Prolonged occurrence of Moraxella bovis infection on a restricted heifer farm.

An epizootiologic study was conducted for infectious bovine keratoconjunctivitis (IBK) on a large grassland farm rearing a total of 1,300-1,400 heifers during May to October in both 1991 and 1992. Heifers were examined for infection with Moraxella bovis, and isolation of M. bovis from the affected eyes and from Asian face flies (Musca bezzii Patton et Cragg) swarming on the heifer face was carried out. During the observation period, 10.7% of the heifers in 1991 and 5.3% of the heifers in 1992 were affected with IBK. Eight M. bovis isolates were isolated from the ocular swabs of affected heifers with IBK for 2 years and identified by biochemical examinations and Southern DNA-DNA hybridization, but no M. bovis was obtained from 331 Asian face flies tested. Plasmid profile analysis showed that two plasmids (35-kb and 4.0-kb) were commonly found in all 8 M. bovis isolates. This indicates that M. bovis isolates with the same plasmid profile were widely disseminated on this farm for 2 years.