Sarcoids are a common condition in horses and can be serious. They are the most common skin tumours of horses, accounting for 40% of equine cancers. They affect horses of all ages.
Often wart-like in appearance, sarcoids are skin tumours (fibrosarcomas) that can be locally invasive, although they don’t usually spread to internal organs. Sarcoids typically do not resolve independently, and affected horses often develop multiple sarcoids at once or serially.
Summary
- Sarcoids are persistent and progressive skin tumours.
- Sarcoids commonly appear on the head, face, chest and groin of horses but can develop anywhere on the skin.
- Sarcoids are a form of cancer and are usually locally invasive but do not spread to other organs.
- Genetically susceptible horses develop sarcoids as a result of exposure to bovine papillomavirus (BPV).
- There are different types of sarcoids, and because they each behave differently and respond differently to treatment, they need careful veterinary assessment. An individual horse may have a mixture of different types.
- There are various treatments, and the correct treatment must be chosen for each sarcoid because inappropriate treatment can make sarcoids more aggressive and may ultimately make the situation worse.
Causes
Bovine papillomavirus (BPV), primarily types 1 and 2, has been found to cause sarcoids in some horses. Horses can become infected with the virus through wounds, abrasions, or insect bites, which allow the virus to enter the skin. While not all horses with BPV will develop sarcoids, some horses appear to have a genetic predisposition to the condition and form sarcoids if infected with BPV.
Horses that are particularly vulnerable to sarcoid development are those that are both genetically susceptible and have weakened or dysregulated immune systems (due to stress, illness or poor condition). With their immune system compromised, they are unable to clear the virus or keep the abnormal cell growth in check.
If horses are genetically susceptible to contracting sarcoids from BPV, they will likely develop the issue multiple times, even after treatment. This is because the virus becomes incorporated into the DNA of infected skin cells, causing the transformation of these cells into tumour cells.
Did you know?
Certain breeds such as Arabians, Appaloosas and Quarter Horses seem more prone to sarcoids.
Types
Sarcoids have a range of appearances and behaviours. Different types of sarcoids have various levels of aggression and need different treatments. It is therefore vital to accurately identify the type of sarcoid your horse has. Inappropriate treatment can make sarcoids more aggressive, particularly when treatment fails and the sarcoid grows back. In this situation, the sarcoid will often reappear in a more rapidly growing form and may change its behaviour, making it more challenging to treat.
There are six types of sarcoid, each of which can be found in all equid species (i.e. not just horses).
Occult
Sometimes mistaken for rub marks or ringworm, occult sarcoids present as circular, hairless areas of skin. While they may remain static for years and not present any clinical symptoms, if occult sarcoids are accidentally traumatised, they may develop into fibroblastic sarcoids, which are much more serious. Occult sarcoids are commonly seen on the nose, side of the face and the inside of the thigh.
Verrucous
Verrucous sarcoids often appear grey and wart-like. They are the least aggressive sarcoid. These kinds of sarcoid are sometimes seen in isolation but can group together and form larger lesions. Although generally not painful for horses, verrucous sarcoids can develop into a more aggressive form, such as fibroblastic or malignant sarcoids, following interference or inappropriate treatment.
These kinds of sarcoid are commonly found on the face, groin, sheath and around the legs.
Nodular
Nodular sarcoids are usually well-demarcated lumps that are covered by a layer of skin, although they can ulcerate. They have a spherical appearance and may have a broad, flat base or a narrow stem-like base. They have a medium growth rate. Like occult and verrucous sarcoids, if nodular sarcoids are accidentally interfered with or given inappropriate treatment, they can develop into more severe types, like fibroblastic sarcoids.
You’ll commonly find nodular sarcoids in the eyelid, inside thigh, armpit and groin areas of horses.
Fibroblastic
One of the more severe types of sarcoid, fibroblastic sarcoids, can be found anywhere on a horse’s body. They are aggressive tumours, which appear as fleshy masses and often have ulcerated surfaces. Fibroblastic sarcoids can proliferate, bleed easily, and develop rapidly from other types of sarcoids or at wound sites. These sarcoids are commonly locally invasive, potentially invading the tissues beneath the skin. They are often not well demarcated and can occur in clusters of tumours of variable sizes and shapes.
