Thelazia lacrymalis in horses in Kentucky and observations on the face fly (Musca autumnalis) as a probable intermediate host.

This research paper examines the infection and prevalence of Thelazia lacrymalis, a parasitic worm, in horses in Kentucky, with a particular focus on the role of face flies (Musca autumnalis) in hosting and transmitting the parasite.

Study Procedure and Findings

• The researchers observed eyes from 114 out of 376 horses that had passed away over a one-year period (from April 1975 to, April 1976). They found out that those eyes were naturally infected with adult Thelazia lacrymalis. An infection from another worm, Thelazia skrijabini, was also discovered in one of the horses.
• Additionally, they successfully transferred adult T. lacrymalis from the dead horses to the eyes of three out of four Shetland ponies, which were free of helminths (parasitic worms).
• They observed larvae from female Thelazia lacrymalis progressing in experimentally infected, laboratory-raised face flies. They managed to retrieve third-stage larvae 12 to 15 days after exposure. These larvae were growing in length, ranging from 1.82 to 2.94 millimeters.

Association between Face Flies and Thelazia

• A total of 866 naturally occurring face flies were collected from the head region of the horses. The researchers found that twelve of the flies were harboring larval stages of Thelazia species.
• Intermediate larvae of Thelazia lacrymalis, which matches the form detected in experimentally infected, laboratory-raised flies, was also identified in the face fly’s larvae.
• When the researchers introduced third-stage larvae from one face fly to the cornea of a pony which was parasite-free, they could retrieve one male T. skrjabini 242 days later.

Other Parasites in the Face Flies

• Besides the Thelazia, the researchers identified several other parasites in the face flies. These included Heterotylenchus autumnalis (a type of roundworm), a first life-stage beetle (of the Rhipiphoridae family), and teenager developmental stages of astigmatid mites.
• According to these findings, Thelazia lacrymalis and probably other Thelazia species use the face fly as an intermediate host. The results highlight potential parasites within the Kentucky horse population and yield insights on how Thelazia spreads in this region.