Sarcoids are a benign tumour that grow on the skin of susceptible horses. While they do not spread to any other organ in the horse’s body, they can certainly spread throughout the skin. There are no rules with sarcoids- in some horses they can appear as a single lump that never changes while in others they can grow rapidly and spread quickly, sometimes after having been present for years in a dormant state. Professor Knottenbelt at Liverpool large animal hospital says “the only predictable thing about sarcoids is that they are unpredictable!” While this is sadly almost true, there are a few things we do know for certain about sarcoids.
The first is that they are caused by the bovine papillomavirus. This is a virus that normally causes warts in cattle but illicits a different response when it infects horses.
We also know that many horses have an immunity to the sarcoid virus so that whatever their exposure to the virus, they will remain sarcoid free. Other horses appear to have very little immunity and these horses are the ones in which sarcoids spread rapidly. Between these two extremes, there are horses that have different degrees of immunity where those with greater immunity will develop fewer sarcoids than those with less. Occasionally, horses suffering from sarcoids can actually develop an immunity so that their sarcoids actually shrink and even disappear- it doesn’t happen often but is very welcome when it does!
Finally we know for certain that the papillomavirus is spread by flies. This may explain why susceptible horses that sustain a wound frequently have a sarcoid develop exactly at the site of the wound at a later date- indeed, sometimes the wound never actually heals but “turns into” a sarcoid. The suggestion is that the open wound attracts flies, some of which are carrying the virus which is deposited at the site.
We consider there to be 5 different types of sarcoid, based on their appearance:
- Nodular- a round shiny growth
- Verrucous- a raised area, warty in appearance
- Occult- a small, often round, hairless scaly patch of skin that resembles ringworm
- Fibroblastic- a really horrible looking growth, prominent like a cauliflower and open and bleeding
- Mixed – a combination of any of the above.
Sarcoids rarely affect horses of less than 3 years of age and if a horse has not developed a sarcoid by 8 years of age, they are unlikely to do so.
Sarcoids are found anywhere on a horses body but certain areas are most commonly affected- the inside of the elbows and the thighs are especially common sites but the face (particularly around the eyes and ears), the underside of the belly and around the penile sheath of males are also frequently affected. Although horses can have a solitary sarcoid or have literally scores of them, most affected horses have between 3 and 7 lesions.
The list of possible treatments for sarcoids is long and varied. No treatment is 100% effective 100% of the time and even when the treatment is effective, regrowth can occur at a later date.
Here are a few of the treatments more commonly used by Vets. None of these treatments are cheap but those treatments involving surgery (and therefore general anaesthetic) tend to be more expensive (think a few £1000!) while creams and injections are less so (but still think £100s!):
1) Surgical removal. This is fraught with danger since if a sarcoid is incompletely removed, the sarcoid tissue left behind can grow and spread rapidly and create a much greater problem than the original sarcoid posed. The surgeon will choose to operate on only those sarcoids which are surrounded by a large area of normal skin and he/she will take care to ensure the entire growth is removed. After surgical removal the skin is usually allowed to heal together without being stitched.
2) Laser removal. A surgical removal but the cutting instrument is a laser, not a blade.
This has the advantage of sealing with heat any blood vessels at the edge of the wound and therefore possibly reducing the likelihood of sarcoid spread. Currently we consider that laser surgery is in most instances the preferred treatment for sarcoids. Usually it is done under general anaesthesia.
3) BCG injection. This is the vaccination we were all given when at school to protect us against TB. It is especially useful for nodular sarcoids that develop around the eye. The sarcoid is repeatedly injected with BCG on several occasions over the course of a few months and often disappears without a blemish
Often sarcoids are treated because they are present in such sites that they rub against other body parts (e.g. when growing between the hindquarters) or when they rub against items of tack (e.g. a sarcoid at the edge of the mouth, irritated by the bit or one on the chest where the girth runs). The rubbed sarcoid becomes painful and often bleeds, irritating the horse. Some sarcoids will bleed even without being traumatised and this of course attracts flies which can be a source of constant irritation to the horse in the warmer months of the year. Many owners will simply choose to have sarcoids treated as they are ugly in appearance and failure to remove them increases the likelihood of them spreading to other areas of skin.
Unfortunately if your horse has a low immunity to the virus responsible for causing sarcoids, it is very likely they will develop them at some point. However, given that we know flies play at least some role in the spreading of sarcoids, then it follows that protecting your horse from flies may limit the number of sarcoids that develop. Fly rugs and repellents are advised and it is also important to reduce fly populations on the stableyard with good hygiene limiting the areas where flies can breed – covering the muckheap, clearing up loose hay or straw lying around and draining any stagnant water can help.
One very exciting prospect is the development of a sarcoid vaccination which is currently in the early stages of trials in the US. The initial indications are apparently encouraging so there may come a time in the not too distant future when a course of injections may be all that is needed to protect your horse against developing these extremely unwelcome growths!