I'm not a veterinarian, but believe that a cows eyes are similar to our own because humans are also susceptible to pinkeye. I had to have a piece of wire from a grinder surgically removed from my eye once. The Dr. explained to me the importance of potassium and sodium in my tears for fighting bacteria and infection. The explanation came as the reason the piece of wire had rusted completely within a few minutes of me getting it in my eye. Knock on wood, I have never had to deal with a single case of pinkeye in me, my family or any livestock.
I always try and control the flys for many reasons and keep a salt mineral block out for the cows. Reading the label on the block tells me that it has potassium, sodium and chloride in it. Do I know for a fact that the mineral block is preventing pinkeye? No, but something is working and the cows are healthy..