Pink eye can occur year-round but is more common during summer, probably because of fly transmission, sunlight, and irritants. This variation also explains why sensitivity to specific antibiotics is inconsistent. What works in one eye may not be as effective in another. The virulence of the bacteria is enhanced by ultraviolet radiation. Eye irritation from dust, plant chaff or seed heads, and insects augments the bacterial infection.
Flies are the primary transmitters of the bacteria from herd to herd as well as from eye to eye.
The bacteria can exist on the fly for as long as four days and in fly intestines for two days. As the number of flies doubles from approximately 6 to 10 per animal head, the incidence of pink eye also doubles. Animals that recover from the disease may show no recurring signs but remain infected indefinitely and provide a source of infection for others next season. As for all diseases, the diagnosis should be confirmed. Nutritional imbalances apparently correlate with the onset and intensity of the disease process.