Photosensitization is a serious skin condition characterized by “sunburned” crusty white or non-pigmented skin that dies and sloughs away. It is usually caused by plant pigments that the cow or horse has eaten, but the skin problem does not appear until the animal is exposed to sunlight, which causes the pigments circulating in the skin to damage the non-pigmented skin.
Dr. Stan Casteel, professor emeritus of toxicology at the University of Missouri, explains that this hyper-reactivity of skin to UV rays in sunlight is usually a symptom of liver disease (in which the skin photosensitization is secondary) or a primary condition due to ingestion of a unique photosensitizing compound (usually of plant origin). Three factors contribute to the development of photosensitization: presence of a photo-activating substance in the skin circulation; exposure to ultraviolet light; and lack of skin pigment (allowing absorption of UV light).
A few plants contain pre-formed photo-reactive compounds and these cause problems when ingested. This situation is termed primary photosensitization. Most cases in cattle, however, are due to liver damage—since the liver is the body’s main filter, screening out and eliminating harmful substances. “In ruminants, this syndrome is most commonly secondary to a forage-induced liver disease,” he says.
Primary photosensitization occurs when there is a photo-reactive compound present within a specific plant (consumed by the animal). St. John’s wort is probably one of the most well-known, along with buckwheat. “In cattle, primary photosensitization is very infrequent, but when they do experience this it is very severe,” says Casteel.
“Light-skinned cattle have the worst problem. Black-hided cattle (unless they have a white face) don’t get photosensitization. The melanin in skin pigment is protective against the UV rays that set it off.
Secondary photosensitization
In an animal with diffuse liver damage, normal body metabolism may cause photosensitization because byproducts of plant digestion accumulate in the skin tissues instead of being cleared through the liver.
When liver function is impaired, byproducts of chlorophyll such as are phylloerythrin are not properly eliminated and spill into the blood, reaching the capillaries in the skin, where they produce the photosensitization reaction. “When too much phylloerythrin moves into the skin, photosensitization occurs when sunlight hits the lightly pigmented skin. Although this type of photosensitization is most common when grazing green pastures, it can also occur in animals fed hay,” he says. There is enough chlorophyll in hay to produce critical levels of phylloerythrin in tissues of animals suffering from liver damage.
“Of the cases of photosensitization I’ve dealt with over the years, 90 percent of the time these cattle have liver disease. If you see cattle in a herd situation that are severely photosensitized (with many individuals affected and not just one or two), more than likely this is secondary to a diffuse liver disease. The liver damage/failure can potentially kill the animals even if they don’t show photosensitization (such as dark-colored animals with pigmented skin),” says Casteel.
Moldy forage
Some forages such as alfalfa or red clover—which are usually harmless—are sporadically associated with outbreaks of secondary photosensitization in cattle. This can happen when these legumes are growing during periods of excessive rainfall, often resulting in delayed harvest. The photosensitization is due to molds that produce liver-damaging mycotoxins in the hay.
“As a toxicologist I often wonder about feeding moldy forage to cattle. About 95 percent of the time they can handle it, though it will lack some of the nutrients because the fungi in the forage consume many of the nutrients. That is always part of the problem. But about 5 percent of the time, like we see in very wet years like this, with delayed harvesting, there is potential for a different kind of mold that only occurs in these wet years and causes profound liver disease,” he says.
“I suspect we will be seeing more cases of photosensitization next winter due to delayed harvesting and moldy forages. Typically, it is legumes (alfalfa, clover) that become overly mature and stemmy. We think that the stemmy material (that stays wet and never completely dries out before it is put up) is invaded by the fungus. It produces a toxin that attacks within the liver in cattle. One of the major functions of the liver is to produce bile, and this function is impaired,” he explains.
“The animal becomes very depressed, stops eating, loses weight, and will die if nothing is done to correct it. Often there is not much that can be done to correct liver damage. This is generally a prolonged clinical course that may last several weeks. The animal just continues to lose weight and finally expires,” says Casteel.
“The photosensitization in these cases is secondary to the liver failure. You can ease the discomfort from the photosensitization—which is like a severe sunburn, but affecting the deeper layers of skin. Once it gets bad enough and they become photosensitized it becomes a chronic, progressive disease that gradually takes them down,” he explains.
“Of the cases of photosensitization I’ve dealt with over the years, most have been due to liver disease. Those that didn’t eat much of the moldy hay or refused it will have mild disease and survive,” says Casteel.
A veterinarian can take serum samples from affected cattle and send them to a lab. “The serum test tells you first of all whether they have serious liver disease or just mild,” he says. Your veterinarian can help with diagnostics and prognosis after sending in the serum samples.
Treatment
The animals experiencing photosensitization must be removed from the feed that is causing it, and put into a shady area, out of the sunlight. “This works for both primary and secondary photosensitization. Put them in the shade/barn, and let them graze at night. If they have severely cracked, fissured skin you need to keep the flies off them or there will be maggots in those fissures. Antibiotics may be necessary to prevent bacterial infections,” says Casteel. — Heather Smith Thomas, WLJ correspondent
