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Pink eye can occur year-round but is more common during summer, probably because of fly transmission, sunlight, and irritants. Moraxella bovis, the bacterial cause of pink eye, includes several strains that differ widely in virulence (the potential to cause disease). 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. At least one case of "pink eye" was actually induced by the infectious bovine rhinotracheitis (IBR) virus. Nutritional imbalances apparently correlate with the onset and intensity of the disease process.
Late summer is an opportune time to remind producers that this economically important disease will likely return next year. Vaccination (which should include paired doses) may be an option and will have to be worked into the cattle-handling and processing strategy next spring. For problem cases, culture and sensitivity tests may be useful in determining an effective antimicrobial. Pasture management (clipping and dragging to break up manure pats) is an important component of control, as is fly control. Perhaps some improvement in cattle-handling facilities will ease the labor and cattle stress of treatment, especially when repeated treatment is involved.
Producers should be urged to consult with their veterinarians to devise the most appropriate prevention techniques and treatment strategies and methods.
G. L. Meerdink, DVM
Veterinary Extension & Diagnostic Laboratory
University of Illinois College of Veterinary Medicine
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