One in four dogs and one in six cats will be affected by cancer in their lifetime, but it’s not yet known exactly how common it is in horses. Cancer isn’t often associated with equines, and they aren’t as susceptible to cancer as dogs, cats and humans. However, there are three main types that commonly affect them – sarcoids, melanomas and squamous cell carcinomas.
Sarcoids are a type of skin cancer and are the most common form of cancer in horses. They often develop in multiple sites and, although they can occur at any age, they’re usually first seen between two and 10 years of age. They affect all types of equine, including horses, ponies, donkeys and even zebra, and colour and gender don’t seem to affect the likelihood of these tumours developing.
There does, however, appear to be a genetic predisposition, with some horses (such as Arabs) being more susceptible and others (such as Lipizzaners) being more resistant. In addition, some families of horses appear to be more prone to developing sarcoids, suggesting that there may be a specific gene or group of genes that give either greater resistance or greater susceptibility to the tumours.
Sarcoids are locally invasive (spread into the surrounding tissues and deform them), but don’t metastasise (spread to lymph nodes or internal organs). Although they’re rarely life-threatening, they can be distressing for the horse, become affected by tack and rugs, and reduce your horse’s resale value.
The cause of sarcoids isn’t completely understood, but it’s thought that a cattle virus is responsible for the disease and that it’s transmitted to horses by flies. This may explain the frequent occurrence of sarcoids around the eyes, sheath and wounds, where flies are commonly seen.
In the summer, when flies are more prevalent, it’s important to look for any changes in your horse’s skin and, if you’re concerned he may be developing a sarcoid, ask your vet to check it. If your horse already has sarcoids, stringent fly control will reduce the risk of him developing any more and limit irritation to the existing ones.
What should I look for?
There are six types of sarcoid and they’re categorised according to appearance…
1. occult (flat, hairless lesion)
2. verrucous (grey, scaly or wart-like appearance)
3. nodular (solid, round lesions)
4. fibroblastic (fleshy masses that bleed easily)
5. mixed (a combination of two or more types)
6. malignant (very rare, this has extensive local spread within the skin and the tissues immediately under the skin)
There are many treatments for sarcoids, including creams, cryotherapy (freezing) and surgery, all with varying success rates. The most difficult to treat are those around the eye, as the options are limited by the location and these are often aggressive, invasive lesions.
However, an exciting new form of radiotherapy is available for sarcoids in the eye area and gives new hope for these difficult lesions. It uses a high-dose radiation technique known as HDR brachytherapy, which is currently only available at the Animal Health Trust and one other veterinary centre in Europe.
During this treatment, specialist imaging techniques are used to form a 3D reconstruction of the tumour so the treatment can be planned. Computer software is used to calculate the dose given to the tumour and the technique is so advanced that different areas of the tumour can be given different doses, enabling a completely bespoke treatment for each horse.
When the horse is treated, the radioactive source is driven from a special shielded safe into the patient via catheters that are implanted directly into the tumour. This method drastically shortens treatment times and it’s possible to deliver the full radiation dose in 5–10 minutes for each session. A typical treatment consists of two sessions delivered a week apart, which is given while the horse is in stocks under standing sedation.
During treatment there is no operator exposure risk, and the horse is completely radiation-free as soon as the treatment is completed. The results to date are impressive and it’s expected that this method will become the standard approach to the treatment of all tumours around the eye.
Squamous cell carcinoma
Squamous cell carcinoma is the second most common form of cancer. It’s thought to develop in response to the damaging action of ultraviolet light in areas where there is no protective skin pigment or hair, although chronic skin irritation and possibly viral infections may be contributing factors. Squamous cell carcinomas can metastasise, however prompt, effective treatment can prevent the spread of this type of cancer.
The most common risk factors are a lack of skin pigmentation and areas of high sun exposure, such as the skin around the eyelids. There’s no gender predisposition to these tumours, however, breeds with little pigment in these areas, such as Appaloosas and Quarter Horses, are predisposed.
Horses with non-pigmented skin around their nose or eyes should be protected from the sun to reduce the risk of developing a squamous cell carcinoma. There are eye masks that protect against UV rays and special sun creams available, which are advisable for any horse who’s at risk.
What should I look for?
Squamous cell carcinomas often have a characteristic appearance, resembling a very small, pink cauliflower. However, they can also look like an ulcer, an unusual pink area or a crusty, non-healing wound, so if your horse has a suspicious skin lesion you should ask your vet to have a look at it.
