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Protect Pastured Animals from Photosensitization


Pet Column for the week of May 5, 2014

Related information:

Related site - Farm Animal Services at the Veterinary Teaching Hospital
Related site - Equine Services at the Veterinary Teaching Hospital

[Horse in pasture]
Areas of an animal that have less hair or pigmentation, such as the muzzle, around the eyes, and teats, are especially prone to photosensitization.

Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
Sarah Netherton
Information Specialist

Springtime brings a welcomed increase in sunshine, but with that comes the potential for a hazardous mix of certain plant compounds and ultraviolet light that sickens horses and cattle.

Photosensitization is an increase in sensitivity of the skin to ultraviolet light due to the presence of photodynamic agents contained in plants, according to Dr. Tina Wismer, a veterinarian at the ASPCA Animal Poison Control Center who lectures at the University of Illinois College of Veterinary Medicine in Urbana. Owners of large animals that are on pasture may need to protect them from getting too much ultraviolet light.

Photosensitization typically occurs during the spring and summer months, when large animals are exposed to intense amounts of sunlight. Areas of an animal that have less hair or pigmentation, such as the muzzle, around the eyes, and teats, are especially prone to photosensitization since they are less protected by hair.

Photosensitization can occur as either a primary or secondary process.

“In primary photosensitization, the photodynamic agent from a plant is absorbed into the animal’s circulation, and this agent will react with UV light at the surface of the skin,” explains Dr. Wismer. “These reactive molecules will attack the molecular structure of the skin—amino acids, membrane lipids of the capillaries, and nucleic acids—and cause skin sloughing, blisters, and tissue necrosis, meaning death of the tissue.”

In this case, the compound the animal has ingested is causing the problem, and there is no liver involvement. Plants that can cause primary photosensitization due to toxins they contain include: St. John’s Wort (fresh and dried), buckwheat (fresh and dried), and smartweed.

Secondary photosensitization is the most common type of photosensitization and frequently occurs in livestock. This occurs when animals ingest plants that contain phylloerythrin (phylloerythrin is what chlorophyll is broken down to by the body).

“Phylloerythrin is normally excreted in the animal’s bile, but when there is liver damage this phylloerythrin will accumulate in the plasma, since the animal’s liver can no longer break down the chlorophyll,” states Dr. Wismer. “Phylloerythrin, that is normally excreted, can now reach the surface of the skin and will react with UV light, causing a bad sunburn and skin sloughing.”

Plants involved in secondary photosensitization and damage the liver include alsike clover, comfrey, rattle box, fiddleneck, and ragwort.

In addition to serve sunburns, skin sloughing, itchy and red skin, photosensitization may cause mothers with burned, painful teats to refuse to let their young feed. In this situation, the young animals are in danger of dying if the animal owner does not quickly recognize the situation.

Treatment for either primary or secondary photosensitization includes getting the animals out of the sun and removing the suspected plants from the pasture. The veterinarian may also prescribe antibiotics to treat the skin lesions.

It may take weeks for animals to recover from photosensitization, just as healing from a bad sunburn in people takes time.

Preventive measures include putting sunblock on an animal’s muzzle or other exposed areas before it goes to pasture for the day, keeping animals out of intense sunlight, and ridding a pasture of photodynamic agents.

For more information about photosensitization in animals, speak with your veterinarian.