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Editorial Information

David Rendle BVSc MVM CertEM(IntMed) DipECEIM MRCVS

Published 2012

Sarcoids


Aetiology

Sarcoids are the most common tumour of the horse and are a tumour of fibroblasts, a type of skin cell. They may also be referred to as fibromas, fibropapillomas or fibrosarcomas. Their cause is uncertain but a bovine papillomavirus (BPV) is suspected as DNA from these viruses is found in virtually all sarcoids. Contact with the virus alone does not result in tumour production and a combination of factors including skin damage, genetic predisposition and a weakened immune system probably all contribute to sarcoids developing in a particular animal. All breeds and genders may be affected although some studies suggest that Appaloosas, Arabians and Quarter Horses are at increased risk with Standardbreds being at decreased risk. Sarcoids typically occur in animals between 1 and 7 years of age.

Clinical Signs

Sarcoids may occur almost anywhere on the body but typically occur at sites of previous injury, around the eyes, ears and lips, on the neck, between the front legs, on the inner thigh and under the belly. They can vary in size from only a centimetre to enormous debilitating growths; the latter are thankfully rare. Horses that develop one sarcoid often develop others nearby or at distant sites. Sarcoids can be locally invasive or change very little over many years. Although their behaviour is notoriously difficult to predict the majority are progressive and spread locally. They commonly spread to other sites in the skin but do not spread internally. Sarcoids are typically categorised into 6 types:

  1. 1. Occult (Flat) Sarcoids.

fig 1

Figure 1:  Occult sarcoid on the muzzle

These are the most superficial and least concerning type of sarcoid because they rarely cause clinical problems and less commonly progress to more aggressive types. They appear as areas of thick, scaly and hairless skin that may become highly pigmented. They may be confused with the lesions caused by ringworm. They are generally slow growing and may remain unchanged for years. If they do progress it tends to be to verrucous types unless they become traumatised in which case they may become the more aggressive fibroblastic type. They often remain unchanged for years and may resolve without treatment.

2. Verrucous (Warty) Sarcoids. These raised, crusty sarcoids look very similar to the viral warts seen in young animals. However, they may occur at numerous sites on the body in animals of any age whereas viral warts tend to occur around the mouth and ears in young animals. Unless they are traumatised they are unlikely to be invasive or aggressive.

fig 2

Figure 2:  Verrucose or warty sarcoid

3. Nodular Sarcoids most commonly occur around the eyes, groin and sheath. They are firm and well well-circumscribed either within or beneath the skin. The overlying skin may lose its hair as the sarcoid grows and the nodules may develop a stalk at their base and become pendulous. If they are traumatised they have a tendency to become aggressive.

fig 3

Figure 3:  A nodular sarcoid over the fetlock

4. Fibroblastic (Flesh-like) sarcoids may occur at various sites and often develop as a result of trauma to other types of sarcoids. They are often ulcerated and bleeding and may have the appearance of granulation tissue (proud flesh).

fig 4

Figure 4:  Fibroblastic sarcoid and occult sarcoid around the lips

5. Malevolent (Malignant) Sarcoids tend to occur around the head, between the front legs or on the inner thigh. They often develop following trauma or failed treatment and become deeply and rapidly invasive. They are often ulcerated and bleeding with thickened cords of tumour spreading under the skin.

fig 5

Figure 5:  Ulcerated nodular sarcoid on the belly

6. Mixed Sarcoids. It is common for more than one type of sarcoid to occur at the same site.

fig 6

Figure 6:  Extensive mixed sarcoid under the jaw

fig 7

Figure 7:  Typical appearance of different sarcoid types around the inner thighs

Diagnosis

The diagnosis is typically made on the characteristic appearance of the tumour. In tumours that are atypical in appearance, biopsies can be taken and examined under the microscope to confirm the diagnosis. However, taking biopsies traumatises the sarcoid which increases the risk of it transforming into a more aggressive tumour type. Biopsies should only be performed therefore if there is an intention to treat the lesion immediately if it turns out to be a sarcoid. If tumours can be removed they are often removed entirely without a biopsy and the tumour sent off to confirm the diagnosis.

Treatment

Small, less aggressive types that do not cause discomfort or impede tack may be left untreated and monitored carefully. However, as the majority of cases will progress and spontaneous remission is rare, treatment is generally recommended. If treatment is unsuccessful it may result in the tumour becoming more aggressive and will reduce the likelihood of successful treatment in future. It is therefore important that treatment is not undertaken half-heartedly and is always aimed at complete cure. Treatment with unproven or home-made remedies is therefore discouraged strongly.

Multiple treatment options are available which is testament to the fact than none are universally effective. Choice of treatment may be influenced by site, size, aggressiveness, number of lesions, previous treatment attempts, availability and the veterinarian's preference. Different studies report different success rates for different treatments and hence there is still a large amount of personal opinion that influences the choice of treatment. In general however, most treatments report success rates of 60-90%.

1.  Surgery. Surgical excision needs to be complete with wide margins. If excision is incomplete a more aggressive sarcoid will likely result. Surgery can be performed with a knife, by freezing (cryosurgery) or with a laser. Surgery can only be performed where there is enough spare tissue to remove. Sarcoids around the eye respond very badly to surgery and mostly recur.

2.  Radiotherapy. The use of radioactive implants is especially effective for tumours around the eyes. However, the treatment poses considerable risks for the operator and its availability is therefore limited.

3.  Chemotherapy. Toxic creams that kill tumour cells (and also normal cells) are the most popular form of therapy in the UK and can be used on most tumours. They tend not to be used on tumours around the eyes because of the risks of contact with, and damage to, the eye itself. Some creams may be injected into Sarcoids if they are nodular. All chemotherapeutic creams are toxic and should be used with extreme care. Possible options include cisplatin, 5-fluorouracil, AW4-Ludes (Liverpool Cream), zinc chloride, bloodroot extract (Xxtera cream) or imiquimod.

4.  Immunotheraoy. The human tuberculosis vaccine (BCG vaccine) is the most effective treatment for nodular sarcoids around the eyes. The vaccine stimulates an inflammatory response within the tumour rather than acting as a true vaccine.

Welfare Implications

The welfare of affected horses is generally unaffected unless tumours become exceedingly large or infected. Large tumours over the limbs or around the eyes are especially problematic. Sarcoids may interfere with tack and become sore or even prevent normal athletic pursuits. The presence of sarcoids reduces an animal's value and on rare occasions may result in animals being euthanased.

Disease Control and Prevention

Outbreaks can occur and some family lines seem more predisposed than others. Little is understood of how sarcoids spread although flies are suspected of being involved in transmission of viral particles. Although infectious transmission is suspected, contact with cattle or other horses with sarcoids does not obviously increase the risk of the disease. It is prudent to attempt to reduce the presence of flies around horses with sarcoids; especially those that are ulcerated but practically this is rarely possible.

Vaccines designed for use in cattle or made from sarcoids themselves are not effective.

Summary of Key Learning Points

1.  Sarcoids are the most common tumour in the horse; young horses are most commonly affected

2.  Sarcoids are restricted to the skin and whilst they may be locally aggressive and occur at other sites on the skin they do not spread internally

3.  The majority of Sarcoids are a cosmetic blemish or may interfere with tack, occasionally they become extremely large and cause discomfort

4.  Horses that have a sarcoid are likely to develop more

5.  Biospy should only be performed if treatment is intended

6.  Treatment should always aim at complete cure

7.  A number of treatment options are available which are all associated with good, but not guaranteed, success

8.  The behaviour of sarcoids is notoriously unpredictable

9.  There are no practical means of prevention

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