What is a sarcoid?

Sarcoids are the most common type of tumour in equine species worldwide, and account for more than 50% of all equine skin tumours. It is estimated that up to 12% of all horses are affected by sarcoids; donkeys are also particularly prone. They are caused by the bovine papilloma virus (BPV) which contains an oncogene (a gene which has the ability to cause cancer). The oncogene allows cells infected with BPV to evade the immune response, allowing tumour tissue to grow. Sarcoids can develop in any horse, however there is known genetic breed predisposition. There is also the potential for hereditary predisposition, although this has not been fully established.

How is a sarcoid diagnosed?

Diagnosis of sarcoids is largely made on clinical examination and the visual appearance of a tumour, however histology of the tissue (analysis under a microscope) can be used to confirm a diagnosis if required. Biopsy of suspected sarcoids must be performed with great care, as interfering with the tissue can cause aggressive tumour growth in some cases. If there is an option to do so, removing the entire mass and sending it for analysis is the preferred option.

There are six different types of sarcoid, some more aggressive than others:

• Occult – this is the earliest form of the disease – identified by subtle changes in the hair coat, usually a circular area of hair loss which then becomes crusty on the surface.
• Verrucose – “wart-like” lesions with a scaly texture – these may be accompanied by a surrounding occult halo, and the overlying skin many crack or ulcerate.
• Nodular – firm lumps under the skin (the overlying skin may or may not be freely moveable) which may or may not ulcerate on the surface – may be a single nodule, or multiple nodules giving the appearance of a bunch of grapes.
• Fibroblastic – these have a fleshy and ulcerated appearance – they may or may not have a stem/pedicle attaching them to the body, and will have varying degrees of roots which can make removal particularly challenging.
• Mixed – most sarcoids are in fact a mixture of the above four types of sarcoid.
• Malignant – these are the most aggressive type of sarcoid, but thankfully are relatively rare – if they do develop they tend to be around the elbow, inner thigh or face – they are sometimes referred to as ‘malevolent’ rather than malignant because they do not spread to internal organs as other malignant tumours would.

Where should I look for sarcoids?

Sarcoids can occur anywhere on a horse’s body, but are more likely to develop where the skin is thin, where there is little or no hair, where sweating occurs or where flies are attracted to feed. It is also possible for sarcoids to develop in areas of skin trauma such as a wound site, an injection site, fly bite site, or an area which has been traumatised by rubbing (either self-inflicted or for example from poorly fitting tack). Common sites include:

• eyelids
• lips
• prepuce (sheath)
• ear
• chest
• trunk
• abdomen
• flank
• castration sites
• groin

Sarcoids usually involve the local skin and sub-cutaneous tissue, but if extensive they may also involve deeper structures such as the underlying muscle, or a joint capsule. They rarely resolve without treatment, although very occasionally occult sarcoids will resolve spontaneously (this is most commonly observed in younger horses).

Are sarcoids contagious?

Sarcoid tissue itself is not contagious, in that if a horse touches another horse with sarcoids, this direct contact will not result in the development of lesions. However, sarcoids can be spread from one part of a horse to another site on the same horse, and also be spread between horses. This is due to fly transmission of the BPV, which contributes to the development of new sarcoid lesions. As mentioned above, genetic predisposition may also play a part. A horse with sarcoids is more likely to develop more sarcoids in the future because of fly transmission, and the longer a horse’s sarcoids are left untreated, the more likely this spread becomes.

Why else should I worry about sarcoids?

• they can become painful, ulcerated or infected
• treatment options may be limited depending on the location/type of sarcoid
• sarcoids can sometimes lead to systemic effects such as anaemia
• they may interfere with tack or rugs
• sarcoids can affect a horse’s performance, especially if the sarcoid involves an eye or a joint
• flies are attracted to sarcoids especially during summer months, which may irritate your horse
• they may reduce the re-sale value of your horse
• occasionally horses require euthanasia due to sarcoids

What treatment options are available?

Treatment of sarcoids is challenging due to the incredibly high risk of recurrence, and there is no one treatment which is one hundred percent effective or one treatment which is suitable for all types of sarcoid – which explains why there are so many treatment options are available! Although some sarcoids are very small, or may appear inactive for many months or years, the benign neglect of lesions is strongly advised against. It is highly unlikely that lesions will spontaneously resolve and given the significant potential consequences of leaving sarcoids untreated, they should be treated as soon as they are identified. Costs can vary widely, from £100-200 for a simple sarcoid banding, to £700-1000 for laser excision of a single sarcoid without complications, to several thousands of pounds if there are multiple sarcoids to treat or if a more advance treatment technique is required. Treatment options to discuss with your veterinary surgeon, and their potential associated risks, may include:

• Conventional surgical excision – usually performed under standing sedation, high risk of recurrence.
• Laser surgery – performed under standing sedation or general anaesthesia using a laser to cut and cauterise the tissue, up to 60-80% success rate depending on the type of sarcoid lesion.
• Ligation/banding – involves applying a tight band around the base of the sarcoid to cut off blood flow – only suitable for selected nodular sarcoids with no significant attachment to underlying structures.
• Cryotherapy – similar to freezing a wart or verruca, has a poor success rate unless combined with other treatment.
• Intralesional cisplatin – this is where a chemotherapy agent is injected into a sarcoid, and should be combined with debulking (removing as much of the tumour tissue as possible) prior to injection if the sarcoid is large.
• Electrochemotherapy – this involves the use of electrical pulses to increase the penetration of chemotherapy agents into a sarcoid.
• Topical chemotherapy cream e.g. bleomycin, 5-fluoro-uracil, AW5/Liverpool cream (a combination of 5-fluoro-uracil, heavy metals and thiouracil) – AW5 is a commonly used treatment and has a good success rate, usually applied multiple times over 1-2 weeks.
• Immunotherapy i.e. BCG injection – currently difficult to source due to a worldwide shortage.
• Topical immune modulators – used to stimulate the immune response to kill off sarcoid cells:
• blood root ointment - no controlled studies have been performed to assess efficacy, but is a low cost treatment
• imiquimod – only one small study that found it resolved 60% of tumours
• Topical anti-viral agents e.g. aciclovir – low cost and most useful for small and superficial sarcoids, one study found 68% success rate in small occult lesions.
• Topical tazarotene (a retinoid) – can help to reduce the wart-like surface of verrucose/occult sarcoids, but rarely results in a cure.
• Radiotherapy i.e. brachytherapy – this has a high cost but high success rate, limited facilities are available in the UK.
• Many natural or homeopathic remedies for sarcoids (e.g. turmeric supplements or thuja cream) have no scientific evidence to support their use.

Most treatments cause some degree of discomfort and swelling, often requiring pain relief, and can leave healing areas that require regular bathing and monitoring. They must also be carefully monitored for sarcoid re-growth.

Should I buy a horse with sarcoids?

This may depend on the number, severity and location of sarcoids present. You should consider that sarcoids may affect the re-sale value off any horse, and depending on their location may affect a horse’s ability to perform if left untreated. If you are considering buying a horse with sarcoids they would be considered by an insurance company as a pre-existing condition, and therefore any examinations or treatment would be excluded on your insurance policy and you would need to cover the cost of this yourself. Be sure to consult your veterinary surgeon for further advice.

Article written for www.aqueos.co.uk by Jessica Putnam BVMedSci(Hons) BVM BVS(Hons) MRCVS