SCCs are malignant tumours from the group of carcinomas that originate in the epithelia of the skin and mucous membranes. Such tumours can also grow from other types of tissue that do not themselves have a squamous epithelium, but have epithelia with the ability to metaplasize squamous cells. SCCs grow infiltratively and spread across contiguous organ borders. In horses, SCCs usually metastasize only in the advanced stages of the disease. In this case, the regional lymph nodes are particularly affected.
Squamous cell carcinomas account for 24.6% of all tumours in horses and are therefore the second most common tumours after equine sarcoids. The cause for the development of SCCs is multifactorial. Depending on the anatomical location, other factors come to the fore. A correlation between EcPV-2 and SCC of the penis was observed. Equine papillomaviruses are also believed to be an etiological agent for other localizations. UV light can lead to cell degeneration via the formation of thymidine dimers in DNA and RNA. Higher prevalence of SCCs in sunny areas has been demonstrated in equines. UV-light induced SCCs are particularly common on unpigmented skin and mucous membranes. There is also a breed predisposition in breeds with many markings, e.g. Appaloosa. Chronic inflammation, irritation and wounds promote the development of SCCs.
Typical locations of SCCs in horses are the eye, especially the conjunctiva, cornea, sclera and the third eyelid. The external genitals, as well as the bladder neck, stomach (cardia), sinuses and pharynx/larynx are also affected.