Springtime brings the warming sun and lush plant growth, but it can also bring danger to some grazing cattle. The culprits are certain plants that result in a skin disease called photosensitization.

Eating these plants makes some cattle more sensitive to the sun. The penetration of light into the sensitized skin causes cell death, swelling and itching. Patches of skin may even slough off.

The damage is due to sensitization of unpigmented skin, which is unprotected from ultraviolet rays, according to Stan Casteel, a veterinary toxicologist at the University of Missouri.

The cause of this reaction, says Salmon, ID, veterinarian Robert Cope, is usually plants that create liver damage.

"When the liver malfunctions, toxins build up instead of being filtered out and some get in the blood," says Cope. When these toxins get in the blood they cause photosensitization when they reach the skin, he adds.

Iowa State University toxicologist Tom Carson agrees. In an animal with a damaged liver, normal body metabolism may cause photosensitization because by-products of plant digestion accumulate in the skin instead of being cleared through the liver, he says.

Carson points to poisonous plants as a primary culprit in damaging the liver. Although in the Midwest the specific cause of photosensitization is often not identified, Carson says.

Causes Of Liver Damage According to Washington State University's Patricia Talcott, the plants that can cause liver damage are: 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, bermudagrass, alfalfa, buttercup, bishop's weed, spring parsley and lantana.

Talcott says although less common, some plants can cause photosensitization directly because they contain preformed photo-active compounds. They are St. Johnswort, buckwheat and smartweed.

In addition to plants, an impaired liver may be due to leptospirosis or obstruction of bile ducts by flukes, tapeworm cysts or tumors. Idaho's Cope feels many liver problems in the Northwest are caused by fluke damage.

Whatever the cause, Talcott says "anything affecting the liver's ability to metabolize or excrete harmful compounds through the bile system predisposes an animal to photosensitivity."

When To Watch For It The number of photosensitization cases will vary with weather and growing conditions. Talcott says, "Every spring and summer we see cases, but it generally affects only a small number. It's often a springtime problem when livestock are put on pasture. The liver may have difficulty metabolizing all the chlorophyll (the green matter in plants that accomplishes photosynthesis), especially if there's impairment from previous injury."

Iowa State's Carson says photosensitization can even occur on wheat stubble, if cattle were on hay all winter then turned in on a green field. "With the sudden change in forage, they don't have a handle on excess chlorophyll or poor metabolism of it at that time. You get the same thing as if there were liver damage, but there isn't. The ruminant animal needs time to adapt to the green feed."

He says, "We've also seen cases in April when it was cloudy all spring. Then we had clear weather and the cows - still on hay - developed photosensitization due to the sudden bright sunshine."

This problem can also be common in the summer when sunlight is intense and animals are eating green plants. "There is more ultraviolet light penetrating the atmosphere in summer," says Cope. He's also seen cases occur in spring when grass is short and animals eat strange plants.

Though most common on green pastures, it can also occur in animals fed entirely on hay. There is enough chlorophyll in hay or hay pellets.

"Toxicity of hay varies with growing conditions and stage of maturity at harvest. We see the most problems in wet years or if hay is overmature and put up in wet, humid conditions," says the University of Missouri's Casteel.

Symptoms Photosensitivity is most common in white-faced or white animals but can affect colored cattle, too.

Unpigmented skin exposed to the sun, such as an animal's face, back or udder, is most affected. Talcott says if the condition is severe, pigmented areas can be affected, too.

"The easiest way to diagnose photosensitization is to see if there's a pattern to the lesions," says Cope. "There'll be abrupt differences where light and dark skin meet."

The skin may be red and swollen, developing blisters that may crack and ooze or have painful scabs. Patches of skin may slough off. In a white face, eyelids may swell and eyes water. Breathing can be impaired if there's much facial swelling. If skin lesions are extensive or severe, temperature and pulse will rise and in serious cases the animal may go into shock.

An afflicted animal will go off feed, lose weight and be in pain when in sunshine. Teats can become cracked, so a cow may kick her calf or be hard to milk, says Carson. Mastitis may even develop.

Horses can also be affected by photosensitivity, Carson says. It's not uncommon for the white blaze on a horse's face to slough off from photosensitization, he says.

Treatment Remove affected animals from the feed that caused the reaction and put them in a barn out of the sun - to halt further reaction in the skin. Let the animal out at night to graze, says Talcott.

Steroids (such as dexamethasone) reduce inflammation, soreness and swelling, but may cause abortion if given in late pregnancy. Since these drugs suppress the immune system, Carson says steroids should only be given if there's a lot of discomfort.

Talcott says washing the lesions and keeping the skin clean can help. If a large area is affected she uses Banamine to ease pain and a systemic antibiotic such as LA-200 to combat infections. Ointments on raw areas can ease pain and prevent infection. Intravenous fluids should be given if there is depression and shock. Most affected animals make a complete recovery.