Subconjunctival nodule due to Setaria equina erratic migration in a horse: First case report

An 18-month-old Arabian-English filly resident in southwest France was referred for evaluation of a conjunctival mass in the right eye (OD). A pink, solid, and mobile nodular formation, measuring approximately 1.2 × 0.8 cm was found under the superior nasal bulbar conjunctiva during an ophthalmic examination that was otherwise normal. The mass was surgically removed using a standing procedure. Cytological examination of fine-needle aspirates from the mass revealed a mixed eosinophilic-lymphocytic inflammation. Histological examination confirmed the dense and diffuse eosinophilic-lymphocytic infiltrate of the mass, and it revealed several cross sections of a parasitic nematode. The morphometric diagnosis identified an immature form of a filarial worm, and molecular analysis of the mitochondrial cytochrome c oxydase subunit 1 (cox1) and 12S rRNA gene sequences led to further identification of the specimen as Setaria equina. Microfilaremia was not observed on fresh blood smears. There have been no signs of local recurrence after 18 months, nor any evidence of intraocular involvement. To the authors' knowledge, this is the first documented case of subconjunctival setariasis due to S equina in a horse.