Sarcoids in Horses

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Latest posts by Eleanor M. Kellon, VMD (see all)

How are sarcoids in horses caused?

Sarcoids in horses are caused by infection with the bovine papillomavirus, BPV-1 or BPV-2 strain, which causes warts in cattle. Biting flies or flies feeding on wounds is the likely route of entry.

The virus also circulates in the horse’s white blood cells. An Austrian study published in March 2011 also found a link between BPV and the frog disease canker.

There’s a saying, “Once a sarcoid horse, always a sarcoid horse”.

This is based on the observation that horses with sarcoids often have recurrences when masses are removed. The virus is circulating in their immune system cells (neutrophils) and also present in the normal skin around lesions.

While it is true that a horse infected with the BPV virus may remain infected for life, that doesn’t mean that the lesions cannot be effectively treated. However, we’re a long way from knowing how to beat this virus.

As with melanomas, immunotherapy has received a lot of attention but the results from vaccines have been less than exciting: Horse Talk Article

What do sarcoids in horses look like?

Sarcoids in horses are classified by their appearance and behaviour. The table below lists those classifications and types of sarcoids:

CLASSIFICATIONDESCRIPTION
OccultCircular hairless patches, often on the face. Occult sarcoids are also common on the inner thighs but easily missed there. Skin flaking/scaling is common. May regress with no further changes, or change to more obvious sarcoid changes.
VerrucousWart-like lesions alone or in groups. Verrucous sarcoids may occur as a group of lesions that coalesce.
NodularNodular sarcoids have raised nodules under the skin, often on the armpit, inner thigh/groin or around the eyes. Overlying skin appears normal.
FibroblasticFibroblastic sarcoids are masses that look very much like proud flesh/exuberant granulation tissue. Bleed easily.
Malevolent/malignantThis is a very aggressive growth that causes an inflammatory reaction in all adjacent tissue and can grow in size very quickly. Malevolent sarcoids / malignant sarcoids are very rare and factors other than the BPV virus may be involved.
MixedSkin lesions that have more than one type of sarcoid, e.g. occult lesion and adjacent verrucose sarcoids/mass.

Sarcoids may remain unchanged for prolonged periods, even the horse’s life, or may grow suddenly, even transform their type. Trauma/irritation usually causes them to flare up, and this may explain why they often show up in areas like the armpit or groin folds where there is a lot of friction.

What treatment options are available?

There are sharply divided opinions regarding what (if anything) to do when a horse has a sarcoid. Veterinarian Derek Knottenbelt in the UK is an expert who developed the classification system and is strongly in favour of early treatment.

However, veterinarians in the field are more inclined to take an initial conservative approach. The fact of the matter is that spontaneous cure/remission is possible, maybe even “common”, although there are no solid figures available to define common.

Spontaneous cure rates of up to 15% have been reported during short observation periods of a few months, which is still a very significant number especially when you consider that aggravating a fibroid commonly makes it worse.

The self-cure rate makes it very difficult to evaluate therapies since the horse may have gotten better on its own.

Fibroids usually appear between the ages of 1 and 6 years. It’s reasonable (but not proven) to suspect that horses that develop sarcoids have a defective immune response to the BPV virus. Then again, evidence of BPV virus infection is higher in horses with sarcoids than in the general population at large.

Horses deliberately injected with the BPV virus typically develop lesions that spontaneously resolve:

Sarcoids in horses study

This suggests that self-cure is probably the norm and other factors are at work in horses that develop persistent growths. Suffice it to say here that there is a lot more to be learned. There may be a genetic Achilles heel in horses that go on to develop sarcoids, or it may be their immune system was weakened by another challenge or even a nutritional factor/deficiency.

If a horse has a sarcoid that is not in an area irritated by tack and it seems quiet/not growing, there is no pressing need to try to treat it. In fact, trying to treat it may actually backfire and make the lesion worse. Lesions that are clearly worsening or spreading are a different story and should be treated.

sarcoid in horses
Verrucous (wart -like) equine sarcoid

Before beginning though, you need to understand that the success rate for any therapy is no more than 50%. across the board. Experience of the treating veterinarian with response rates in various scenarios can improve this, but even when two horses seem to have identical sarcoids behaving in the same way, the response to any particular treatment cannot be predicted.

An exception to this is radiation therapy. Brachytherapy (implantation of radioactive wires) or external-beam radiation (radiation delivered similar to taking a radiograph) is used.

Success rates of 100% have been reported for sarcoids involving the eyelids and skin around the eyes; almost 90% for locations elsewhere. Radiation also works when other methods have failed. It can be used either before or after surgery to improve the odds of a cure.

The cost of radiation treatment is high. This is because the radiation must be given in divided doses, called “fractions”, when using an external beam or be left in place for 6 to 10 days when using brachytherapy. During this time, the horse must stay at the treating facility and in the case of brachytherapy must have a special radiation-proof stall.

There are less than 10 veterinary school facilities in this country offering radiation therapy, led by Ohio State and Washington State, but with several others in the late planning stages. Liverpool Hospital in the UK offers this treatment, as does the University of Sydney in Australia.

equine sarcoid on hind leg

For pedunculated fibroids, removal by banding is popular. A tight rubber band or heavy suture is placed at the base to cut off the blood supply. For more broad-based lesions, surgical excision can be done in any area that has sufficient extra skin to close the wound. When available, laser removal has a somewhat higher success rate.

Cryotherapy uses freezing to kill the growth, which then falls off. Recurrences are common with surgery alone.

Chemotherapy, either locally injected or in the form of creams, is a common treatment. Because these are slow-growing tumours, topical therapy is typically continued for several months. With injections, common skin tumours are treated at two-week intervals for up to 3 to 4 months.

Cancer chemotherapy drug/agents used have included 5-fluorouracil and cisplatin. BCG, an immunostimulant, has also been used and most recently there was a report from Austria that the antiviral acyclovir as a cream was successful. These treatments work best with mild diseases. Larger lesions should be surgically removed or reduced first.

The mistletoe extract has been tried with sarcoids in horses, with a success rate similar to chemotherapy:

Treatment of Sarcoids in horses study

Another topical approach is to chemically destroy the sarcoid. The herbal bloodroot (Sanguinaria canadensis) is the most well known and is available as a cream, mixed with zinc chloride solution. Xterra is the most well-known product in the US and is available through veterinarians.

There are others on the market as well. In the UK, there is a product called AW4-LUDES, commonly known as Liverpool Sarcoid Cream, developed by Dr Knottenbelt. It contains 5-fluorouracil as well as caustic chemicals and herbal extracts.

The appeal of the caustics is how quickly they work. Small growths may dry up and fall off in a matter of days. Larger growths may take a few weeks. The downside is the local inflammation and pain they produce.

When the destroyed sarcoid falls off, you are left with an open wound that must heal by granulation. There are no formal studies available but the success rate appears to be higher than either surgery or other topicals used alone. However, recurrence is possible.

Finally, there are sporadic anecdotal reports from owners that toothpaste applied topical treatment works. Some claim it has to be Crest, but other brands have been used.

A theory is that fluoride may be an effective agent when applied as a topical treatment. Some have claimed success with fluoride mouthwashes. Responses are seen in the same time frame as with the caustics above. There is no information on the success rates or recurrences.

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Last Updated on January 27, 2022 by Forageplus Team

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