Occasionally, damage to the eye can be severe enough for blindness to be permanent. It is poor management and unacceptable from a welfare standpoint to allow severe cases to progress to this stage without treatment. The infection can spread very rapidly and the economic impact due to weight loss and lowered milk production can be considerable.
Pinkeye is caused by the bacterium Moraxella bovis, which infects the eye and produces a toxin. The toxin attacks the surface of the eye (cornea) and the surrounding membranes (conjunctivae), eroding the surface and causing severe inflammation. Important factors that predispose cattle to infection include:
Pinkeye outbreaks are most frequently seen in summer and autumn when flies are more prevalent and ultraviolet radiation is high. This also coincides with the time when mature dry thistles and dusty conditions are more likely.
Pinkeye persists in a herd in the eyes of carrier cattle that do not show any signs of disease. Eye irritation from dust, bright sunlight, thistles and long grass can then cause lachrymation (tear production) which attracts flies. The flies feed on infected secretions and move from animal to animal, spreading the bacteria. Carriers may also carry infection in the nose and vagina, so that discharges from these areas are also a source of infection.
As always, prevention is better than cure, and should include an integrated approach: