Alfalfa hay can trigger primary photosensitization in horses, researchers from the University of California, Davis, have found.
However, the growing and climate conditions that result in the presence of photodynamic compounds in the hay remain unknown.
Photosensitization, sometimes called photodermatitis, can occur when phototoxic or photoactive substances build up in the skin and interact with sunlight, resulting in crusting, itching or painful dermatitis in unpigmented or lightly haired skin areas.
Primary photosensitive reactions occur through eating the phototoxic compounds, while secondary photosensitivity can arise when a horse’s liver cannot properly excrete phylloerythrin, a byproduct of chlorophyll degradation.
Brigit Puschner and her colleagues, who reported their findings in The Veterinary Journal, noted that primary photosensitization, caused by direct ingestion of photosensitizing agents, has been reported anecdotally in horses after eating alfalfa (lucerne) hay.
Between 2004 and 2014, several large outbreaks of primary photosensitization in horses fed mainly on alfalfa hay were investigated in California.
Alfalfa hay samples were collected and checked for known photosensitizing plants and pesticide residues but none were identified.
Select hay samples were evaluated for unusual fungal infestations and for phototoxicity using a specific laboratory test for the fungus Candida albicans. Results were negative.
During the 2004 outbreak, a feeding study was conducted with three horses exclusively fed alfalfa hay suspected to have caused the outbreak.
Two weeks after eating the hay, two of the horses developed skin lesions in non-pigmented areas which were consistent with photosensitization, suggesting the alfalfa hay in question was responsible.
In the 2014 outbreak, seven different implicated alfalfa hay samples were tested for chlorophyll a and b, and pheophorbide a – three compounds suspected to play a role in alfalfa-induced primary photosensitization.
The levels were compared to other hays and found to be similar, which ultimately led the researchers to eliminate the compounds as possible causes for alfalfa-hay induced primary photosensitization.
Alfalfa hay induced primary photosensitization in horses
B. Puschnera, X. Chen, D. Read, V.K. Affolter.
The abstract can be read here.