The Big 'C' | Trainer Magazine | European Trainer Article Index

Oct 18

Oct 18 The Big 'C'

What is cancer? In normal healthy tissue, damage is repaired by new cell growth which stops when the damaged cells have been replaced. However, in cancers, there is a transformation of normal body cells into cancer cells, whose growth is unregulated leading to the formation of tumours. The transformation of a normal cell into a cancer cell involves a change in the cell’s genetic make-up – its deoxyribonucleic acid or ‘DNA.’ The exact cause of this change is unknown but it is probably a result of the faulty repair of damage to the cell, which can be caused by a number of things. Certain cancers have been linked to viruses, whilst others are thought to have a genetic link. In addition, high incidences of cancer can also be linked to high doses of ionising radiation or certain carcinogenic agents (the most well known examples of which in human medicine are tobacco smoke and asbestos). These altered cells pass on inappropriate genetic information to their progeny cells and begin to reproduce and multiply in an abnormal and destructive way. As the cancer cells continue to multiply, they form a growing mass of abnormal tissue called a tumour. These tumours can be benign, which means that they are not growing quickly and they will not spread to other parts of the body, or they can be malignant, which is much more serious. As the malignant tumour grows, it ulcerates and begins to shed cells that spread the disease locally or to distant sites. Locally, this spread or ‘metastasis’ occurs by the invasion of cells into neighbouring tissues, destroying and displacing normal cells and taking their place. However, cancerous cells can also enter the blood or lymphatic vessels and so be carried to other parts of the body. Diagnosis If a tumour starts to form near the surface of the body then the cancer can be picked up quite early on because it is visible and therefore easily detectable. However, if the cancer is deep within the body – in the chest or abdomen, for example – then it is not usually detected until the tumour starts to impede the horse’s vital functions. In both circumstances, lumps and associated lymph nodes are either biopsied or removed for examination under the microscope to search for abnormal cancer cells which identify the individual cancer. Treatment Although the treatment options for equine cancer are similar to that of human medicine – surgery, chemotherapy, and radiotherapy – the frontline therapy for cancer in horses is surgery. However, the treatment of specific cancers often depends upon the type and location of the cancer. For example, a skin tumour can be removed surgically or have substances applied to it or injected into it whereas tumours deep within the chest or abdomen are hard to access and treat. Hopefully, all the cancer research that is being undertaken in human medicine will eventually be carried across into the treatment of cancer in horses as regrettably, unless the tumour is benign or detected at a very early stage, the long-term prognosis for most horses with a serious cancer is poor. Papillomas or ‘warts’ Perhaps the least significant benign tumour the racehorse can get is the papilloma, which is more commonly referred to as a ‘wart.’ These are usually found in horses aged two years or less and are thought to be caused by the Equine Papillomavirus. They appear quickly and can look quite unsightly but they are harmless and usually disappear in a relatively short period of time, so if they persist then the strength of the horse’s immune system should be questioned. They are most frequently found around the horse’s face and muzzle but can also appear in other locations such as the lower leg. Sarcoids The most common skin tumour in the horse is the sarcoid. Sarcoids are rather strange skin growths that can occur in all ages, breeds, and colours of horses and mainly affect the head and legs. The exact cause is unknown but research strongly suggests that Bovine Papillomavirus has some involvement. Due to the fact that sarcoids often grow at sites where the horse had previously suffered a wound, it has been suggested that flies may play a part in spreading the disease, probably by passing the virus from one horse to another. As a result, work is currently being carried out to try to produce a commercial vaccine. There are several types of sarcoid and some are more serious than others. Horses can have one inconspicuous, small sarcoid on the inside of a back leg or can be affected by several malignant sarcoids that spread quickly and eventually result in death. A small, settled sarcoid in a place where it is not causing a problem should be left alone; however, if a sarcoid is in an important place, or if it is particularly active and growing, then treatment may be attempted. There are many treatment options which include a rubber ring to kill the root of a round nodular sarcoid, chemotherapy, radiation therapy, cryotherapy, the injection of BCG, surgery to remove the sarcoid, or the application of sarcoid cream. Fortunately, the majority of sarcoids are manageable but they are very important when it comes to pre-purchase veterinary examinations – it is impossible to predict which sarcoids will end up causing problems and so no horse with a sarcoid can completely pass a vetting. Melanomas Perhaps the most well-known type of equine cancer is melanomas in grey horses, and this is because up to 80% of grey horses aged fifteen years or more have at least one melanoma. They are most frequently found underneath the tail and around the dock, but they can also occur behind the jaw, around the ear, and in the eye. Fortunately, most equine melanomas remain small and benign hence it is best to leave them alone if possible. However, sometimes even benign melanomas can cause problems by physical obstruction, such as making it difficult for the horse to pass faeces. In addition, some melanomas are malignant and can spread rapidly throughout the body, transforming into an invasive, destructive cancer and it is for this reason that so many people are reluctant to buy a grey horse with a