Research from the University of California–Davis confirms that alfalfa hay can induce primary photosensitive reactions in horses, but the exact mechanism remains unknown.

Primary photosensitive reactions occur when phototoxic compounds are ingested by a horse and accumulate under his skin. (In secondary photosensitivity, a horse’s liver cannot properly excrete phylloerythrin, a byproduct of chlorophyll degradation.) When exposed to sunlight through pink skin, the compounds cause painful blistering, crusting and inflammation.

St. John’s wort, buckwheat and several other plants have been found to contain compounds that can cause photosensitive reactions. Similar reactions have been linked to alfalfa hay anecdotally but it has not been found to contain the phototoxic substances.

To investigate further, the Davis researchers performed a series of case studies, examining samples of alfalfa hay associated with seven outbreaks of equine photosensitization from 2004 to 2014. The tests revealed no known photosensitizing compounds, pesticide residues or fungal infestations.

After the 2004 outbreak, three test horses were fed the same alfalfa hay associated with the photosensitivity reactions. Two developed skin blistering, confirming that the alfalfa was the culprit.

The Davis researchers also tested seven samples of alfalfa hay associated with a 2014 photosensitization outbreak for two phototoxic agents: chlorophyll and pheophorbide. They found that levels of both substances were similar to those found in orchard grass hay, as well as control bales of locally grown alfalfa not associated with any photosensitive reactions. This, the researchers say, rules out those compounds as a causative agent.

The researchers advise owners who feed alfalfa to remain alert to any changes in the horse’s skin—particularly after switching to a new batch of hay—and to stop feeding it immediately and call a veterinarian if a photosensitive reaction is suspected.

Reference: “Alfalfa hay induced primary photosensitization in horses,” The Veterinary Journal, May 2016

This article first appeared in EQUUS issue #466

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