Two common forms of skin cancer in horses are squamous cell carcinoma and melanoma. Squamous cell carcinoma is a malignant
skin tumor of the outmost layer of the skin. It's typified by reddened, roughened or ulcerated skin.
Melanoma in horses isn't associated with exposure to the sun, but squamous cell carcinoma is, says Ginn. "Squamous cell carcinoma
is something we see often here [in Florida] on the eyelids, nose and genital region and in areas of poorly pigmented, lightly
haired skin. It's usually preceded by actinic dermatitis characterized by erosions or ulcers. Most clinicians are tuned in
to what it might be and often biopsy it. You basically have to protect the horses from sun. We also see lesions of habronemiasis,
pythiosis and actinic dermatitis, which precedes squamous cell carcinoma."
In Texas, Rees sees patients with melanoma, but squamous cell carcinoma is more common, especially in lighter-coated horses
such as Palominos or grey or white-coated horses. In fact, it tends to be a chronic problem. Squamous cell carcinoma affects
horses with greater UVA exposure to intense sun in southern climates such as in Texas, New Mexico, Arizona and Florida.
Some horse breeds, such as Appaloosa, get it at the tail. These tend to be more benign, Rees notes. But some are more aggressive.
"Some people will biopsy them to determine their mitotic rate or only remove them if they're located in a bad spot, such as
the penis, the rectum or an area affecting a bodily function. They're a little different from those seen in people in that
in some horses, especially grey horses, they aren't as aggressive a cancer."
As for treatment, Rees says some veterinarians have tried a melanoma vaccine, or they treat with cimetidine and trimediazine
to try to shrink the lesions. "Some claim it helps, though I haven't seen that it does," she says. "The only way to properly
treat melanoma is surgically."
Injection site reaction
While uncommon, some horses do show sensitivity to injections, such as from the silicon coating on the needle, and develop
a small nodule at the injection site, White says.
Ginn says she frequently sees vasculitis in horses. "In Florida, we see a lot of sarcoids." Sarcoids, which frequently occur
in areas subject to trauma, are associated with bovine papillomavirus. It's speculated that the virus may be spread by biting
flies or fomites. Sarcoids manifest as wartlike, ulcerated, nodular areas or flat plaque surfaces, usually on the ears, lips,
neck and ventral abdomen, or around the eyes.
Dermatophilosis is most likely to develop after a particularly rainy season on a horse has been left out in the moisture.
Also known as rain scald or mud fever, it's a bacterial infection aggravated by prolonged exposure to moisture, coupled with injured skin. Chronically infected
animals are usually the source of the infection for other animals; transmission may occur by flies, ticks, grooming equipment
and tack. The lower layer of hair is firmly matted in small scabs that leave a raw surface when removed (Photos 4 and 5).
The hair roots can protrude from the crust and may have a covering of yellowish-orange pus. The affected areas are sore to
the touch but are not pruritic.
Photo 4: A close-up view of dermatophilosis.
This is an abnormal inflammatory skin reaction to the sun's UV rays that occurs in areas of white hair or pink skin. The condition
is different from simple sunburn. It may occur due to ingestion of photodynamic agents (e.g., St. John's wort, buckwheat)
from certain plants or chemicals. When the horse consumes specific pigments, they may circulate to the skin where they're
exposed to UV light and cause oxidative damage to skin cells.
Photo 5: A cytologic sample of Dermatophilus congolensis, a gram-positive bacterium causes dermatophilosis.
By knowing what may be triggering a horse's skin condition, you can determine the best treatment option to help alleviate
discomfort and bring the horse back to optimal health.
Ed Kane, PhD, is a researcher and consultant in animal nutrition. He is an author and editor on nutrition, physiology and veterinary medicine
with a background in horses, pets and livestock. Kane is based in Seattle.