melanoma. Surgery can be successful in the case of the removal of a ngle tumour that is causing a physical obstruction providing that it is not malignant. However, the prognosis for horses affected by malignant melanomas is usually hopeless. Squamous cell carcinomas The last common cancerous disease that can be found when examining a horse from the outside is the squamous cell carcinoma. This type of cancer usually occurs at the junction between the skin and mucous membranes, for example, the eyelids, mouth, and genitals. Squamous cell carcinomas of the eye are generally found in older horses, especially if they lack eyelid pigmentation – surgery can be successful but it should be performed quickly to improve the chances of a full recovery. Squamous cell carcinomas of the genitals are also quite common, especially affecting the penis in older geldings. Although this tumour is usually malignant, the spread of the disease occurs late in its progression and as a result surgery is often successful. Cancers that cause colic such as strangulating lipomas Unfortunately, unlike the tumours mentioned above, cancers of the intestine and abdominal organs usually only become apparent once the disease is in an advanced state. This is because internal tumours within the abdomen can become very large without any outward sign of a problem. It is usually only when the cancer begins to obstruct the intestines that the horse starts to show signs of the disease. In many cases, affected horses suffer from recurrent bouts of colic and the intervals between episodes then become shorter. However, some horses with massive cancer infiltration of their intestines can slowly become less able to absorb nutrients from their food and hence show slow but progressive weight loss, often despite eating extra feed. Other abdominal cancers such as splenic lymphomas Regrettably, most abdominal tumours are not detected until they have reached an advanced stage because they are very difficult to find. As the horse is so large, modalities such as ultrasound, magnetic resonance imaging (MRI), and computed tomography (CT) scans, which are employed in other species, often do not penetrate deeply enough to be useful. Consequently, abdominal tumours are often only found when a horse starts to show signs of failure of an organ because the tumour has taken over so much of the organ that there is little normal tissue left. Unfortunately, this usually means that it is too late to save the horse and this is exactly what happened to 2005 Eclipse Award-winning US sprinter Lost in the Fog. When the sad news first broke that Lost in the Fog had a splenic lymphoma, the American public could not understand how this was possible. How could such a robust four-year-old colt that had won the Grade Three Aristides Breeders’ Cup Handicap at Churchill Downs just two months earlier now be terminally ill with cancer? In fact, Lost in the Fog had three large tumours – one in his spleen, one in the ligament supporting the spleen, and one just next to his spine. It was impossible to know how long the tumours had been growing but they must have been there when he won his last race on 3rd June 2006. Sex hormone related cancers Sex hormone related cancers are not uncommon and there is a ‘textbook’ example for both sexes. A rig or cryptorchid refers to a male horse with only one descended testicle. If the horse is young, for example, less than two years old, then it may yet descend. However, if the testicle never descends, then it is probably located in the horse’s abdomen and this puts it at a higher risk than normal of cancer because the temperature in the abdomen is higher than the temperature in the scrotum where the testicle is meant to be located. Laparoscopic abdominal surgery to remove the testicle usually cures the problem unless the cancer is malignant and has spread. Cancerous cells not only display abnormal growth but the function of the new cells can also be abnormal. An example of this is a mare with an ovarian tumour known as an equine granulosa cell tumour which produces excessive levels of testosterone. This male hormone, normally present in very low levels in mares, leads to stallion-like behaviour and aggression towards other horses. Fortunately, this particular type of cancer is usually treatable via removal of the cancerous ovary, and the mare returns to normal after surgery. Keratomas By far the most common tumour of the equine musculoskeletal system is the keratoma, which is a tumour that arises from the hoof wall. They are usually found in horses that are seven years old or more during a lameness investigation because the growing mass can cause considerable pain, although they can sometimes also result in abnormal hoof growth. An x-ray diagnoses the problem and surgery is usually successful providing that the growth can be completely removed. Conclusion In summary, horses suffer from cancer just like any other species. However, equine cancer receives very little publicity for two main reasons. Firstly, the risk of serious cancers increases dramatically with age and illness in older horses is only really newsworthy in champions such as Doyoun. Secondly, the cancers that occur most frequently in younger horses are often not life-threatening, for example, papillomas, benign sarcoids, and benign melanomas in grey horses. This is why it was such a big story in North America when the reigning champion Lost in the Fog was diagnosed with terminal cancer just two months after winning a Grade Three sprint. Each cancer is different and every case must be treated individually. Veterinary surgeons have become quite proficient in the treatment of certain tumours such as sarcoids and keratomas. However, horses like Lost in the Fog show just how helpless veterinary surgeons are in dealing with malignant cancers such as a splenic lymphoma. Every veterinary surgeon and human doctor would dearly love for us to be able to treat malignant cancers more effectively, and many researchers around the globe continue to work tirelessly in the hope that one day there will be a more effective treatment.

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