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Why photosensitization in cattle can be a serious problemWhy photosensitization in cattle can be a serious problem

Plant toxins can cause severe skin and liver damage.

Heather Smith Thomas

October 5, 2015

9 Min Read
Why photosensitization in cattle can be a serious problem

Green pastures and sunshine. What can be more idyllic? There are times, however, when that bucolic scene can be deadly.

It’s not common, but when it occurs, photosensitization in cattle can be a serious problem. Photosensitization is a serious skin and sometimes liver condition characterized by “sunburned,” crusty white or non-pigmented skin on horses and cattle.

While it is usually caused by photo-reactive plant pigments that the cow or horse has eaten, the skin problem does not appear until the animal is exposed to the ultraviolet (UV) rays in sunlight, causing the pigments to damage the non-pigmented skin, which dies and sloughs away.

There are two general types of photosensitivity problems — primary photosensitization, in which unique photosensitizing compounds in plants cause a skin reaction when the animal is exposed to sunlight.


Then there’s a deeper, more serious problem. In ruminants, this hyper-reactivity of skin to UV rays in sunlight is usually a symptom of forage-induced liver disease, meaning the skin problem is secondary, says Stan Casteel, professor of toxicology, College of Veterinary Medicine, University of Missouri.

“Three factors contribute to the development of photosensitization: presence of a photo-activating substance in the skin circulation, exposure to light and lack of skin pigment [allowing absorption of UV light],” he says.

Primary photosensitization

“There are some plants that cause photosensitization when eaten by healthy cattle [without liver damage],” says Anthony Knight, professor emeritus with the College of Veterinary Medicine and Biomedical Sciences at Colorado State University.

These plants contain unique photo-reactive pigments that, when eaten by the animal, are absorbed from the digestive tract into the bloodstream and react in non-pigmented skin when exposed to UV light. This causes cell damage and sloughing of the white-skinned areas, he says.

Casteel says primary photosensitization is very infrequent in cattle, but when it does happen, it is very severe. “Light-skinned cattle have the worst problem. Black cattle [unless they have a white face] don’t get [primary] photosensitization. The melanin in skin pigment is protective against the UV rays that set it off,” according to Casteel.

“The classic plants that cause primary photosensitization are buckwheat and Saint-John’s-wort. “Buckwheat can actually be more of a problem than Saint-John’s-wort, because buckwheat is sometimes used as a cover crop or as a grain crop,” says Knight.

“If cattle, horses or sheep get into those fields and graze the buckwheat — which they like to eat — or happen to eat Saint-John’s-wort, they can develop a severe photosensitization. Once removed from the buckwheat or the pasture containing Saint-John’s-wort, and given protection from sunlight in a barn, they fully recover over a period of several weeks, because there is no underlying damage to the liver,” he says.

“Saint-John’s-wort is a native plant [considered a weed] that grows in some parts of the Northwest, and as far south as Colorado and Wyoming. Smartweed is another weed that may cause problems if animals eat it, and it grows more commonly in the Eastern and Southern parts of the country,” Knight says.

“Another form of primary photosensitization that we sometimes see in cattle occurs when they are turned out into lush, green fields or are fed very green, good-quality leafy alfalfa hay, or if they get into an alfalfa field — especially if they have been on poor-quality hay or pasture. There are also some forage crops that can cause primary photosensitization, such as birdsfoot trefoil and cicer milkvetch,” he says.


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These are legumes with good nutritional value. When they are green and cattle eat them in the spring, after they have been on dry hay through winter, they may become photosensitized within a week. The green plants have so much chlorophyll that it overloads the liver, causing a flush of a compound called phylloerythrin that produces the photosensitization, he says. One of the rumen metabolites of chlorophyll is phylloerythrin, which is produced when an animal eats green forage. It’s a photodynamic agent, which means it reacts with ultraviolet light if it reaches non-pigmented skin.

“This type of photosensitization can also occur in a wet fall with lush, green regrowth of pasture. It can occur any time that you suddenly switch from a drier feed to a lush, green feed,” says Knight. Bringing cattle off a dry summer pasture in the mountains and putting them into a lush, green, irrigated field containing legumes can cause problems. 

“The good thing about this problem is that the excess phylloerythrin does not damage the liver. The animals will recover if provided shade and fed hay,” Knight says.