Malevolent
The most aggressive form of sarcoid, malevolent sarcoid, can rapidly spread over a wide area and quickly grow in size. While these types of sarcoids can result from repeated, incomplete, or unsuccessful treatments, they can also develop spontaneously. They tend to spread locally via lymph vessels, producing lines of sarcoids extending from the original tumour site. Malevolent sarcoids are rare, and treatment options are minimal. While rare, you’ll most commonly find malevolent sarcoids on a horse’s face, inside the thigh and elbow regions.
Mixed sarcoids
It is common for sarcoids to display mixed characteristics of two or more sarcoid types. While mixed sarcoids can develop anywhere on your horse’s body, they’re most commonly found on the head, armpit and groin area. Sometimes multiple sarcoid types can present at different sites around the horse’s body.
Diagnosis
Since there are so many types of sarcoids, each with different appearances, behaviours and treatments, it is vital to diagnose the types of sarcoids your horse has correctly. Diagnosing sarcoids usually occurs upon inspection of the appearance and location of the affected area. Other potential conditions should be ruled out, such as fungal infections and warts. A biopsy is not recommended, as any interference of this kind can potentially aggravate the sarcoid and make it worse. Your vet should be able to correctly diagnose the type of sarcoids by appearance and recommend the appropriate treatment.
Treatment
There is no universal best treatment for sarcoids, which has ultimately resulted in many different treatments being proposed and used. The different behaviours of sarcoids mean that different treatments are appropriate for different circumstances, and a ‘one size fits all’ approach to therapy is not suitable. In some cases, it may be best to simply ‘watch and wait’ with sarcoids and alert your vet again if anything changes.
As well as the type of sarcoid, factors affecting treatment options include the location and extent of sarcoids, treatment cost and the horse’s temperament. For example, some anatomical sites, such as the skin around the eye, are usually not suitable for surgical treatment because of the risk of deforming the eyelid. They are also not suitable for some topical chemotherapy treatments because of the risk of collateral damage to the eye.
Medical treatments include injecting the immune stimulant Bacillus Calmette Guerin (BCG) vaccine (which is used to prevent tuberculosis) into the tumour (particularly common for sarcoids close to the eye); injecting chemotherapy drugs like cisplatin and Mitomycin C into the tumour, which act by interfering with DNA copying in tumour cells; applying topical chemotherapy cream like AW4-LUDES (‘Liverpool cream’); and applying ointments containing extracts of the blood root plant.
Other treatments continue to be translated from human medicine, including photodynamic treatment, in which a chemical is applied to the surface of the tumour and then exposed to a specific type of light, which activates the chemical and kills tumour cells. Similarly, brachytherapy where radioactive implants are placed in the tumour can be very effective particularly for periocular sarcoids (those around the eye), but this treatment tends to be prohibitively expensive in the veterinary setting.
Surgical treatments include surgical excision, cryosurgery (freezing) and laser surgery. Surgical excision without additional therapy has poor success rates, as often cells are left behind. Surgery followed by freezing (cryotherapy) improves success rates somewhat; however, most sarcoids still recur following this approach. In most cases, adjunctive chemotherapy is applied after the surgical procedure o reduce the risk of reoccurrence.
Electrochemotherapy (ECT) is another option. It involves the combined use of injecting a chemotherapeutic drug (cisplatin) into the sarcoid, followed by the application of high-voltage electric pulses (electroporation). This increases the drug concentration in the cells of the sarcoid by 70 times, thereby increasing its effect. Due to the electric shock, the procedure is completed under a brief (usually 15-minute) general anaesthetic.
Are sarcoids contagious?
Although there is a possibility that sarcoids may be contagious, this has yet to be proven. Bovine papillomavirus (BPV), which can lead to horses developing sarcoids, may be spread by biting flies. However, most horses will develop BPV at some point in their lives, and only those genetically predisposed will develop sarcoids. Some owners of sarcoid-affected horses have had difficulties getting horses into livery yards because of the fear of transmission to other horses, but at present, there is no evidence to suggest that horses affected by sarcoids are any threat to others.
Prognosis
If your horse is genetically predisposed to developing sarcoids, they will likely develop them multiple times throughout their life, even after treatment. Horses treated at an early stage of the disease, when the lesions and tumours are smaller, may have a better prognosis than those treated later. Again, it’s important for the sarcoids to be correctly identified by a vet, as inappropriate treatment could worsen the problem.
Prevention
Currently, there is no vaccine against sarcoids. However, some work is being put into one for the bovine papillomavirus, which could also protect against sarcoids in the future. You can also ensure you follow correct wound management for your horse, especially during warmer seasons when there are large numbers of flies, as this might help prevent infection.