Face flies can carry infective bacteria for three days after feeding on ocular discharges of afflicted animals. Clinical Signs Initially, the eyelid is swollen, and large amounts of watery tears flow from the eye. Photophobia, sensitivity of the infected eye to light, causes the animal to seek shade or refuge in a darkened shed. The entire cornea of one or both eyes may be involved within six days after the first tearing is observed. As an outbreak progresses, the disease frequently intensifies as the virulence of M. bovis increases in the spread among susceptible calves.
Some animals recover with no permanent damage to the eye.
Some have a permanently scarred cornea but retain a degree of sight. A few are blind in one or both eyes. Paradoxically, the year following a severe outbreak, any of several preventive measures may appear successful. The apparent success results from the high level of herd immunity that persists for at least a year and which markedly reduces the number of carrier animals. Fewer carrier animals reduces the shedding of M. bovis and exposure of susceptible calves.