Unlike humans and other animals, horses aren’t typically affected by tumours. The majority of tumours that horses do develop are of the skin. Three types of skin tumours affect horses: sarcoids, squamous cell carcinomas and melanomas. They are more than just “bumps” and should be taken seriously.
“In general, all of these tumours would be considered by most people as cancers. They can all cause disfigurement, disability and, in some cases, the death of the horse,” said equine tumour expert, Bruce Wobeser, Assistant Professor in the Department of Veterinary Pathology at the Western College of Veterinary Medicine, Saskatoon, SK.
Sarcoids
Sarcoids are a tumour of the normal cells of the tissue beneath the skin called fibroblasts, which can be transformed and become cancerous. Rarely deadly, they are persistent, unsightly and are “by far, the most common tumour that occurs in horses,” Wobeser said.
They are believed to be caused by infection from bovine papillomavirus (BPV). “This is unusual because papillomaviruses are normally species-specific and so a bovine papillomavirus would not be expected to cause disease in a horse. The virus seems to drive replication of the fibroblasts in an uncontrolled manner. Over time, these proliferating fibroblasts stop responding to normal cellular control mechanisms and become a cancer,” explained Wobeser.
The exact cause of transmission is unknown. Researchers generally believe the virus is transmitted by flies. There could be a connection between wound sites on the horse and fly bites. There is also some speculation it could be passed on by shared tack or equipment.
Sarcoids can develop anywhere on a horse, but are most frequently found on the ears, around the eyes, on the legs and abdomen. Six basic types of sarcoids have been identified and are described based on how they look clinically.
Types of Sarcoids
Occult – grey, scaly, circular areas of hair loss, often mistaken for ringworm;
Verrucous – flat, wart-like, scaly, grey-black;
Nodular – cylindrical bumps lying in and under skin;
Fibroblastic – raw, ulcerated mass, often mistaken for proud flesh;
Mixed – has the characteristics of two or more of the above;
Malignant – aggressive, affecting surrounding tissue.
Sarcoids are very difficult to deal with because they can transform from the more benign occult-type to a fibroblastic sarcoid-type in what Wobeser calls “an unpredictable fashion.” And, in very rare situations, these tumours can also resolve spontaneously. “We don’t understand why this transformation occurs or why they will resolve spontaneously,” he said.
“Some horses that develop sarcoids may develop dozens of these tumours. This suggests that there may be a genetic component to their development, but we don’t entirely understand what this is.”
Successful treatment is difficult because sarcoids usually reoccur – sometimes growing even faster and worse than before action was taken. For this reason, veterinarians often recommend steering clear of any type of management, instead monitoring the tumour regularly for growth or changes. Treatment is also generally time-consuming and expensive.
Surgically excising or debulking (cutting away) the growth with a scalpel is the most common treatment. In order to be successful, however, every tumour cell must be removed or destroyed – and that is a difficult task. For this reason, many veterinary surgeons find cryosurgery, in which liquid nitrogen is used in freeze-thaw cycles to remove the tumour, or laser surgery more effective at removing all the cancerous cells.
Surgery is often followed up with chemotherapy or chemotherapy is used on its own to obstruct the growth and spread of malignant cells. The anti-cancer drug Cisplatin is used most often. Cisplatin is injected directly into the tumour several times within two to five weeks or slow-release beads are injected once, releasing the drug over the course of a month or so.
In some cases, veterinarians will prescribe a topical chemotherapy such as 5-fluorouracil cream that kills the affected tissue. For sarcoids around the eye, the chemotherapeutic drug, Mitomycin C, which is used in human ophthalmology, is sometimes used to shrink growths.
Non-chemotherapy topicals include the anti-viral Acyclovir, which is considered safe and effective treatment for mild sarcoids and Xterra, an ointment made from bloodroot extract and zinc chloride.
Immune system stimulation – or immunotherapy – is also used to treat sarcoids. The intention is to provoke an immune reaction from the horse’s body to destroy or reject the sarcoid tissue. One such method of immunotherapy is injection of BCG (Bacillus Calmette-Guerin – a material used for human tuberculosis) into the tumour. BCG is used mainly for sarcoids around the eye. It is not recommended for use on limbs because it can make sarcoids much worse. Treatment can take several months.
Topical immunotherapies such as Aldara (Imiquimod) cream are also widely employed, but treatments are often lengthy and can cause serious and painful inflammation.
Another form of immunostimulation, called autogenous vaccination, involves grinding piece of sarcoid and injecting under the mane to prompt the horse’s body to produce an immune response. Although, according to equine sarcoid expert Derek C Knottenbelt of the University of Liverpool in the United Kingdom, with autogenous vaccination “overall results are singularly poor; it sometimes makes the condition even worse.”
Various radiation therapies are probably the most effective sarcoid treatment. In fact, one study from the University of Liverpool reported a success rate of 98 per cent in the treatment of sarcoids around the eye. At this point in time though, it is not widely available. Radiation is expensive and requires the horse to be treated in a facility that is licensed for radiation therapy. These are few and far between. Safer and more specific means of using radiation are being developed, so this could be more widely-used treatment in years to come.
“As far as prevention goes there is no way to predict which horses are likely to develop these tumours,” said Wobeser. “You would think that because they are caused by a cattle virus that exposure to cattle would be a risk factor, but this is not always the case. Exposure to bovine papillomavirus seems to be common in horses regardless of whether they are associated with cattle or not. Why some horse get these tumours and others don’t is not known.”
