CASE REPORT: Intraocular Setaria equina in a horse

Setaria equina parasites have a wide geographic distribution and are normally found in the peritoneal cavity of ungulates and some other mammals.  Their life cycle is typical of filarial worms so they rely on the seasonal activity of the mosquito vectors.  They rarely cause intraocular infections in horses and donkeys and this is the first documented case of subconjunctival aberrant migration in a horse in Europe.

An 18-month-old Arabian-English filly presented a pink, solid and mobile conjunctival mass in the right eye. There was no sign of ocular discomfort in either eye or changes in ocular responses and reflexes. Fluorescein staining was negative and no general abnormalities were noted.

When the application of a corticosteroid-antibiotic ophthalmic ointment twice daily did not create an improvement, the horse was hospitalized so that the mass could be removed and analyzed. Fine-needle aspirates cytology revealed a mixed eosinophilic-lymphocytic inflammation and histology showed not only a diffuse eosiphilic-lymphocytic infiltrate, but also numerous sections of the nematode. PCR amplification and sequencing showed a high percentage of homologies with Setaria equina, confirming the diagnosis.

Eighteen months after the surgical procedure there was no recurrence or intraocular impairment. No further treatment was prescribed as microfilaremia was not observed on fresh blood smears.

The authors conclude that, despite its rarity, this condition should be included in the differential diagnoses of nodular lesions in the subconjunctival space.