PHOTO: Dr. Anthony Knight
Heather Smith Thomas
March 29, 2019

Q: One of my cows appears to have severe sunburn, and her skin is actually peeling off. What can I do?

A: Your cow is suffering from a type of sunburn photosensitization. This serious skin condition generally affects nonpigmented skin such as white areas on the animal or cattle with light-colored skin. Black skin is less affected because it has more protection from the sun’s rays. Nonpigmented skin dies and sloughs away because the damage is much deeper than ordinary sunburn that affects just the top layers.

Pigments in plants that the cow eats cause it. If these get into the blood and travel to the skin, the damage occurs when sunlight causes the pigments to fluoresce and “burn” the nonpigmented skin.

In cattle, this hyperreactivity of skin to sunlight’s ultraviolet rays often signals forage-induced liver disease (in which the skin problem is secondary). This is even more serious. The liver is a filter, screening out and eliminating harmful substances. If the liver can’t filter out the plant pigments, those pigments enter the bloodstream, travel to the skin and react with sunlight to form this bovine sunburn.

Photosensitization can be a primary condition if a healthy cow eats certain plants containing a photo-sensitizing compound called phylloerythrin. A few plants, including buckwheat, smartweed and Saint John’s wort, contain these photoreactive compounds. Buckwheat is sometimes used as a grain crop, and cattle like it. The other photosensitizing plants are considered weeds.

Primary photosensitization can also occur if cattle are put into lush green legume pastures (alfalfa, bird’s-foot trefoil, cicer milkvetch) or fed green, leafy alfalfa hay, especially if they have previously been on poor-quality hay or pasture. The green plants have so much chlorophyll that it overloads the liver with a flush of phylloerythrin that produces the photosensitization. Primary photosensitization can occur when cattle suddenly switch from a drier feed to a lush green feed. The more they eat the offending plants, the more sunburn damage occurs. If cattle are in the sun without any shade, it can become a full thickness burn.

Once removed from the offending feed or pasture and given protection from sunlight (and fed nonlegume hay), affected cows can fully recover from the sunburn—with new skin growing—over a period of several weeks, if there is no damage to the liver.

If your cow shows signs of photosensitization, get it out of the sun and off the feed that caused the problem, then treat the inflammation and any secondary infection. Put the cow in a barn, and let it graze at night.

If the skin is severely cracked/crusty from the sunburn or it sloughs away and leaves raw areas, keep flies away or maggots will soon inhabit those areas. You might need antibiotic salve to prevent bacterial infection. If her udder and teats are affected, milking or nursing a calf will be painful. You might have to apply soothing ointments over the raw areas.

A veterinarian can tell whether the photosensitization is primary or secondary. Generally, if an animal shows severe photosensitization secondary to liver damage, the prognosis is poor. There is no effective treatment for severe liver damage, and the animal will eventually die. In either case, you should investigate what the cow ate and remove that feed from her diet or remove the toxic weeds or photosensitizing plants from your pastures to keep this from happening again.

This story originally appeared in the July/August 2018 issue of Hobby Farms.

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