It is often difficult and frustrating to ascertain the cause of your horse's eye irritations. The culprit may be the least likely source.
Something natural to your horse's environment that you would never think of.....the seedheads of the plants he lives among. It is very likely that pieces of hay or seeds carried by the wind as they float through the air can find their way into your horse's eye. The irritants can become trapped under the eyelids, scratching the lens of the eye. Once the source of irritation is removed, the eye can go back to normal health.
It is important to know the plants in your fields. Burdock (Arctium lappa) seems to be the most likely culprit in the US since it commonly grows as a field plant. The seed heads are composed of hooked bracts that form prickly, round burr heads. The flowers mature by mid to late summer. These are the plants that you may commonly carry into the house with you on your pants or jacket sleeves. It seems that fall and early winter are the problematic times for your horses since that is when the seeds are ripe and ready to fall from the plant.
Symptoms of problem may be discharge and weepiness, sensitivity to light, contraction of the pupil or being unresponsive to other treatments like antibiotics. Staining the eye with fluorescein can help discern the foreign substance that may be in the eye. Fluorescein is a yellow vegetable dye that fluoresces even in visible light.
The most natural way to eradicate the plant would be to pull them up by their roots in September or October, avoiding the use of herbicides. The burdock root is a respected plant by herbalists. The root is known for its alterative (blood purifier), diuretic and bitter properties. It looks very similar to a thin taut carrot. It is a most valuable remedy for clearing skin conditions. It can also be found in blends composed to deal with rheumatic conditions. The bitter principles stimulate the secretion of digestive juices, aiding in appetite and digestion. Once you harvest the plant you can save the roots, chop them up and feed them to your horse. If you have a local herbalist in the area, they would be very happy to take the plants off your hands to make burdock tinctures. Perhaps you can barter: you bring them the plant in trade for some of their finished products.
Equus Magazine #321, July 2004.