is the most common and harmful to cattle in the Old World, but it has not been recently reported in North America. The prevalence of Thelazia spp in livestock has declined in at least some areas where macrocyclic lactone endectocides such as ivermectin and doramectin are in common use. Thelazia spp are also found in pigs, sheep, goats, deer, water buffalo, dromedaries, hares, dogs and cats (see Small Animals), birds, and people.

The face fly, Musca autumnalis, is the vector of T lacrymalis, T gulosa, and T skrjabini in North America. Feeding habits of this fly include a preference for ocular secretions, which are ideal for transmission. The life cycle of Thelazia is as follows: female worms are ovoviviparous and discharge larvae into the ocular secretions; the larvae are ingested by the fly and become infective in 2–4 wk. Infective third-stage larvae emerge from the labellae of infected flies and are mechanically deposited in the host's eye by the fly during feeding. Development of sexually mature worms takes 1–4 wk in cattle, depending on worm species, and 10–11 wk for T lacrymalis in horses. Infections may be found year-round, but clinical disease outbreaks, particularly in cattle, usually are associated with warm season activities of the flies. Thelazia sp larvae may overwinter in face flies. Infection rates generally tend to increase with advancing host age, although some studies report maximal levels in hosts 2–3 yr old.

The lacrimal gland and its ducts are common sites for T lacrymalis and T gulosa, with the glands of the nictitating membrane and the nasolacrimal ducts less so. T skrjabini is normally found within the lacrimal ducts of the nictitating membrane. Superficial locations on the cornea, in the conjunctival sac, and under the eyelids and nictitating membrane are more typical for T rhodesii, but T lacrymalis, T skrjabini, and T gulosa may be found in these sites, too. Worms may also be found on the periorbital hair or skin during anesthesia or following migration after death of the host. Localized irritation and inflammation is likely due to the serrated cuticle of the worms, especially for T rhodesii. Invasion of the lacrimal gland and excretory ducts may cause inflammation and necrotic exudation. Inflammation of the lacrimal ducts and sac has also been reported in horses. Mild to severe conjunctivitis and blepharitis are common. Also, keratitis, including opacity, ulceration, perforation, and permanent fibrosis, may develop in severe cases, particularly with T rhodesii infection in cattle.

Asymptomatic infections in horses and cattle appear to be typical of thelaziasis in North America. Infection may be encountered incidentally during surgery or at necropsy. However, Thelazia infections in cattle in North America may not always be innocuous. They may produce mild conjunctivitis, excessive lacrimation, localized edema, corneal clouding, and occasionally, subconjunctival cysts. In Europe and Asia, thelaziasis is commonly associated with severe clinical manifestations, including conjunctivitis, photophobia, and keratitis. Characteristically, there is chronic conjunctivitis with lymphoid hyperplasia and a seromucoid exudate.

A clinically feasible technique for reliable detection of adult eyeworms is lacking. Gross inspection of the eyes may reveal the worms and is generally recommended for T rhodesii, commonly found in the conjunctival sac. However, T gulosa and T skrjabini in cattle, and T lacrymalis in horses, tend to be more invasive and are less apt to be seen. Topical anesthetics allow for tissue manipulation and are useful for detection and recovery of worms. Microscopic examination of lacrimal fluids for embryonated eggs or larvae may be attempted.

Clinical signs may be helpful in differential diagnosis. Thelaziasis tends to cause a chronic conjunctivitis. In cattle, infectious keratoconjunctivitis (see Infectious Keratoconjunctivitis) is an acute, rapidly spreading infection of the cornea. In horses, infective larvae of the stomach worms Draschia and Habronema sp may also produce ophthalmic lesions. These tend to occur near the medial canthus of the eyelid and are raised, ulcerative granulomas, often containing characteristic yellow, plaque-like “sulfur granules” 1–2 mm in diameter. Likewise, microfilariae of Onchocerca sp invade the eye and may result in ophthalmic manifestations. Small (<1 mm), raised, white nodules in the pigmented conjunctiva adjacent to the temporal limbus are pathognomonic of Onchocerca infection. Depigmentation of the bulbar conjunctiva in this area is also common. Other lesions of onchocerciasis involve the cornea and include edema and punctate or streaking opacities of the stroma, superficial erosions, and a wedge-shaped sclerosing keratitis emanating from the temporal limbus. Intraocular structures also may be affected by microfilariae of Onchocerca sp (see Onchocerciasis). Worms may be identified morphologically. In addition, PCR and sequencing assays have been developed to confirm the identity of some species, but these are not used routinely.

Mechanical removal with forceps after instillation of a local anesthetic is useful for T rhodesii in cattle. This also may be feasible for the more invasive T gulosa or T skrjabini in cattle or for T lacrymalis in horses. Irrigation of the eyes with 50–75 mL aqueous solution of 0.5% iodine and 0.75% potassium iodide has been recommended for T gulosa and T skrjabini. This also may be effective for T lacrymalis in horses. Topical application of 0.03% echothiophate iodide or 0.025% isoflurophate (both organophosphates) has been successful for T lacrymalis in horses. Concurrent use of antibiotic-steroid ointment for the inflammation and secondary invaders is recommended. These topical agents should also be useful for T gulosa and T skrjabini in cattle. Certain systemic anthelmintics have exhibited activity against eyeworms. In cattle, levamisole at 5 mg/kg, SC, and ivermectin and doramectin, both at 0.2 mg/kg, SC or IM, have shown activity against Thelazia spp. Pour-on formulations of ivermectin or doramectin, delivered to achieve a dosage of 0.5 mg/kg, are also effective. Doramectin has been approved in the USA for treatment of adult eyeworms in cattle. For T lacrymalis in horses, single doses of the commonly used anthelmintics, including ivermectin, administered via stomach tube at 0.2 mg/kg, have had limited, if any, effect on eyeworms. In contrast, the multidose regimen of fenbendazole (10 mg/kg/day for 5 days) is efficacious against T lacrymalis.

Fly control measures, directed especially against the face fly, aid in the control of thelaziasis in cattle and horses. Cattle on dry, open pastures have fewer face flies than those on pastures where shade and water are present.

Last full review/revision April 2013 by David G. Baker, DVM, MS, PhD, DACLAM

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