Secondary photosensitization

When liver damage occurs, however, the problem becomes more severe. “Plant or algae toxins sometimes cause liver damage,” says Knight. Once it is damaged by the plant toxins (pyrrolizidine alkaloids), the liver is unable to excrete the various products of chlorophyll metabolism, which include phylloerythrin.

Green forage plants generally contain a lot of chlorophyll. “In the healthy animal, phylloerlythrin is absorbed in the circulation that goes through the liver. If the liver is doing its job, it conjugates the phylloerythrin and excretes it in bile, routing it back into the intestines for elimination. It never gets into peripheral circulation and doesn’t get to the skin,” explains Casteel.

When liver function is impaired, however, the byproducts of chlorophyll are not properly eliminated. They spill into the blood, reaching capillaries in the skin, where they produce the photosensitization reaction.

“When too much phylloerythrin moves into the skin, photosensitization occurs when sunlight hits the skin. Although this type of photosensitization is most common when grazing green pastures, it can also occur in animals fed hay,” he says. There can be enough chlorophyll in hay to produce critical levels of phylloerythrin in tissues of animals suffering from liver damage.

“This is a much more serious form of photosensitization because [the skin damage] is secondary to severe liver damage,” says Knight. “This damage can be caused by a multitude of things, including blue-green algae. When it blooms on standing water and ponds in late summer, it contains a toxin that affects the liver,” he says. 

“Of the cases of photosensitization I’ve dealt with over the years, 90% 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 or failure can potentially kill the animals even if they don’t show photosensitization [such as dark-colored animals with pigmented skin],” says Casteel.

“More commonly, secondary photosensitization occurs when certain plants containing pyrrolizidine alkaloids are consumed over a period of weeks, damaging the liver,” Knight says. These alkaloids are common in some wildflowers such as the Senecio species (groundsel). There are about 400 to 500 species of senecio, including tansy ragwort.

In Colorado, Kansas and Nebraska, there is a senecio called Ridell’s groundsel that has been associated with liver damage and secondary photosensitization in horses and cattle. In the Southeast, a species of Crotalaria, rattlebox, causes problems. In the Southwest, similar problems occur with fiddleneck (Amsinckia spp.). 

“Some of these plants become invasive in pastures and can remain toxic in hay after the plants are dry,” says Knight. Thus, photosensitization can also occur in winter when hay is contaminated with plants containing pyrrolizidine alkaloids. “An invasive weed that has spread to many areas of North America is houndstongue [Cynoglossum officinale], which contains these alkaloids. It often invades hay meadows and remains toxic when fed in hay,” says Knight.

Cocklebur will also cause liver damage, and it usually produces enough damage that the animal dies before it shows signs of photosensitization. Other plants that have been associated with photosensitization include horse brush, panic grass and smartweed species.

Another plant in parts of the West and Midwest that can cause liver damage and photosensitization is kochia weed. This is a different plant than forage kochia. Some years, kochia weed causes photosensitization in cattle due to liver damage, but not consistently. This suggests there may be an unknown mycotoxin involved. Moldy grain infected with the fungus Aspergillus will also cause liver damage and secondary photosensitization. Various types of mold and fungi can damage the liver.

What to do

“The more the animal keeps eating the offending plant, the more damage occurs. Out in the sun without any shade, it can be a full-thickness burn. Damaged capillaries in the skin results in skin loss,” Knight explains. It is crucial to get the animal out of the sunlight and off the feed that caused the problem, and to treat any inflammation or infection secondary to the photosensitization.

“You can put these animals in a barn and let them graze at night,” says Casteel. “If they have severely fissured skin, or it sloughs away and leaves raw areas, you need to keep the flies off — or there will be maggots in those fissures. Antibiotics may be necessary to prevent bacterial infection,” he says.

“Inspect the pasture and hay that the animals have been consuming in the present and past,” says Knight. “It may take many weeks for sufficient liver damage to occur before the animal shows signs of liver damage and photosensitization.”

Your veterinarian can do liver function tests and possibly a liver biopsy to see if the photosensitization is primary or secondary. The prognosis is poor if there is liver damage. “Generally, if an animal is showing marked photosensitization secondary to liver damage, approximately 80% of that animal’s functional liver is permanently destroyed. There is no effective treatment for severe liver damage, and the animal will eventually die,” Knight says.

Heather Smith Thomas is a rancher and freelance writer based in Salmon, Idaho.

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