What The Muck Is That? Photosensitization

Each week we will investigate one equestrian conundrum in “What the muck is that?” This week: Photosensitization.

WARNING: This post contains graphic photos of a very serious skin condition.

Horses in California — as many as 70 of them, some say — are suffering from a mystery ailment that many believe is photosensitization.

So what is Photosensitization?

Merck Manual reads:

Photosensitization is a condition in which skin becomes overly sensitive to ultraviolet light (sunlight). This condition is not sunburn, although the difference can be difficult to distinguish. Photosensitization occurs when certain compounds that are activated by light are present in the skin, and the skin is then exposed to ultraviolet light. The molecules present in the skin are energized by the light. When the molecules return to a less energized state, the energy released causes chemical reactions in the skin. Many chemicals, including some that are fungal and bacterial in origin, may act as photosensitizing agents. Affected areas are usually those that are lightly pigmented or that have little hair, such as the lips, eyelids, and tips of the ears.

In laymans terms, the horse eats something it should not, exposes its white parts to sunlight, and then…

There is redness, swelling, scabs and crusts.  More severe cases can have open draining sores and peeling skin.

Photodynamic agents (what causes the skin to react) can be found in:

  • Chemicals (fly sprays, coat conditioners, etc.)
  • Plants (Clover, St. John’s Wort, and Rye among others)
  • Wood preservatives
  • Moldy feed
  • Veterinary medicines (tetracycline and phenothiazine tranquilizer)

St. Johns Wort

Perennial Rye Grass

Alsike Clover and moldy, aphid-infested Alfalfa are the most well-known culprits.

What’s the prevention and treatment plan?

Prevention starts in the pasture and feed room.  Overgrazed pastures contain a lot of weeds.  Rotational grazing and periodic applications of herbicides can prevent weed development.  Also, always check your hay bales and grain for mold before feeding.

If you suspect photosensitization, remove the horse from the pasture immediately, check all your feed sources carefully for mold, and stop the application of all chemical sprays.  The horse should be kept indoors away from direct sunlight and fed only clean hay.

The areas of affected skin should be cleaned with antiseptic soap daily and if needed, a cream such as Desitin can be applied to soften the scabs and soothe the skin.  In severe cases, steroids can be administered by a veterinarian.  Full recovery can take months.

If a horse develops photosensitization more than once, the liver is most likely involved.  The Horse states, “You need to have your veterinarian perform a thorough work-up, which includes blood testing to assess whether there is liver impairment. That will markedly direct the work-up thereafter and make a tremendous difference in prognosis.”

Go Riding!

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