Sarcoid is a form of skin cancer with a viral basis. The development of an effective vaccine that will resolve the aggressive growth of these warty nodules — or prevent them in the first place — has so far proved a forlorn hope. Direct inoculation of the virus into the skin neither resolves the tumours nor protects a horse from developing them. Additionally, the majority of horses with even the most severe sarcoids fail to produce any protective antibody immunity to the virus involved in the process of the disease.
The main reason for this is that tumours can disguise themselves from the host’s immune system. All animals are protected from tumour development by this system, which is an extremely potent and effective process. It is when this goes wrong that cancer develops its “camouflage” and becomes undetectable.
It is not a matter of deficiency of an immune system; this runs at 100% unless a specific disorder leaves us “immunocompromised”, in which case tumours and infections can be much worse than they would otherwise be. The whole point about cancer is its ability to hide within the body and not be recognised — and the thrust of treatment is to encourage the body to recognise the tumours.
For this to happen, there has to be a significant change in the tumour cells or something that triggers the immune system to “see” them. Once the immune system recognises these rogue cells, they are rapidly eliminated.
An amazing thing that can happen with equine sarcoids is spontaneous and complete “resolution” without interference. Left untreated, the occasional sarcoid simply disappears.
Why is this important? Spontaneous resolution of any tumour is unusual, but it reveals that, in these cases, the immune system for some reason suddenly recognises the cells and eliminates them. From studies at the University of Liverpool over the past 30 years, we believe that this occurs in around 1% of UK cases.
Interestingly, there is no recorded case of a horse whose sarcoids have spontaneously resolved who then develops any subsequent sarcoid. This signifies that there is an immunological aspect to the disease.
It is unlikely this is anything to do with the virus itself; it is more likely to be linked to the principles of cancer cell recognition in the body. The immune system produces some mechanism whereby the camouflage of the tumour cell is recognised.
If we can identify what happens to cause this spontaneous resolution, we can try to induce that response. We will then have a means of resolving more cases immunologically.
Should an owner ignore a sarcoid, in the hope that it resolves of its own accord? The answer is not straightforward. If the lesion is very small, static and of a non-serious type, there is the possibility that it can be left — but not ignored.
Leaving a sarcoid alone is a high-risk strategy. Some do remain static for many months, or even years, but more than 75% are known to get worse. Almost all sarcoids traumatised in any way show significant exacerbation in size, invasive nature and pathological character.
Neglect is not a strategy that is the norm in any other cancer. We know that the disease needs to be treated promptly and robustly, with the best available system. Leaving cancer alone is dangerous and ill-advised — this applies to equine sarcoids as to every other cancer in every other species. Professional opinion should be sought at the earliest possible point and the advice of the vet should be followed implicitly.
Currently, we do not have effective immunological methods of treatment, so we are reliant upon dealing with individual lesions as they occur. The fact that there are more than 40 different treatments for sarcoids that have been published or suggested indicates that none of them are universally effective.
Radiation is the gold standard treatment for most lesions, but this is very expensive and available at only a few centres globally. Fortunately, one of these centres is the Animal Health Trust in Newmarket, where the team are working hard to develop a new and effective system that does not require the horse to be isolated or undergo general anaesthesia.
Surgical or laser excision is a good approach in some cases but, since almost every sarcoid has an ill-defined margin, recurrences are common. Laser surgery involves a bloodless “scalpel”, yet there are still recurrences and technical difficulties that also apply to more conventional scalpels and plasma knives.
Chemotherapy of some sort is used either alone or alongside the surgery, cryosurgery or laser surgical excision, usually applied directly on to the tumour or the post-surgical site. Topical and intralesional chemotherapy drugs and ointments can also be useful. As with all cancer treatments, nothing is easy — nor is any one method effective in every case.
Much research has been diverted to the investigation of the cause of this disease, but very little to its treatment.
Now at least, we are starting to view it as a cancer rather than a wart. While we might ignore a wart on our own skin, none of us would ignore a cancer.
Certain breeds are more liable to sarcoids. Any horse that has, or has had, them is genetically liable to the disease — this remains for life, so recurrences and new lesions are always to be expected. In addition, the more sarcoids a horse has, the more he will get. It follows that the fewer he has, the fewer he will get — another compelling reason for treatment.
The only circumstance in which it seems that a horse becomes totally resistant is when spontaneous resolution occurs over a relatively short time (months, rather than years). Such cases will probably not recur. Resolution must be total, however, not just the gradual improvement of one or two lesions.
Rather than simply waiting for a miracle cure, we should treat sarcoids as we would a skin cancer and seek qualified medical advice as a first course of action. Isn’t that what we would do for ourselves?
Ref Horse & Hound; 29 March 2018