A new review published in Equine Veterinary Journal has examined whether equine sarcoids can disappear without treatment, concluding that spontaneous regression appears to be an exceptional and uncommon event.
Sarcoids are benign but locally aggressive skin tumours and are widely recognised as the most common tumour affecting horses and other equids. They are caused by infection with bovine papillomavirus (BPV), primarily types 1 and 2, and are often associated with additional factors such as trauma and genetic predisposition.
Although sarcoids do not metastasise, they can have a major impact on horse health and welfare. Their location and severity can affect a horse’s suitability for sport, breeding or work, and in severe cases the condition can lead to euthanasia.
The review, written by Sabine Brandt, explores existing evidence surrounding reports that some sarcoids may resolve on their own without treatment.
Persistent disease driven by viral infection
Sarcoids occur when bovine papillomavirus infects cells in equine skin, particularly dermal fibroblasts. Viral genes promote cell survival and proliferation, allowing infected cells to multiply and form tumours.
Unlike bovine papillomas, or “cow warts”, which commonly regress after several months in cattle following an immune response, sarcoids in horses are typically persistent. This is thought to be partly due to viral mechanisms that interfere with the horse’s immune response, allowing the infection to evade immune detection.
For example, viral proteins can suppress immune signalling pathways and inhibit the presentation of viral antigens to immune cells, reducing the ability of the horse’s immune system to recognise and eliminate infected cells.
Experimental lesions can resolve
Early experimental studies showed that sarcoid-like lesions could be induced in horses by applying bovine papillomavirus to scarified skin. These experimentally produced lesions, sometimes referred to as “pseudo-sarcoids”, often regressed spontaneously within several months.
More recent experimental work confirmed similar results. In one study, four young horses developed lesions after intradermal inoculation with BPV1 virions, but all lesions resolved completely within 36 weeks. Evidence suggested that regression in these cases was linked to a strong cellular immune response.
However, these experimental models involved exposure to very high numbers of viral particles, which may stimulate an immune response not typically seen in natural infections.
Evidence from naturally occurring cases
Reports of spontaneous regression in naturally occurring sarcoids do exist, but the review highlights important limitations in much of the available evidence.
Several studies have reported cases where lesions diagnosed as sarcoids resolved over time without treatment. In one retrospective study of 120 horses, 10 untreated lesions resolved spontaneously within six months to seven years. Other studies have reported similar observations in small numbers of horses.
However, the review notes that in many of these reports the diagnosis of sarcoid was made based only on clinical appearance, without confirmation through histopathology or PCR testing for bovine papillomavirus.
Because other skin conditions can resemble sarcoids, this makes it difficult to confirm whether the lesions that resolved were truly sarcoids.
Mild lesions may occasionally resolve
The available evidence suggests that when spontaneous regression does occur, it is usually limited to milder forms of the disease.
Occult sarcoids, which appear as slightly thickened hairless patches of skin, and verrucous sarcoids, which have a warty appearance, may occasionally resolve without treatment. Nodular, fibroblastic or more aggressive forms are far less likely to regress.
Exceptional rather than expected
Overall, the review concludes that while spontaneous regression of equine sarcoids can occur, it appears to be rare and likely limited to certain mild lesions.
The author concludes that current evidence suggests spontaneous regression is an exceptional event, rather than a typical outcome for sarcoid disease.
