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Professional Version

Eyeworms of Large Animals



Domenico Otranto

, DVM, PhD, DipEVPC, University of Bari

Reviewed/Revised Sep 2021 | Modified Oct 2022
Topic Resources

Thelaziases are caused by spirurid nematodes (also known as eyeworms), which cause infestation of the orbital cavities and associated tissues of several animal species. These whitish nematodes are transmitted by nonbiting dipteran muscid flies or drosophila fruit flies (intermediate hosts) while feeding on the lacrimal secretions of infected animals. Adults of Thelazia spp localize under the eyelids and the nictitating membrane, as well as in nasolacrimal ducts, conjunctival sacs, and excretory ducts of lacrimal glands, depending on the species targeted. Thelaziases may be responsible for eye disease, with signs and symptoms of varying severity (eg, lacrimation, ocular discharge, epiphora, conjunctivitis, keratitis, and sometimes corneal opacity or ulcers). Levamisole and ivermectins (eg, doramectin) administered either subcutaneously or intramuscularly, are useful for the treatment of thelaziasis in cattle and horses. Some Thelazia spp may also infect humans.

Etiology and Epidemiology of Eyeworms in Large Animals

Eyeworms (Thelazia spp) are common parasites of horses and cattle worldwide. Horses are infected primarily by T lacrymalis, whereas cattle are infected mainly by T gulosa, T skrjabini, and T rhodesii, particularly in Africa, and Asia and Europe. The prevalence of Thelazia spp in production animals 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, many species of wild carnivores, and humans.

Thirteen species of Musca have been incriminated in the transmission of eyeworms in cattle and horses; however, only face flies (ie, M autumnalis and M larvipara) have been demonstrated, in both experimental and natural conditions, to act as vectors in the US, Canada, and Italy. 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 weeks. Infective third-stage larvae emerge from the labella of infected flies and are mechanically deposited in the eye of the host by the fly during feeding. Development of sexually mature worms takes 1–4 weeks in cattle, depending on worm species, and 10–11 weeks for T lacrymalis in horses. Infections may be found year-round; however, clinical signs, particularly in cattle, are usually 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 years old.

Pathogenesis of Eyeworms in Large Animals

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 common. 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 of targeted animals are more typical for T rhodesii; however, 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. Human cases transmitted by T gulosa have been recently reported in the US, although T callipaeda is the most common agent of infection in humans worldwide.

Clinical Findings and Diagnosis of Eyeworms in Large Animals

  • From asymptomatic to conjunctivitis, excessive lacrimation, keratitis, corneal clouding, and subconjunctival cysts

  • Ophthalmic examination to identify the presence of nematodes

  • Removal of nematodes by means of swabbing with cotton or saline lavage

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 examination. However, Thelazia infections in cattle in North America may not always be innocuous. They may produce mild conjunctivitis, excessive lacrimation, localized edema, keratitis, corneal clouding, and occasionally, subconjunctival cysts. In Europe and Asia, thelaziasis is commonly associated with severe clinical signs, 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 occur. 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.

Treatment and Control of Eyeworms in Large Animals

  • Environmental measures to control flies

  • Administration of ivermectins and levamisole

  • Good hygiene in stables

Mechanical removal with forceps after instillation of a local anesthetic is useful to treat 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 administration of antimicrobial-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, administration of 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 US 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 per 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.

Key Points

  • Thelaziases (ie, eyeworm infestations) are caused by spirurid nematodes, which live at the adult stage in the orbital cavities (under the lids, in nasolacrimal ducts, in conjunctival sacs) of large and small animal species and may also infest humans.

  • Nematodes are transmitted by nonbiting dipteran muscid flies (zoophilic drosophilas for T callipaeda in small animals), while the flies feed on the lacrimal secretions of infected animals.

  • Clinical signs may be of varying severity (eg, lacrimation, ocular discharge, epiphora, conjunctivitis, keratitis, and even corneal ulcers).

  • Administration of levamisole and ivermectins (eg, doramectin, either subcutaneously or intramuscularly) are useful for the treatment of thelaziasis in cattle and horses.

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