Squamous cell carcinomas
Squamous cell carcinomas (SCC) are malignant tumours of the epithelial cells of the skin. They are locally invasive and can be very destructive. SCCs are slow-growing, typically wart-like, and are often not spotted until the lesions are more advanced. Any changes in pigment or to the surface of the skin should, therefore, be examined by a veterinarian immediately because, if left untreated, SSCs can metastasize to lymph nodes and internal organs and become fatal.
It has been traditionally thought that SCCs occur when ultraviolet light from the sun penetrates skin cells, damaging DNA and causing abnormal cell growth. Horses with non-pigmented or white skin (such as Paint horses and Appaloosas) are susceptible to these skin tumours because they don’t have protective melanin pigmentation, particularly in areas where skin meets mucous membranes. They are the most common tumour in and around the eye. SCCs can also be found at the mouth, anus and genitals, and “fleshy ulcerated masses that can be quite large on the penis,” according to Wobeser.
One of Wobeser’s current research projects investigates the possibility that SCCs may, like sarcoids, be caused by a papillomavirus. “In this case though, it’s an equine papillomavirus that is thought to be the cause,” he said. “If caused by a virus, there is the possibility that these could be a sexually transmitted disease in the same way that cervical cancer in women can be a sexually transmitted disease through infection with a different papillomavirus.”
Treatment can be difficult because of the hard-to-access locations that are typically affected. Nevertheless, aggressive action can prevent eventual metastasis.
Surgery by standard excision, cryosurgery or laser can be successful if the SCC is caught early. It’s not always possible to remove all the affected cells, however, and chemotherapy is a common follow-up. The anti-cancer drug Cisplatin is reportedly not as effective for squamous cell carcinomas as it is for sarcoids. Mitomycin C eye drops are sometimes prescribed for the surface of the eye and surrounding areas.
Radiation therapy is said to be very effective in SCC removal, but, again, is expensive, difficult to find and also can prove risky to practitioners.
It is suggested that a degree of SCC prevention might be obtained by using sunscreen, flymasks, flysheets on exposed white areas and avoiding access to ultraviolet light. Regular cleaning and inspection of the genitals, especially the sheath and penis, can also aid in spotting irregular skin changes that should be examined by a veterinarian.
Melanomas
A melanoma is a tumour of melanocyte cells, which live in the skin and produce the pigment melanin.
Although melanomas can occur in horses of any age, they are more common in older horses, particularly grey or white animals. Published research shows that the majority of grey or white horses over the age of 15 will develop these tumours. It is not known what causes melanomas, although it is believed there is a genetic or hereditary component based on mutations linked to coat colour. There is speculation that sunlight could also be a factor.
Melanomas are generally located under the tail, on the dock, groin and around the anus.
“Initially they appear as small dark skin nodules. If left untreated, over a period of time (often a few years) approximately two-thirds of these will become malignant, progress to multiple large masses and can metastasize through the body and will kill the horse,” explained Wobeser.
Treatment options are few and depend on the number of melanomas, their size, location and effect on the daily lives of the horses and their owners. While some veterinarians prefer to simply monitor smaller lesions, others are more aggressive in their approach.
As literature from Virginia Maryland Regional College of Veterinary Medicine’s Center for Comparative Oncology, which has several equine melanoma research projects underway, states: “early treatment of small lesions is desirable (before they grow extensively).”
As is the case with other forms of equine tumours, surgery under local or general anesthesia, is often the first line of treatment with or without some form of chemotherapy follow-up.
Oral Cimetidine (a human anti-ulcer medication) is sometimes used to reportedly good effect by some veterinarians to slow growth, decrease tumour size and prevent development of new melanomas. Resistance to the drug can occur.
There is no preventative treatment available for melanomas.
Early Detection Essential
No matter what type of skin tumour, or skin tumours, you suspect your horse may have, early detection is essential. Owners should call a veterinarian right away to examine a suspicious area because, as Wobeser explained, “Many of these tumours can look the same and may resemble other conditions. For example, sarcoids can resemble ringworm infections, warts, SCC, ulcerated wounds or proud flesh depending on the type of sarcoid. So to make a definitive diagnosis typically requires a surgical biopsy.”
There is no absolute preventative measure for any of the three tumour types, but, according to Wobeser, as sarcoids and SCCs may be caused by papillomaviruses, it is possible that vaccinations could be developed to prevent these tumours “like the vaccines that have been developed to prevent cervical cancers in women, which are also caused by a different papillomavirus,” he said.
Meanwhile, Wobeser explained, a vaccine to treat melanomas has been developed for use in dogs and there exists the possibility for development of a similar vaccine for treating melanomas in horses. An active ingredient in the vaccine targets tyrosinase, a molecule present in melanoma cells. It tells the immune system these cells are foreign and stimulates the body to find and eliminate them.
Many unanswered questions remain about why these skin tumours occur, their prognosis, treatments and prevention. Although rare, they are stubborn, disfiguring, potentially performance-inhibiting, and occasionally fatal. That certainly means they are more than just your average bump on the skin. Continuing research and understanding of into equine tumours may one day render them diseases of the past.
Interesting Treatments To Note
A French study released in 2011 showed that electrochemotherapy shows promise as a safe, effective treatment either when used alone or in combination with surgery. Electric pulses allow Cisplatin to penetrate better and in higher concentrations than when the drug is merely injected. After four years of study, the non-reoccurrence rate was 99.5 per cent for individual tumours. General anesthesia is required and that can drive up the cost.
Another possible future treatment is thermochemotherapy, which uses heat and chemotherapy in conjunction and supposedly is a highly targeted therapy that does no damage to surrounding healthy tissue.
In a study released in 2011 in the Journal of Veterinary Internal Medicine, Swiss researchers found an extract of mistletoe called viscum album showed promise. In the horses involved in the study, the sarcoids completely or partially regressed in 41 per cent of the horses treated.
