Thelazia lacrymalis in Horses in Kentucky and Observations on the Face Fly (Musca autumnalis) as a Probable Intermediate Host

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Abstract
Eyes from 114 (30.3%) of 376 dead horses, examined from 3 April 1975-3 April 1976, were naturally infected with adult T. lacrymalis; 1 horse was also infected with 1 male T. skrjabini. Adult T. lacrymalis from dead horses were successfully transferred mechanically to the eyes of 3 of 4 Shetland ponies raised helminth-free. Larvae from gravid female T. lacrymalis underwent development in experimentally infected, laboratory-raised face flies (M. autumnalis) and 3rd-stage larvae ranging from 1.82-2.94 mm in total length were recovered at 12-15 days postexposure. A total of 866 naturally occurring face flies were collected from the head region of horses. Twelve of the face flies harbored larval stages of Thelazia spp. One of the larvae resembled 3rd-stage T. lacrymalis that were recovered from the experimentally infected, laboratory-raised face flies. Introduction of three 3rd-stage larvae from 1 face fly onto the cornea of a pony raised helminth-free resulted in the recovery of 1 male T. skrjabini 242 days later. In addition to the eyeworm larvae, other parasites recovered from the face flies included Heterotylenchus autumnalis, hypopi of astigmatid mites and a 1st instar beetle (Coleoptera: Rhipiphoridae). Face flies are probably an intermediate host for T. lacrymalis and probably other species of Thelazia in this part of the country.

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