Rosie Naylor MRCVS is a consultant in equine medicine at Newmarket Equine Hospital, where she sees a broad range of referred internal medicine cases in horses of all disciplines. She holds the ACVIM Diploma in Large Animal Medicine and recently completed a PhD at the Royal Veterinary College. She reports on a promising new therapy for melanoma, shown to increase survival rates in dogs, that could work for horses, too.
Melanomas are one of the most common tumours that affect horses. They are usually found in the skin, particularly (but not exclusively) of grey horses. Affected horses typically develop firm, darkly pigmented nodules in characteristic locations — underneath the tail, around the anus, adjacent to the lips, on the sheath or in the throatlash region.
While many melanomas are benign and slow-growing, seeming to cause few problems, they can become locally invasive and also spread to other parts of the body.
In severe cases they can interfere with vital bodily functions such as passing droppings, head flexing or swallowing, leading to problems ranging from constipation or colic to neurological deficits.
Melanoma can also affect the internal organs such as the intestinal tract, salivary glands, guttural pouches, liver and spleen.
Local treatment of melanomas in the skin is often performed, using a variety of chemotherapeutic agents such as cisplatin or 5-fluorouracil. These can be injected directly into the tumour, with some success, however, even if a reduction in tumour size is observed, tumour cells may still metastasise.
A medicine called Cimetidine is sometimes prescribed, with often disappointing results. Surgical management is occasionally attempted but can be problematic if the tumour is close to vital structures.
Target practice
A new melanoma treatment vaccine with the trade name Oncept could provide the answer.
This DNA-based vaccine stimulates the horse’s own immune system to mount a response to tyrosinase, a protein found in high concentrations in melanoma cells.
This is an attractive approach as it will target all melanoma cells — internal and external — including those not accessible with other treatments.
The vaccine was originally developed for the treatment of malignant melanoma in dogs, where it has been shown to increase survival rates significantly. It is now licensed for use in dogs in the US.
Pioneering work has subsequently been performed in the USA, at the University of Tennessee, to evaluate the possibility of extending this therapy for the treatment of melanoma in horses. Research has shown that equine melanoma express similarly high levels of tyrosinase, the target protein, to those in canine melanoma, and that the vaccine is safe to administer to horses.
After a course of four vaccinations at fortnightly intervals, the horses developed a detectable immune response. A significant reduction in tumour size was observed in four out of five horses within six months.
The vaccine is currently being trialled in a larger group of horses with melanoma, in North America.
Although the final results have not been released, the clinical impression suggests a positive response in a proportion of horses.
A shot of hope
Due to these promising reports, the vaccine is now available to vets in the UK with specialist qualifications (diploma holders). As the vaccine is currently not licensed for use in Europe, vets must obtain authorisation from the Veterinary Medicines Directorate — the organisation that regulates drug use in the UK — to import it.
The vaccine is administered into the chest area using a specialised high-pressure device. With time, it is hoped that it will become a realistic and readily available option to control melanoma growth.
