Neoplasia is generally an uncommon occurrence in horses. "As a species, horses appear to have less of a predisposition to
cancer," says John Robertson, VMD, PhD, director of the Center for Comparative Oncology at The Virginia-Maryland Regional
College of Veterinary Medicine. "The overall incidence of neoplasms in horses is lower than in other long-lived species, i.e.,
humans, cats and dogs."
Melanoma as parotid mass.
Though rare, several forms of cancer are found in equids, most commonly tumor types of the skin, including primarily melanomas
in gray horses, equine sarcoids and squamous cell carcinomas.
Horses don't get many tumors, but it isn't because they don't live long enough to get them. They appear to be resistant to
them as a species. The main target organ for neoplasia in the horse is the skin, and it is thought there are differences by
breed and by pigmentation, just as there are in dogs.
At a recent AAEP session, one veterinarian noted that the answer to why horses rarely get cancer is that they are herbivores.
"That's not a statement that can be backed up with published research results." Robertson says. "To counter that thought,
by the age of 15, 80 percent of horses that have a pale pigmentation in their coat will have melanoma. We're talking about
a significant number of animals. In fact, there may have been selection against the gray coat color based on an early recognition
that melanoma might develop in these animals. This has been described in the literature for a couple-thousand years. The Romans
actually recognized the fact that melanoma was out there. This is an old problem in search of modern answers."
Gross appearance of melanoma on intestinal tract at necropsy.
Though in relatively low incidence in horses compared to dogs, cats and cattle, it has been reported that 3 percent of horses
presenting either for treatment of clinical disease or for necropsy had neoplasms of various organs and tissues. Cutaneous
neoplasms are the most common type of equine tumors, accounting for 45 percent to 51 percent in some studies. In several studies
of equine surgical biopsies, between 56 percent and 72 percent were sarcoids, melanomas or squamous cell carcinomas. The incidence
of skin neoplasms in horses is very likely to be significantly higher than what is published in the veterinary literature
because some of the most common tumors (melanomas, sarcoids) do not routinely undergo biopsy, sources say.
Of 3,115 equine admissions in 2005 that were diagnosed by biopsy, there were 15 squamous cell carcinomas, 11 melanomas, and
two others, notes Sue Lindborg, veterinary technician at the University of Pennsylvania New Bolton Center.
We see relatively few numbers of cancer patients in the hospital, says Fairfield Bain, DVM, MBA, Dipl. ACVIM, ACVP, ACVECC,
Hagyard Equine Medical Institute. "But I do have the opportunity to review biopsies from several patients from the field as
well as the few we do see here in the hospital. Sarcoids represent the most common lesion through my biopsy service. Squamous
cell carcinomas, usually involving the eyelids, nose or other non-pigmented area of skin, are next in frequency."
Peritoneal lipoma: The development of peritoneal lipomas can result in strangulation of bowel and the immediate need of surgical
Though Bain says he suspects melanomas are fairly common, his practice doesn't see a great deal, presumably because of their
"Very rarely will we see a patient with nodular skin lesions that end up being cutaneous lymphoma," he says. "I have seen
three adult mares over the past 10 years with this diagnosis, clinically presenting with widespread cutaneous nodules and
plaques of varying size. Their clinical course waxed and waned for a period of time until the disease became more progressive.
A rarely diagnosed skin tumor is a basal cell tumor fitting in the miscellaneous category. I can only recall seeing ovarian
adenocarcinoma in one patient in the last 23 years."