When it comes to animal health care, cancer is much more frequent in pet dogs and cats than it is in horses.
However, cancer can occur in horses, particularly older ones.
Tumours can arise from any cell type in the body ranging from those that make up the skin to the cells that line the inner guts.
Fundamentally, tumours start when mistakes in the genetic material of a cell are passed down when the cell divides. These genetic changes can alter the proteins made in the cell, including those that regulate how it responds to injury and when it dies.
Cancerous cells stop responding to normal signals from the body to stop dividing. They also disregard the organizational structure of how they attach and interact with the surrounding tissues. Ultimately, these rogue cells proliferate unchecked, invading healthy tissues and even spreading to lymph nodes and other organs. The behaviour and progression of cancer depends on the type of cell involved.
Horses are frequently diagnosed with skin cancers, likely because these are the most visible and easily diagnosed of all the various cancer types that can affect this species. If a horse is going to develop a tumour, by far the most likely is a skin tumour known as a sarcoid.
These tumours can arise anywhere on the skin as single or multiple masses that can ulcerate and bleed. There is some thought that sarcoids occur at locations of previous injuries in association with infection by a bovine paillomavirus. These tumours can be challenging to manage because they aggressively invade the surrounding tissues. Treatment may include removing the mass surgically and supplemental therapy to modify the immune system and/or freeze the affected tissues.
Horses can also develop skin tumours of the melanin-producing cells, called melanoma. Grey horses are particularly prone to melanomas.
Other common horse tumours worth mentioning are fat tumours that arise in the belly. Another name for fat tumours is lipoma. These benign tumours can prove deadly for horses. Fat tumours can grow to be fist-sized and can dangle on the string of connective tissue among the guts. If the tumour and its stalk wrap around a piece of gut, it blocks it off, preventing the feed material from moving through and the blood from draining out. When this occurs, the condition is called strangulating lipomas. The gut tissue dies as it is rapidly starved of oxygen.
Horses with this condition present with colic (belly pain). The only treatment option is surgery and the decision to try surgery must be made quickly because horses with this condition can deteriorate rapidly. The veterinary surgeon will remove the fat tumour and the dead pieces of gut and then attach the healthy gut ends, re-establishing the tubular pathway for feed to come through.
Under the skin, tumours are much more challenging to identify and diagnose in horses. While for dogs and cats there are amazing technologies available such as MRI and ultrasound, horses are often too big for these diagnostic techniques or their use too expensive. Signs of internal cancer are also vague and non-specific, including weight loss, fever, lack of appetite, poor performance and lethargy.
Diagnosing cancers involves understanding the clinical progression and patient factors.
Laboratory testing is essential to confirming the diagnosis and identifying the type of tumour involved. This may include blood tests and biopsies, which are sent to veterinary pathologists.
It is important to remember that not all cancers are deadly and the treatment options really depend on the cancer type. For instance, some benign skin tumours can be cured by surgical removal. Other types may require more intensive therapy like surgery to remove most of the mass and then additional treatment with radiation or chemotherapy.
The true rate of cancer in horses is probably vastly underestimated because horses that are put down for other causes such as lameness or old age may not have a full autopsy that would find tumours. But it is a good thing that rates of cancer in horses remain remarkably low compared to other animals.
Dr. Jamie Rothenburger, DVM, MVetSc, PhD, DACVP, is a veterinarian who practices pathology and is an assistant professor at the University of Calgary’s Faculty of Veterinary Medicine. Twitter: @JRothenburger