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Equine sarcoids – a common problem which can be difficult to treat

Equine sarcoids are spontaneous, locally invasive tumours of the skin of horses, mules and donkeys, affecting all breeds, ages, colours and sexes and they are the most commonly encountered equine tumours

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Sarcoids are variable in appearance, location and rate of growth and although they seldom affect a horse’s usefulness, they may do if they are in a position likely to be rubbed by tack

They are, however, unsightly and may cause considerable discomfort to the horse. They are commonly seen in moist areas of skin such as the groin, chest, neck and face and particularly in younger animals (one- to six-year-olds) and geldings.


Sarcoids are frequently associated with wound sites and often multiply. Transfer and spread by flies has been implicated. Familial tendencies have been identified and a genetic susceptibility has been suggested.


They may proliferate in size and number and may develop at other sites. Treatment is not always necessary, but where required it can prove difficult and expensive and re-growth may occur after treatment.


With regards to sarcoids identified on pre-purchase examinations, it depends on the purchaser’s intended use. If sarcoids are not in a position which affects a horse’s performance for jumping/eventing, they may not prevent the purchase.


However, if they interfere with the horse’s performance or are unsightly, they may compromise the horse’s use and affect its resale value and insurance cover.

Clinical signs

There are six types of sarcoids classified according to their appearance. This is helpful as the treatment recommendations are influenced by the type of sarcoid. They are very unpredictable and less severe forms can develop into more aggressive forms, particularly if traumatised such as with surgery by taking a biopsy.


Sarcoid types

  • Verrucous – These are slow growing, wart-like proliferations of the skin.
  • Fibroblastic – These are fleshy proliferative growths, often ulcerating and locally invasive with two subtypes: Type 1 has a narrow stalk and large fleshy outgrowth and may be amenable to banding or ligation and type 2 has a broad base with extensive spread through surrounding skin. This is the type which often appears at the site of wounds and frequently gets secondary infection which could be mistaken for granulation tissue.
  • Flat (occult) – In this type, single or multiple patches of hair loss appear which may contain nodules and they often become locally aggressive.
  • Nodular – These are hard, marble-like growths in the skin measuring about 5-20mm (0.2-0.8ins) in diameter, found particularly in the groin, sheath or eyelid. Two subtypes are recognised: Type A has no skin involvement and the skin moves freely over the nodule, whereas type B is fixed to the overlying skin and is prone to becoming more malignant if aggravated by trauma or surgery.
  • Mixed – These are very common and are transitional between verrucous and fibroblastic.
  • Malevolent – These are highly aggressive, multiple invasive tumours spread along lymphatic vessels and lymph nodes


The prognosis for all cases is very guarded and there are many approaches to the management of equine sarcoids, suggesting no one treatment is invariably effective. The choice of treatment depends on the site of the lesion, size of the tumour, aggressiveness of the tumour, cost and treatment facilities available.


The identification of sarcoid tissue should be carried out before treatment to differentiate sarcoids from other lesions such as scar tissue or skin cancers (squamous cell carcinoma or melanoma).


Treatments include surgical removal by blade or laser, cryotherapy (freezing), application of cytotoxic creams, application of human antiviral/anticancer therapies, immunomodulation (injecting bacterial cell wall extracts) and implanting radioactive rods.


The insertion of radioactive implants into the sarcoid is particularly effective for eyelid tumours, but can only be carried out in registered premises such as veterinary universities where handling radioactive chemicals is done under strictly controlled conditions, but consequently it is very expensive.




The prognosis is always guarded, however, the following cases have been treated successfully:


This mixed sarcoid appeared in the groin of an eight-year-old mare and was removed surgically. No relapse has been noted three years after the operation.


However, 50 per cent of cases which are operated on grow back, sometimes in a more aggressive form and can take up to five years to recur at the site of previous treatment.


Nodular sarcoid

Nodular sarcoid

This case had a nodular sarcoid surrounded by an occult sarcoid which partially responded to the cytotoxic cream from Liverpool Veterinary School.


However, cryotherapy on two occasions a month apart caused the sarcoid to regress completely although the affected area has remained hairless and no relapse has been noted two years after treatment.

A sarcoid near the eye

A sarcoid near the eye

This sarcoid in front of the eye was treated with BCG injections under guidance from Professor Knottenbelt of Liverpool Veterinary School. It responded fully and no relapse was noted three years later.


Injecting BCG into sarcoids can provoke an anaphylactic reaction so pre-treatment of patients with anti-inflammatory injections is essential to minimise the risk of an adverse reaction.


Eyelid tumours are particularly difficult to manage because of the proximity to the eye. Cytotoxic chemicals are contra-indicated because they will damage the eye and interfere with vision and surgery may distort the eyelid. 


Other approaches to managing sarcoids which have proved effective include the use of human antiviral medication Acyclovir. This has to be applied topically several times a day every day and will take several weeks to get the sarcoid to regress. It requires a high level of commitment from the owner to ensure it is applied correctly but has proved to be 100 per cent successful.


Aldara (imiquimod) is a human product, but is used to treat some types of viral infections and skin cancer and has to be applied several times a week for several months but has proved to be very effective provided the course is completed. 


Mitomycin C is another human drug used to treat bladder cancer and in the management of human glaucoma surgery which has been used successfully by injection to treat sarcoids around the eye.  


The fewer sarcoids a horse has, the fewer it will get, so sarcoids are best treated early with the best available option with the highest chance of success used from the start.


If a treatment approach fails, subsequent attempts to treat the sarcoid will be much less effective. The large number of different treatments available indicates just how difficult sarcoids are to treat.


Some products which are available from non-veterinary sources have not necessarily been tested scientifically and may even aggravate the tumour and make it worse.


However, improvements in behaviour and performance have been seen when lesions have been successfully treated so it is always worth getting an opinion from someone experienced in treating sarcoids and tackling them at an early stage.


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