WHEN we get a reader request at Horse Sense we do our very best to get an expert answer: this time it’s the problem of sarcoids. Helen Sharp spoke to vet Karen Dunne about how to spot them and what the current options for treatment are.
What exactly are sarcoids?
Sarcoids are the most common skin tumour affecting horses. While sarcoids don’t spread elsewhere in the body, they are often problematic in terms of their location, and are difficult to cure. Once a horse has even a single, small sarcoid the likelihood is high that they will develop more, and reoccurrence is common.
Sarcoids most commonly appear between two and ten years of age. Their exact cause is unclear, but biting insect spread of the papilloma virus that causes warts in cattle is believed to be involved, along with a genetic and breed disposition (Arabs and Quarter Horses are reported to be more prone to them than thoroughbreds).
The tumours are variable in appearance: many start off as a faint circular area of hair loss and greyish, scaly skin and slowly progress to form bumpy nodules (figure 1). The rate of growth is very unpredictable – some remain small and inactive for years, while others may rapidly develop into large, ulcerated and locally invasive lesions (figure 2). Some horses have only a single growth while others may develop multiple tumours. Sarcoids can occur anywhere, but the most common sites are on the head, along the lower chest and abdomen and on the limbs. Lightly haired areas of thin skin where flies feed freely seem to be higher risk, e.g., around the eyes, muzzle and between the hindlimbs. Sarcoids may also develop at the site of a wound, especially on the limbs.
How can sarcoids affect our horses?
Sarcoids may interfere with a horse’s ability to work if they are located where they will be rubbed by tack. If the lesions become inflamed and ulcerated, they are also painful, and attract flies and biting insects during the summer. They may make it uncomfortable for a mare to allow her foal to nurse if they are present around the udder (figure 3). The presence of a sarcoid may reduce the horse’s economic value as the prognosis is always somewhat unpredictable.
What should we do if we discover a sarcoid?
If you find a potential sarcoid on a horse always show it to your vet as soon as possible to allow good management decisions to be made as early as possible. If the animal has just one or two small, inactive sarcoids that are not affecting them they may be best just left alone and monitored for any sudden changes in appearance or size. It may be helpful to photograph and measure the lesions every 6-12 months to keep track of progress. Try and avoid rubbing or irritating the lesion, as any interference increases the risk of a flare up.
How are sarcoids treated?
When it comes to treatment, there are multiple approaches that have been tried. While this might seem encouraging, it is important to note that in medicine the more treatments there are for a condition, the less likely it is that any of them are highly successful!
All sarcoids are different and each case should be evaluated individually, as unfortunately there is no simple and effective cure-all approach. The best results (success rates of 80-100%) have been reported following the use of various forms of radiotherapy – similar to techniques used to treat some skin cancers in humans. Unfortunately, radiation therapy is typically not available for use in animals on the grounds of cost, safety and practicality (the patient may be radioactive and require strict isolation for a period after treatment).
The tumours typically have a base that extend like a web into the surrounding skin. If surgically cutting out a tumour it is usually very difficult to remove all these strands, and the remainder may grow back far more rapidly and aggressively than the original sarcoid. Surgical removal alone, or with the use of a rubber ring to remove the top of the tumour, is therefore usually reserved for individual tumours that have not broken the skin.
Surgical lasers are now widely available and can be used to excise the tumour with much less bleeding (figure 4). The challenge of extracting all the tumour tissue remains however, so it’s typical to remove a portion of the normal-looking tissue around the tumour as well. This reduces the risk of regrowth but is difficult to achieve in all locations, e.g., near a joint. Once the site has healed it should be monitored closely for any sign of sarcoid regrowth. Seek veterinary advice immediately if suspicious, as if the tumour does regrow it is likely to do so aggressively.
Cytotoxic drugs are those that kill cells and their use is referred to as chemotherapy. These substances may be injected or implanted into the tumour, or applied as a cream, e.g., “Liverpool sarcoid cream”. Treatment is typically repeated several times over a period of days or weeks depending on the size of the sarcoid and the specific drug being used.
Cytotoxic substances must be handled very carefully as they may also damage any normal skin and tissue that they come in contact with, resulting in painful and non-healing injuries. Horses are usually admitted to a veterinary hospital during treatment to minimise the risk of harm due to accidental contact with handlers, children, pets et cetera. The need for repeated periods of hospitalisation adds to the hassle and expense of sarcoid management for owners. There are also some creams available that contain an immune response modifier substance, such as imiquimod. These creams have the advantage of being less harmful to normal tissue, so they can usually be safely applied in the horse’s normal environment. Unfortunately, many of these treatments have become increasingly difficult to source in recent years due to regulatory and supply issues, such as those caused by Brexit.
Key take homes
In summary, all sarcoids have the potential to grow aggressively and there is no one simple and effective cure. Key points if you encounter them include: