The role of face flies in bovine pinkeye

Face flies, Musca autumnalis DeGeer, supposedly transmit the hemolytic bacterium, Moraxella bovis, the major causative organism of infectious bovine keratoconjunctivitis (IBK, pinkeye), but the role of the fly in the spread of the pathogen between cattle has not been clearly delineated. To establish "effective contact" between face flies and the eyes of cattle, we confined flies fed fluorescein isothiocyanate over calves' eyes and detected fluorescence in and around the eyes after the flies were removed. This indicates that the flies regurgitate into the eyes as they feed. We fed female face flies on the surface of the eyes of cattle with clinical IBK symptoms, and recovered M. bovis from their crops. We recovered the bacteria from the flies' crops up to 48 h after ingestion. M.bovis-fed face flies transmitted the bacteria to the eyes of susceptible calves. These results indicate that the face fly is a competent vector of M. bovis, and that ingestion of the bacteria from the surface of infected eyes and regurgitation into the eyes of susceptible cattle is the mode of transmission.