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Sarcoids are caused by bovine papilloma virus (BPV). However, it appears that the virus requires genetically susceptible horses in order to cause sarcoids; in other words, not every horse exposed to the virus will develop sarcoids whereas those that are genetically susceptible are likely to keep developing sarcoids.

As the susceptibility remains, horses that have been successfully treated for sarcoids often get them again. The virus becomes incorporated into the DNA of infected skin cells and causes transformation of those cells into tumour cells. This is not unique to the horse: papilloma viruses do this in other species, notably in humans where papilloma viruses cause cervical cancer in women.

Are sarcoids a contagious disease, spread from horse to horse or cattle to horse? It is possible that sarcoids spread contagiously and this is something that has worried some people but, as yet, the ability for sarcoids to transmit by either direct horse-to-horse contact or indirectly by flies is unproven. Although parts of the virus (DNA and protein) have been detected on flies, infectious (whole) virus has not been detected. Some owners of sarcoid-affected horses have run into difficulties registering horses at livery yards because of fears of transmission to other horses but, at present, there is no evidence to suggest that horses affected by sarcoids are a threat to others. 

Sarcoids have a range of appearances and behaviours. Different types of sarcoids have different levels of aggression and need different treatments. It is therefore vital to identify which type of sarcoid your horse has. Inappropriate treatment can make sarcoids more aggressive, in particular in situations when treatment fails and the sarcoid grows back. In this type of situation the sarcoid will often reappear in a more rapidly growing form and may change its behaviour making it more difficult to treat, which underlines the importance of correct identification.

Biopsies are not worthwhile unless there is any doubt about the lump is a sarcoid or not. The reason for this is that all sarcoids look alike under the microscope and biopsy can make sarcoids more aggressive.

The main types are sarcoids are:

• Verrucose – These are slow growing and have flat, scaly tumours and look like scars or ringworm. They are the least aggressive sarcoid type. 
• Nodular – These are well demarcated lumps, which may be covered by normal skin or may be ulcerated. They have a spherical appearance and may have a wide, flat base or narrow stem-like base. They have a medium growth rate and their behaviour may change over time. 
• Fibroblastic – These are aggressive tumours that grow rapidly and are locally invasive, possibly invading down into the tissues underneath the skin. They might not be well demarcated and often occur in clusters of tumours of variable size and shape. They have an irregular appearance and because they grow rapidly are often ulcerated. 
• Mixed sarcoids are combinations of the above three types and it is fairly common for horses to develop multiple sarcoid types in one region or for there to be multiple sarcoid types present at different sites around the horse.
• Occasionally horses develop malignant sarcoids which are highly aggressive and spread locally via lymph vessels producing lines of sarcoids spreading from the original tumour site. 

There is no universal best treatment for sarcoids which has resulted in many different treatments being proposed and used. The different behaviour of sarcoids means that different treatments are appropriate for different circumstances and also that a ‘one size fits all’ approach to treatment is not appropriate.

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 risk of deforming the eyelid and are also not suitable for some topical chemotherapy treatments because of the risk of collateral damage to the eye itself.

Medical treatments include the immune stimulant Bacillus Calmette Guerin (BCG) vaccine (which is used to prevent tuberculosis) injected into the tumour; the injectable chemotherapy drugs cisplatin and Mitomycin C which act by interfering with DNA copying in tumour cells; the topical chemotherapy cream AW4-LUDES (‘Liverpool cream’); ointments containing extracts of the blood root plant, and various other natural remedies. Other treatment 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.

Surgical treatments include surgical excision, cryosurgery (freezing) and laser surgery. Surgical excision without additional therapy has poor success rates. Surgery followed by freezing (cryotherapy) improves success rates somewhat but the majority of sarcoids still return following this approach.

Electrochemotherapy (ECT) is available at the RVC offering an affordable and safe treatment 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 into 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. 

• Sarcoids are persistent and progressive skin tumours
• Genetically susceptible horses develop sarcoids as a result of exposure to bovine papilloma virus (BPV)
• Sarcoids commonly appear on the head, face, chest and groin 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. 
• There are different types of sarcoids and because they behave differently, and respond differently to treatment, they need careful veterinary assessment 
• There are different types of sarcoids and 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 make the situation worse