Horse owners and veterinarians should be aware that the Arizona Veterinary Diagnostic Laboratory has recently received four inquiries regarding the occurrence of a photosensitization-like disease which has occurred in numerous horses in the Kingman/Lake Havasu City, Prescott/Mayer and Phoenix areas. Veterinarians and owners in all the cases believe the disease resulted from consumption of certain lots of new alfalfa hay. All horses were being fed alfalfa hay. The Kingman/Lake Havasu City and Prescott/Mayer and at least one of the Phoenix cases occurred after purchase of new lots of alfalfa hay. Owners reported that in several cases, skin lesions on white-haired or unpigmented areas appeared within 24-72 hours of feeding of a newly purchased lots of alfalfa hay and resolved after the horses were taken off the hay. Veterinarians and owners involved estimated that approximately 40-50 horses were involved in the Kingman/Lake Havasu City area, approximately 95 in the Prescott area and in excess of 35 horses in two locations in the Phoenix area. Sources of the hay were growers in the Mohave Valley and Phoenix areas. First and second cuttings were associated with the Kingman/Lake Havasu City outbreak, according to the local county agent.

Photosensitization, also known as photodermatitis, occurs when phototoxic or photoactive substances accumulate in the skin and interact with sunlight to produce a sometimes severe, crusting, itching or painful dermatitis in unpigmented or white-haired areas of the skin. Local swelling and skin damage can be mild to severe. Affected skin may weep serum and slough or peel. The extent of the skin damage is typically limited to the unpigmented skin on an animal that is exposed to sunlight and does not occur on darkly pigmented areas. A number of different plants as well as certain drugs are known to have photosensitizing potential. Alfalfa (Medicago sativa) and related Medicago sp. are listed among the many plants having photosensitizing potential. Photosensitization associated with consumption of alfalfa or other Medicago sp. appears to be uncommon, at least in Arizona. The photoactive substance that it occasionally contains is unknown and it is not known why or how it is formed.

Photosensitization can occur in other livestock but so far the recent problem in Arizona has been reported only in horses. It occurs in two general types which are called primary and secondary. Primary photosensitization occurs when photoactive substances present in certain forage plants consumed by a horse accumulate in its skin. Interaction of the substances with sunlight then occurs in unpigmented areas exposed to sunlight. Secondary photosensitization occurs when the liver is damaged by toxins and cannot excrete phylloerythrin, which is a photoactive substance produced from ingested chlorophyll. This substance accumulates in the blood and tissues, resulting in photosensitivity, again only in unpigmented areas that are exposed to sunlight.

It has been reported that alfalfa-associated photosensitization has occurred in cattle when aphid-infested clover or flood-damaged, moldy alfalfa hay was fed. The hay associated with the Kingman/Lake Havasu City horse outbreak was heavily infested with aphids and contained a small amount of lovegrass. The hay associated with the Prescott/Mayer outbreak and from one of the Phoenix outbreaks appeared to be of good quality and did not appear to contain weed contaminants. Attending veterinarians reported no abnormalities in liver function tests on a few selected horses in the affected groups, suggesting the presence of a pre-formed phototoxic substance in their feed. However, this represents a limited sample and may not be representative of all affected horses.

We recommend that horse owners having animals affected with crusting, painful skin lesions call their veterinarian for assistance immediately. Samples of suspect forage (a 3-4 inch thick flake is sufficient) can be forwarded to the AzVDL for examination to identify possible contaminant plants that are known to cause photosensitization and for an assay for phototoxicity. The phototoxicity assay is performed for a nominal fee and requires a week or more to complete. 

Affected horses should be taken off suspect forage, protected from sunlight and evaluated for liver damage by a veterinarian. Skin lesions may require local treatment and a laxative may be indicated to remove ingested toxin in severe cases. Continuing to feed photosensitized animals the offending forage will lead to worsening of the condition and possible death of the animals. We recommend that an equine veterinarian be consulted regarding all aspects of diagnosis and treatment of affected horses.

by T. H. Noon, DVM

Arizona Veterinary Diagnostic Laboratory

520-621-2356