Not always UV induced
Equine squamous cell carcinoma Origin, forms, treatment.
von Dr. Jan Kuntz & Team // 30. January 2025
What is squamous cell carcinoma?
Squamous cell carcinoma (SCC) is the second most common skin cancer in horses, after equine sarcoid. Squamous cell carcinomas are tumors that arise from the cells of the upper layers of the skin. This squamous epithelium forms the covering layer of both the skin and mucous membranes. As the outer layer, it is also exposed to special environmental influences, so that keratinocytes can degenerate in these epithelia and form malignant tumors. Preliminary stages of these tumors are often small, rather inconspicuous changes, also known as plaques or actinic keratoses. Such a precancerous change can develop into a SCC, which can range from a non-healing wound to a massive tumor.
How to detect squamous cell carcinoma?
Suspicious for SCC are changes especially on the typical skin areas: Eyelids, unpigmented areas on the nose, nostrils and lip of the horse as well as on the anal cone and genitals. Regular monitoring is especially important in predisposed breeds, horses with homozygous genetic defects, and fair-skinned animals. Caution is also advised in horses in which SCC has already occurred. Clinically, only nonspecific changes such as poorly healing wounds and open, scaly or crusted areas appear at first. Reliable detection is by biopsy, a sample that the pathologist can examine microscopically for malignancy.
Where do equine squamous cell carcinoma most commonly occur?
Ocular Squamous Cell Carcinoma
Squamous cell carcinomas are the most common tumors of the equine eye. They originate from the eyelids, conjunctiva or third eyelid. Ultraviolet radiation is a risk factor, especially in these bright, unpigmented areas. If the tumors are still small and not inflamed, they usually do not affect the horse. However, because they often grow rapidly and invasively, waiting worsens the prognosis. Ocular SCC can metastasize, but in many cases the primary tumor is the problem.
Cutaneous squamous cell carcinoma
The unpigmented areas of the horse's head are more frequently affected by squamous cell carcinomas, some of which grow rapidly. They often proliferate and show an inflammatory, purulent and often malodorous surface. The visible area is often only a small portion, and the change can be palpated well into the depths.
Similar changes may also occur around the anal cone and in the anogenital area. Again, the externally visible portion is often small.
Other localizations
Squamous cell carcinomas can occur in a variety of other locations in the horse and then have very different prognoses. While tumors of the gastrointestinal tract are usually discovered very late and are then often no longer treatable, squamous cell carcinomas of the oral cavity have a better prognosis. Squamous cell carcinomas may also occur in the nasal cavities and then significantly interfere with breathing.
Penile squamous cell carcinomas are more common. Here they can occur throughout the area from the tip of the penis well into the prepuce and in some cases completely prevent excavation. An occasional check of the genitalia should be performed in order to be able to react at an early stage.
Metastasis
While equine squamous cell carcinoma metastasizes late in many cases, local tumor control is often the primary concern. If metastasis occurs, it is usually lymphogenic, i.e. the metastases are initially found in the regional lymph nodes. Nevertheless, distant metastasis is possible and thus metastases in the lung are not excluded.
Which horse breeds are particularly frequently affected?
Squamous cell carcinoma is a form of skin cancer that occurs primarily in horses of certain breeds. These include Appaloosa, Paint and Quarter Horses. In addition, some Haflingers are particularly susceptible to ocular squamous cell carcinoma due to their genetics. Horses with fair skin and those that spend a lot of time in the sun also have an increased risk of developing this type of cancer. It is important that owners of these horse breeds take extra precautions to protect their animals from the sun, such as providing adequate shade and using special equine sunscreens. By taking these measures, they can at least reduce the risk of their horse developing skin cancer.
What causes squamous cell carcinoma?
The first step in the development of tumors is always the mutation of individual cells. As is so often the case with tumors, carcinogens also play an important role in squamous cell carcinomas. Carcinogens are substances and agents that promote the mutation of cells and thus the development of tumors. As in humans, UV radiation plays an important role in equine SCC. In addition, smegma, especially in geldings, and chronic inflammation are tumor-promoting factors.
What are the risk factors?
In ocular SCC, the genetic component plays the major role in its development. Thus, in horses with homozygous gene mutations, several localizations are often affected independently of each other, while conspecifics from the same herd do not show any changes. The decisive role is played by a protein that is responsible for repairing the DNA after it has been damaged. Mutations of the DNA damage-binding protein 2, DDB2 for short, have been testable for several years. This is less relevant for the individual animal, but can be an important basis for decision-making in breeding.
Other risk factors include general inflammatory processes, for example, with increased smegma production and an inflammatory environment in the tube pocket. Squamous cell carcinoma of the hose occurs significantly more frequently in geldings than in stallions.
How can I protect my horse from developing a SCC?
Effective sun protection is the best and simplest measure to prevent the risk of UV-induced tumors in horses. As with humans, this means that horses can seek shade at any time and should not stand in the midday sun on midsummer days. A fly mask with suitable UV protection can also be used at times, especially for horses with the appropriate disposition. For very sensitive horses, sunscreen or night grazing may also be an option. Tumors on the hose can be prevented to some degree by improved hygiene.
Although prevention is the best protection, early workup of abnormal changes improves the prognosis if SCC does occur. The veterinarian should be the first point of contact here. Actinic keratoses, i.e. chronically changed areas with crusts and scales, should also be clarified by a veterinarian.
What are the treatment options for squamous cell carcinoma?
To treat squamous cell carcinoma, the three classical pillars of tumor therapy can be applied.
Surgery
Since the tumor is usually diagnosed very early, metastases are rare at this stage. Surgery is therefore often the first therapeutic option. The challenge is to remove the tumor completely and with a safety margin of 2-3 cm in healthy tissue without damaging important neighboring structures. Especially in the eye, this can be very difficult and the tumor can often only be removed with a marginal hem. The consequence is a recurrence of the original tumor: a relapse. The remedy then is the extensive removal of the tumor and eye, known as enucleation.
Amputation with relocation of the urethral outlet is also possible for penile tumors in advanced stages. Although this is an invasive procedure, patients usually tolerate it well.
Radiation therapy
The equine squamous cell carcinoma is a very radiosensitive tumor. Therefore, radiation is an excellent option for the treatment of squamous cell carcinoma in many cases. It can be used as an adjuvant radiotherapy after surgical removal as well as a stand-alone therapy. It involves treating the tumor with ionizing radiation in up to ten sessions (more on the equine radiation therapy process). The advantage is that the tumors can be treated even in difficult locations and, in the medium term, the risk of recurrence is nevertheless low.
Chemotherapy
In horses, chemotherapy is usually administered locally, i.e. the drugs are applied to, in or under the tumor. In some cases, the use of cytostatic or cytotoxic drugs can influence tumor growth.
Other therapy options
In the case of very superficial tumors and tumor precursors, local treatment with ointments may be indicated. This should only be carried out according to veterinary advice, under appropriate control and not on one's own. Depending on the localization and size of the tumor, photodynamic therapy may also be considered, in which the tumor cells are first made photosensitive by an ointment and then damaged by visible light.
What is the prognosis for equine squamous cell carcinoma?
The prognosis in the treatment of squamous cell carcinoma can range from good to exceedingly cautious. This depends primarily on the size and localization of the tumor and whether it has already metastasized. Other prognostic factors include, for example, the number of unsuccessful attempts at therapy and the malignancy of the tumor. Clinical assessment should not prevent a thorough workup.
