Incidences of neoplasia
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.
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.
Though Bain says he suspects melanomas are fairly common, his practice doesn't see a great deal, presumably because of their benign nature.
"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."