Horse Health

Tainted hay suspected in horses’ sun sensitivity


The lack of water caused by California’s severe drought might have caused the levels of some ingredient in alfalfa hay to increase…

In this photo taken on Tuesday, Aug. 12, 2014, Brian Cockerton pets his horse Crystal at his ranch in Norco, Calif. Crystal and another of Cockerton’s horses are suffering from a skin condition known as photosensitization – a hyper-sensitivity to direct sunlight – and subsequent infections called photodermatitis, he said. Though tainted hay is suspected as the cause, tests are continuing at the California Animal Health and Food Safety lab at UC Davis. ~ photo by Kurt Miller

NORCO, Calif. Tainted hay is suspected of causing sun sensitivity that is leading to rashes and skin infections in horses across Southern California, state officials said.

Vets and horse owners have reported at least two dozen cases of photosensitization in recent weeks, the Riverside Press-Enterprise reported Tuesday.  As many as seven cases are in the Riverside County city of Norco.

Affected horses experience infections called photodermatitis, which can result in hair loss and painful lesions.

The source appears to be tainted alfalfa hay from several distributors, with at least some from the Bishop area in the Eastern Sierra, according to the California Department of Food and Agriculture‘s Animal Health and Food Safety Services Division.

Toxicologists have not definitively identified the exact substance, but officials believe it might be naturally occurring, the agency’s Dr. Kent Fowler said.


Toxicologists have not definitively identified that substance, said veterinary Dr. Kent Fowler, head of the state agency’s animal health branch, describing it as a “very odd occurrence.” ~ photo by Kurt Miller

The lack of water caused by California’s severe drought might have caused the levels of some ingredient in alfalfa hay to increase, officials said.  The problem could stem from the hay being fed to horses very soon after it was cut and baled.  The effect might lessen over time in hay that is kept awhile, the newspaper reported.

The condition typically is not fatal, unless a secondary condition, such as an infection, gets out of control.  A horse owner in the Phelan area reported two old horses died of what she believed was photosensitization in recent months, but an investigation didn’t find that sun sensitivity was involved, Fowler said.

Treatment starts with getting horses under shelter or shade, and sometimes turning them out only at night.  They should be kept out of sunlight until they heal.  Open sores resulting from severe cases might get bacterial infections, which would then most likely require antibiotics administered by a vet.

7 replies »

  1. It can also result in death for young foals. This is something I struggled with when I was raising Paint babies….and it was only the Paints, not the solids. They were not on hay, they were in pasture. So it’s not only a hay problem. I never did figure out what the foals were eating that was causing this. I just knew as soon as I saw signs, to put them up out of the sun during the day. This is very scary!


    • Barb, I had a neighbor whose pastured horse experienced this also a few summers ago. It was his home pasture and the horse was around 12 years old then… but it was a dry summer and this was a tobiano paint horse. I don’t think they ever figured out the cause of the sudden onset and he suffered mightily, then they moved out of here altogether.


  2. That happened in Illinois several years back. Horses also without enough water would suck on the hay instead of chew and had this issue. The onset cleared as soon as dry hay was given. In Illinois u can order hydration hay. Perfect for drought but expensive in quantity. You can give it as long as needed and quicker recovery. The patches can look awful…some horses here in this state were actually slthing off skin like a snake. They were kept in shade and night turnout but also were treated with aloe and vitamin e to rebound the skin. The hay curing process is vital especially in alfalfa and clovers. Hope they recover.


    • Colt, can you describe what hydration hay is? Why not just soak regular hay, for instance. What makes it better/more expensive?


  3. I don’t know if its just in the East – but I remember years ago an incident with some ponies on a farm near here – I think it was a sunny day after a day of rain & the ponies were turned out in a paddock where there was lots of clover! Apparently there was a parasite on or in the clover that irritated any of them that had white on their legs – others were a little bothered but any pony with white legs had sort of a rash & it was VERY itchy. Once we caught them & got them quieted down – we hosed them for some time & that helped – put cream on their legs. They were lucky it didn’t last long – because someone was there and saw it happening! I realize that is a minor thing compared to this photosensitivity – wonder if somehow this hay problem is one more of the same? Possibly a parasite?


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