Normally, squamous cell carcinomas are removed surgically. This is usually an effective treatment, but it can be difficult to get good surgical margins and, therefore, recurrence and spread to lymph nodes is a risk. Other treatments are often given alongside surgical removal or instead of surgery if it’s not possible. These are usually chemotherapy agents that are applied onto or injected into the tumours, or radiotherapy, although radiotherapy has more advantages.
The most effective form of radiotherapy for superficial squamous cell carcinomas is strontium plesiotherapy – currently, the AHT is the only veterinary centre providing this treatment for horses in the UK. Strontium plesiotherapy involves the direct application of a radioactive source to the tumour surface. The major advantage is that the type of radiation emitted penetrates only a few millimetres, meaning that a large dose of radiation can be applied to the surface without adversely affecting deep tissues. This means that the tumour tissue is destroyed, but delicate surrounding structures such as the eye aren’t affected.
Strontium plesiotherapy has been successfully used to treat tumours in many difficult areas in dogs, cats and horses, and represents a real advance in the treatment of these lesions. Depending on the location of the tumours, strontium can be performed under standing sedation or a short general anaesthetic.
Depending on the size of the tumour, treatments are usually given in three sessions, spaced 48 hours apart. Each treatment takes around 5–10 minutes, depending on the size of the area being treated, and between treatments the horse can be treated as normal.
Strontium is suitable for tumours that can’t be surgically removed and those that have been removed, but have small surgical margins or cancer cells along the edge of the margin.
In horses, melanoma is a type of skin tumour that’s usually associated with coat colour, with up to 80% of grey horses developing some form of visible melanoma by 15 years of age. Disappointingly, there’s currently nothing that can be done to stop horses from developing melanomas. Unlike in people, sun exposure isn’t thought to be linked to the development of these tumours. Horses with ulcerated melanomas require excellent fly control in the summer to reduce discomfort and the risk of infection.
What should I look for?
Melanomas are characteristic in their appearance. They’re rounded, raised, black nodules of varying sizes, commonly found under the tail, in the perineal and genital region, on the salivary glands and on the lips. While usually considered to be benign (non-cancerous), some are malignant and may even metastasise throughout the body. Many benign lesions become large and obstructive, and may also ulcerate and form a hole that oozes black discharge. This often causes secondary infections, pain and functional problems, such as difficulty passing droppings. Unfortunately, melanomas in non-grey horses are almost always malignant.
Traditionally, equine melanomas have been left alone instead of actively treated, because they’re usually slow-growing and don’t tend to cause pain or other problems in the early stages. However, the chance of them becoming malignant or causing problems is high and early treatment is likely to slow down or even stop the progression of the tumours.
Surgical removal is often the preferred approach in the early stages, but it can be difficult depending on the location and size of the tumours, so it’s not always a feasible option. Some melanomas can also be treated by injecting chemotherapy agents, but this can be difficult, and the results are highly variable and confined to the individually treated lesion. This method of treatment also brings up many health and safety concerns for the treating vet and the owner, because the drug is very dangerous to people should they come into contact with it.
A promising new treatment for equine melanoma is a vaccine, licensed in the USA for the treatment of melanoma in dogs, which is an option for at-risk horses. It works by causing an immune response against the melanoma cells, helping the body to fight the tumours. This means that all melanomas in the body will be targeted, including those that aren’t suitable for other treatment approaches.
A study has shown that horses vaccinated four times at fortnightly intervals develop an immune response against the melanoma. If the horse shows a positive response, the vaccine is then given at six-monthly intervals thereafter. There is currently a large clinical trial underway in North America to further investigate this treatment, but initial results are good, with a number of horses showing a reduction in growth or even regression of the tumours. This means that specialist vets in the UK can now give owners a new hope for this common and sometimes very debilitating disease.
A seven-year-old homebred gelding called The Baby has a long history of developing multiple sarcoids. Although the sarcoids on his body were successfully treated with topical creams, he developed several lesions around the corner of his eye that slowly grew and ulcerated. A biopsy confirmed that these were aggressive and active sarcoids, so the decision was made to treat him with HDR brachytherapy.
The tumours responded rapidly and completely to the treatment, and the prognosis for sarcoids such as these is excellent, despite their dramatic appearance and difficult location. As with all forms of radiotherapy, the area became slightly sore in the early stages and as it healed, depigmentation developed which gradually reduced in size. Now fully healed, The Baby has a good cosmetic result with no evidence of the sarcoids recurring so far.