You’ll spot the first one from the pickup. A calf standing off in the shade with one eye running, blinking against the light, tears streaking the hair down its face. Give it a day and the eye turns cloudy. Give it a few more and there’s a white ulcer in the middle of the cornea, and the calf is packing its head sideways trying to keep the sun out of it. That’s pinkeye, and by the time you’ve noticed one, the flies have usually spread it to a handful more.
It runs hardest from about now through late summer, when the grass is heading out, the face flies are thick, and the sun is high enough to bake a squinting eye all day. On Montana range that means June into August, and it hits calves worse than cows because a young animal hasn’t built up any resistance yet.
What’s actually going on in the eye
Pinkeye starts as an irritation and turns into an infection of the cornea and the tissue around it. The bacteria that drive it live on the surface of the eye, and they need help getting in. That help comes from anything that scratches or irritates the eye first — face flies feeding at the corner of it, tall seed heads and cheatgrass awns catching the lashes, dust, and flat-out sunlight glaring off a watery eye.
Once it takes hold it moves fast. A clear, weepy eye becomes a squint, then the cornea clouds over gray or blue, and then you get the ulcer — a soft spot in the center that can go all the way through if it’s bad enough. A calf that loses an eye entirely will still make it, but it spent the summer in pain and put on less gain than its pen mates, and that shows up on the scale in the fall.
The reason it doesn’t stay in one calf is the face fly. It feeds on the tears and secretions around the eye, then moves to the next calf and the next, carrying the bug on its mouthparts and legs. One infected calf standing in a bunched-up herd in the heat of the day is a distribution center. That’s why pinkeye tends to sweep through a group rather than show up as a single case.
Slowing the spread before it snowballs
Fly control is the piece you have the most leverage on, and it needs to be going before the first case, not after. Insecticide ear tags put in at turnout or branding help, and they last longer if you also have back rubbers or dust bags set where cattle have to use them going to water or through a gate. Whatever you run, rotate the chemistry class year to year so the flies don’t get ahead of you.
Grazing management matters more than people give it credit for. Mature, stemmy grass and seed heads poke eyes. If you can move cattle through before the range gets rank, or clip the worst of the tall stuff around gathering points, you take away one of the irritants that lets the bug in. Same goes for dusty, overcrowded corners — spread the herd out and you cut the fly traffic between animals.
Some producers vaccinate for pinkeye, and it can help, but the results are honestly mixed. The bugs come in different strains and the vaccine may not match what’s running in your country that year. If you go that route, get the shots in well ahead of fly season so there’s time to build immunity — vaccinating the day you see the first weepy eye is too late to do that calf any good.
Treating the ones you catch
Catch a case early and it responds well. The standard is a long-acting injectable antibiotic labeled for pinkeye — talk to your vet about which one and the dose, and get the withdrawal right. The sooner you hit it, the less chance that eye ulcerates.
The other half of treatment is protecting the eye so it can heal. Gluing a patch over it, or stitching the eyelids partly shut in a rough case, keeps the sun and the flies off and takes a lot of the pain out. A patch also flags the calf so you’re not doctoring the same one twice. It looks like a lot of trouble for one calf, but a shaded, protected eye heals in a fraction of the time.
Pull the worst cases and any calf you’re doctoring away from the bunch if you can. Every infected animal you leave in the middle of the herd is feeding the flies that carry it to the next one. Handle them early in the morning before the heat bunches everybody into the shade, and you’ll do less spreading yourself in the process.
Ride through and actually look at eyes when you check cattle this month. The calf that’s blinking today is the one you want to catch before it’s the calf with a white eye in July.
