The outbreaks in the Midwest are highest during the summer months, in part because of spring calving in cow-calf operations and also because of greater exposure to sunlight and flies. One of the problems associated with this disease is weight losses of as much as one half pound per day in calves with pinkeye. Cause The primary infectious bacteria is Moraxella bovis. The surface of the bacteria is covered by hair-like structures known as pili, which attach or adhere to the conjunctiva or the cornea.
Adhesion prevents the bacteria from being washed away by tears and blinking.
Other bacteria and viruses can produce the red and swollen eyelids or conjunctivae without the involvement of the cornea that is typical of pinkeye. Predisposing Factors Unpigmented eyelids and white hair on the face do not absorb ultraviolet light, which in turn increases the susceptibility of the calf to M. bovis. Another factor which increases the susceptibility of the calf to M. bovis is irritation of the eye by dust or plant material from mature, unmowed pastures. Also, face flies can carry the bacteria from one calf to another, but they are only one way the outbreak is spread. Pinkeye outbreaks occurred long before face flies became a problem.