You can usually spot the first case from horseback. A calf standing off by itself, head down, one eye running and squinted half shut against the light. Ride up close and there’s a dime-sized cloudy spot in the middle of the eye. By the time you’ve got that one roped and doctored, there are three more starting behind it. Pinkeye doesn’t creep in — it lands, and then it spreads.
The disease is a bacterial infection of the eye, and the classic bug behind it thrives in early summer for reasons any Montana cattleman already recognizes. Face flies show up. The grass heads out and cheatgrass and foxtail start throwing seeds and sharp awns. Dust picks up on the trail to water. Sunlight is long and hard. Every one of those things scratches or irritates the eye or carries bacteria from a sick animal to a healthy one, and the calf’s eye does the rest.
Why it hits calves hardest
Mature cows have usually run into pinkeye before and carry some immunity. Calves haven’t. They’re also lower to the ground in the tall grass, they crowd up, and they don’t have the eye pigment some cows do to shade against UV light. A bad outbreak can run through a big share of the calf crop in a couple of weeks.
The cost isn’t just the doctoring. A calf that quits eating because it can’t see and hurts is a calf that quits gaining. A scarred, cloudy eye — or a blind one — knocks dollars off that calf at the sale barn every time, whether the buyer says so out loud or just backs off his bid. And a blind calf on open range is a calf that gets left behind, walks into fence, or doesn’t find water.
Catch it early and it’s usually manageable. The standard is an antibiotic your vet recommends, given the way the label and your vet say to give it. A lot of outfits also patch the eye — glue a cloth or commercial patch over it — to keep out flies, dust, and sun while it heals, and to stop that calf from spreading it to the rest. Don’t guess on the drug or the withdrawal; talk to your vet before turnout so you already know what’s in the doctoring kit and what the meat withdrawal is.
Knock down what causes it
The reason pinkeye and horn flies get talked about together is that fly control does double duty. Face flies are the main way the bacteria travels from a weepy eye to a clean one. Whatever program you’re already running for horn flies — pour-ons, dust bags, oilers, fly tags put in at the right time so they don’t wear out by August — helps hold pinkeye down too. Tags are worth mentioning: put them in too early and they’re spent when you need them most.
The other big lever is the grass itself. Mature, seeded-out forage with sharp awns pokes eyes and gives the bacteria a foothold. You can’t clip a mountain pasture, but grazing management that keeps calves off the rankest, most headed-out stuff, or clipping the worst around water and gates, takes some of the pressure off. Dusty, crowded conditions around a single water point make it worse, so anything that spreads cattle out helps here the same way it helps everything else.
Some producers vaccinate. The trouble is there are several strains and a lot of local variation, and a vaccine that saves one outfit a wreck does nothing for the neighbor. If pinkeye hits you hard every year, that’s a conversation to have with your vet about which product, and to have it early — most of these need time and sometimes a booster to do any good, so a shot given the week the first calf goes cloudy is too late.
Ride through and catch them early
None of this replaces looking at your cattle. When the flies are bad, ride through often enough that you catch the runny, squinty eye before it turns into a white cloud and a calf that’s off feed. Carry the doctoring kit with you so you’re not making a second trip while the infection gets a day’s head start. Watch the calves that stand alone with their head down — that’s the tell.
Pinkeye is one of those things that looks like bad luck but usually isn’t. Flies you didn’t control, grass you turned onto too rank, a vaccine decision put off, or just not riding through often enough. Handle those and most years you’ll pull a calf or two instead of half the bunch.
