Photosensitivity in livestock can be a major problem. Not only can it cause pain and discomfort but it may even result in production loss and in severe cases death.

Photosensitivity is an abnormal skin reaction to direct sunlight exposure. This condition is not the same as sunburn.

Photosensitivity causes skin to become very susceptible to ultraviolet rays. When light penetrates the affected skin it kills cells and causes swelling, itching and may perhaps even patches of skin to slough off. Severe damage can occur when unpigmented skin that is unprotected from the sunlight becomes sensitized.

Photosensitivity is due to the accumulation of photosensitive compounds beneath the skin. It usually occurs in herbivorous or omnivorous and is often due to the ingestion of harmful plants that create liver damage. Most commonly white-faced or white animals with unpigmented skin that is exposed to sunlight, such as skin occurring on the animal’s face, back or udder, are most affected. However colored livestock can also experience photosensitivity.

A healthy liver filters toxins out of the body. When damage occurs to the liver, toxins build up and enter the blood stream. An animal with a damaged liver is unable to metabolize the chlorophyll in hay or green plants causing a build up in the blood stream and accumulation under the skin. It is the build up of this chlorophyll that most often causes photosensitization of the skin.

Plants that may be consumed by livestock that can cause liver damage include: tansy ragwort or groundsel rattlebox, fiddleneck, heliotrope, comfrey, hounds-tongue, blue thistle, paintbrush, cocklebur, sneezeweed, bitterweed, birdsfoot trefoil, Kochia or fireweed, mushrooms, rapeseed, bog asphodel, tarbush, mustard horsebruh, signal grass, Bermuda grass, alfalfa, buttercup, bishop's weed, spring parsley and lantana, Alsike clover, furocoumarins, Lupinosis, Sporidesmin and Lady's thumb.

Plants such as St. Johnswort, buckwheat and smartweed are also dangerous because they can cause photosensitization directly due to the fact that they contain preformed photo-active compounds.

Many other conditions including leptospirosis and the obstruction of bile ducts by flukes, tapeworm cysts or tumors can also cause liver damage and lead to photosensitivity.

Photosensitization is not only limited to the summer months when the sun is strong. In the spring, livestock that are out on pasture may have trouble metabolizing the chlorophyll that is contained in green plants. This can also occur following the winter months. After spending the winter consuming hay the animals’ body initially has trouble metabolizing the chlorophyll resulting in a buildup in the blood and causing photosensitivity. The condition can also occur due to sudden bright sunshine. Livestock fed only hay can also experience photosensitivity. Usually this occurs in wet years or if hay is over mature and put in wet, humid conditions.

A clear indication of photosensitization is an abrupt difference in pattern where light skin and dark skin meet. The skin may be red and swollen and develop blisters that may crack and ooze or have painful scabs. Patches of skin may slough off. Swelling of the eyelids and watering eyes may occur in animals with white faces. If there is a significant amount of facial swelling breathing may also become difficult. If the animal’s condition is severe their pulse and temperature may rise and they may even go in to shock. Affected animals may stop eating and begin to lose weight. The sun will cause them a lot of pain and mothers may even stop letting their newborns feed.

Livestock that are affected by photosensitivity should be kept out of the sun during the day and instead let out to graze at night. As well, the toxic plant or feed should be removed from the animals’ diet. Livestock that experience photosensitivity due to poor metabolic processing of chlorophyll may need time to let their bodies adjust and better handle the green plants they are consuming.

Fortunately most affected animals make a complete recovery.