BY LIESBETH HAEGEMAN / EQUITOM
PHOTOS: EQUITOM

You’re grooming your horse and suddenly notice a small lump on the skin. “Probably just a bite or a small wound,” you think – and you move on. But a week later... it’s still there. Maybe it’s even a little bigger. And then the questions start creeping in. Could this be a sarcoid? Should I call the vet? Should I be worried?

You’re not alone. Sarcoids are the most common skin tumours in horses. While some remain small and harmless, others can develop into a real challenge – and a source of concern for many horse owners.

What is a sarcoid?

A sarcoid is a skin tumour that develops from fibroblasts, the cells responsible for repairing and maintaining the skin. Under normal circumstances, these cells act like tiny repair workers. But in sarcoids, they start growing in an uncontrolled way.
The trigger is a virus – the bovine papillomavirus (BPV) – which typically enters the skin through small wounds, often with the help of flies. That’s why sarcoids are most commonly found in areas such as:
• inner thighs
• belly
• sheath or udder
• around the mouth
• eyelids
These regions are more prone to small injuries and tend to attract flies, making them ideal entry points for the virus.

Recognising a sarcoid

One of the biggest challenges with sarcoids is that they don’t all look the same. They can easily be mistaken for a wound, a scar, or even an insect bite. They can appear anywhere on the body and come in 5 different forms:
• Occult: flat, round, scaly patches with hair loss or changes in hair colour (Figure 1)
• Verrucous: wrath like lesions. They consist of thick crusts that are very resistant and cannot be removed easily (Figure 2);
• Nodular: firm Firm lumps found in the skin, or underneath the skin. Sometimes these masses are completely loose from the overlying skin. The skin on top of the nodule can have a normal appearance or can show hair loss/alteration of the skin colour (Figure 3);
• Fibroblastic: fleshy, granulation type tissue, ulcerated masses that bleed easily;
• Malignant: very aggressive and tend to spread easily.
Because of this variation, it is not always easy to recognise sarcoids immediately. Sometimes it is even difficult to distinguish a real sarcoid from a chronic wound or an ordinary lump, but these 3 signs should always get your attention.
1. It doesn’t go away. A skin lesion that is still present after 1–2 weeks deserves a closer look.
2. It changes over time. If the lump grows, changes shape, becomes irritated or starts bleeding, don’t wait.
3. It’s in a predisposed area. Suspected skin changes and lesions in specific predisposed areas of the body, particularly where the skin is thinner, such as the inner thighs, along the ventral abdomen, around the sheath or udder, and in the head region — especially around the mouth, lips, and eyelids.
It is also important not to manipulate or irritate a suspected sarcoid yourself. Even small trauma can trigger more aggressive growth.
Why sarcoids should not be underestimated
Although sarcoids are benign – meaning they do not spread to internal organs – they are far from harmless.
Some remain stable for years without causing problems. Others, however, can suddenly start to grow aggressively, become irritated or infected, and cause discomfort or pain. When located in sensitive areas, such as around the eyes, they can even interfere with normal function and lead to more serious complications.
What makes sarcoids particularly challenging is their unpredictability. Two horses can have similar lesions, yet their evolution can be completely different. This is partly linked to the horse’s immune system.
A strong immune response may help keep the virus under control and limit tumour development. In contrast, when the immune system is compromised – whether due to stress, illness or overall condition – sarcoids may progress more easily...

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