The role of face flies in an episode of infectious bovine keratoconjunctivitis.

Cattle partially protected from face flies (Musca autumnalis) had much less infectious bovine keratoconjunctivitis and yielded fewer isolated of hemolytic Moraxella bovis than did similar cattle that received no face fly protection. The number of new isolated of M bovis from the unprotected cattle was highly correlated with the number of face files observed on the cattle. Close physical proximity to cattle infected with M bovis was important in the spread of the bacteria to previously uninfected cattle; however, it did not require physical contact among the cattle to spread from herd to herd. Moraxella bovis first began to spread from herd to herd after face fly populations exceeded 10/animal for 1 month. Keratoconjunctivitis was not observed in 53% of the cattle from which M bovis was isolated. Moraxella bovis was isolated from 4 of 45 pools of face files collected from animals in the untreated herds.