The best strategy for treating sarcoids is selecting the treatment method most likely to be successful in the first instance, an expert says.
Sarcoids are believed to be the most common skin tumor of the horse. They affect about 2% of horses worldwide.
Although they do not metastasize (spread to distant locations), they undoubtedly cause welfare concerns, especially in the ulcerated “fibroblastic” form.
This may be through discomfort from the lesions themselves, from fly interference with the lesions, and also from the treatments given.
In their most extreme forms, they can affect eyelid function and lead to secondary ulceration of the eye’s surface.
“Clearly, they are not a ‘benign’ lesion, despite their benign classification, and they should never be ignored,” Dr Anna Hollis writes in the latest issue of Equine Disease Quarterly.
Hollis, from Britain’s Animal Health Trust, says the presence of sarcoids also has financial implications, not only due to the (sometimes very high) cost of treatment, but also due to reduced resale value.
Hollis, who is the trust’s Equine Referral Clinic director, is undertaking research on why equines develop sarcoids in a bid to better understand this cancer.
“There is,” she says, “compelling evidence that sarcoids are caused by a bovine papillomavirus which is believed to be transmitted by flies, most likely from infected cattle, but possibly also from infected horses.
“It remains unclear exactly how the virus leads to neoplastic (cancerous) change, or why the virus is able to cause disease in more than one species.”
Interestingly, she writes, a human papillomavirus also is responsible for the vast majority of cases of cervical cancer and an increasingly large proportion of tumors of the head and neck in humans. Clearly, there is much to be learned about the implications of infection with papillomaviruses in all species.
“There is little doubt that there is a heritable component to the disease,” she says of sarcoids.
“In warmbloods, the heritability has been well demonstrated: Breeding two sarcoid-affected horses vastly increases the risk of producing a horse that develops sarcoids at some stage in its life.
“There also appear to be breed-related differences in sarcoid risk: Thoroughbreds are more likely to develop sarcoids than Standardbreds, and Lipizzaners seem more resistant to sarcoid development.
Unlike melanomas, there is no color predisposition and gender doesn’t affect the chance of developing the disease.
“Specific gene variations, noted in horses with sarcoids, are also associated with virally induced cancers in humans.”
The reasons for this association are unclear, she says, but it is likely due to differences in immune function and may explain the apparent breed-specific variation in sarcoid risk.
Treatment options for sarcoids are numerous, with no one option being suitable for every case.
Traditionally, sarcoids were often left without treatment, but they almost invariably become larger and more difficult to treat. Early intervention is therefore strongly recommended to avoid long-term consequences.
Treatment may involve topical or intra-lesional chemotherapy or immune modulators, surgical procedures, or radiotherapy.
Electrochemotherapy is a relatively new alternative treatment option with apparently very good results, but it unfortunately requires the use of multiple general anaesthetics, so the risks must be carefully considered.
Appropriate treatment selection is largely dictated by the location and type of the lesion.
Hollis says laser surgical removal is her treatment of choice for the majority of lesions in locations other than around the eye. Lesions around the eye are treated via radiotherapy with great success.
Reported success rates for laser surgical excision are in the order of 83% and for radiotherapy are about 90%. Compared to other treatment options they represent the least risk to the horse and lead to the best results.
“However, not every lesion — or horse — is a suitable candidate for these treatments, and the key is to select the method most likely to be successful in the first instance.”
Recurrent sarcoids become increasingly difficult to manage and convey a far worse prognosis than those appropriately treated in the first place.
Equine Disease Quarterly is funded by underwriters at Lloyd’s, London.