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COMMON CANCERS IN PETS
The lymphatic system is part of the body defence system. Fluid which leaks from the blood system within the tissues is collected and travels though small tubes to lymph glands (or lymph nodes, more correctly). These nodes filter any debris, such as bacteria or fragments of damaged cells, from the fluid. The cells in the nodes monitor the fluid for infections and form the first line of defence, setting the rest of the immune system into action against the infection. The nodes also trap spreading cancer cells if they get into the lymphatic system, and secondary tumours can grow in the nodes.
Cancer of the lymphatic system takes several forms. The commonest is where most of the lymph nodes in the body swell with malignant lymphocytes: this is multicentric lymphoma. Otherwise, lymphatic tissue within other organs can develop malignant cells: the commonest sites are the intestine, kidneys, spleen and liver, though any organ can be affected.
The common type of lymphoma in humans, Hodgkin's lymphoma, does not occur in pets: they have the equivalent of non-Hodgkin's lymphoma. There are 2 types of lymphocyte, T and B, and a lymphoma can be composed of either. B cell lymphomas are easier to treat than T cell, but checking the type is fairly expensive.
Treatment of multicentric lymphoma in pets is fairly routine these days. The other lymphomas can be more tricky. The disease is treated by chemotherapy (for more information click here) and 70-80% of patients will have a complete remission and be well. The disease free interval for lymphoma patients varies from 3 months to 4 years. This is because a) T cell lymphomas do not respond as well as B cell and b) there is a variation in the resistance/sensitivity to treatment of cells in the lymphoma and if there are a lot of resistant ones, they will grow despite the treatment.
In cats, lymphomas are commonly caused by Leukaemia Virus. Vaccination of cats against this common virus is very effective protection.Go to top of page
The mammary glands are complicated and are made up of many different cell types. Most mammary tumours are 'mixed cell' tumours. About 50% of mammary lumps in bitches are benign when they first appear, but there is a significant risk of them becoming malignant over about 2 years, so they are always best removed.
Early surgery is absolutely the best approach to mammary tumours. There are some very aggressive tumours in bitches but these are, fortunately, rare. They are usually untreatable and progress very quickly.
In cats, 96% of mammary lumps are malignant, and they tend mostly to be quite rapid growing, so surgery is urgent. However, mammary tumours are quite rare in cats because most female cats are spayed. This is very effective prevention.
Spaying is also excellent prevention in bitches. If a bitch is spayed before her first heat, she will have the same risk of mammary tumours as a male dog: in other words, very, very low risk (but they can occur). Every heat a bitch has increases her risk of mammary cancer.
Other than surgery, it is unusual to use other treatments, though immunotherapy has been shown to be of benefit.Go to top of page
The skin is reasonably visible on pets and, generally, skin tumours are noticed quite early by owners. Many skin bumps are benign tumours or just cysts, but there are some potentially nasty skin tumours, so it is always best to have any lump checked by a vet. The vast majority of skin tumours can be cured surgically if seen early.Go to top of page
Owners do not often look in their pets' mouths, so many mouth tumours are comparatively large when first seen. The commonest symptoms are smelly breath, salivation, discomfort eating and bleeding from the mouth (also symptoms of dental disease). By the time any of these symptoms shows, the tumour is likely to be a fair size. Most vets will check pets' mouths when they see them for booster vaccinations, or during general examination (except for the difficult pets!) and it would be hoped that tumours would be spotted then. If the tumour is at the back of the mouth, it can be difficult to see in the early stages except in a very cooperative pet. Tumours in or under the tongue are tricky, as are those in the throat.
Many mouth tumours are malignant: the main exception is the epulis, though one type of these can invade the bone and become a major problem. If mouth tumours are seen early enough, relatively minor surgery may be sufficient to cure them, but most will need major surgery. This can have cosmetic complications and owners need to be aware that their pet's appearance may be affected by surgery. In some cases owners are unwilling to allow such surgery, in some cases it is not reasonably possible. Many of these patients could benefit from radiotherapy, though there are side-effects such as mouth drying and hair colour changes associated with that. Most mouth tumours are to some extent sensitive to radiotherapy. Chemotherapy is less used, though there are situations where it can be beneficial.Go to top of page
Pets tend not to show symptoms until tumours in the intestine are quite large. Vomiting is often seen with gastric tumours at a fairly early stage, but these are not easy to diagnose except by endoscopy. Some intestinal tumours do not grow as lumps, but as a general thickening of the intestine: most patients with this type of disease will have diarrhoea.
Surgery is the first line of treatment for tumours of the stomach and intestines. Radiotherapy or chemotherapy are less useful except in intestinal lymphomas.Go to top of page
Major organ tumours.
The liver, kidneys, spleen, lungs, heart, reproductive organs and brain can all develop tumours. Removal can be difficult, and will depend on detailed assessment of the site and type of tumour. Animals cannot survive without a liver, heart or brain; one kidney or part of a lung can be removed if the remaining parts are in good condition. The surgery can be complex.
The spleen is quite commonly the site of tumours. Some of these can enlarge very rapidly and may rupture to lead to abdominal bleeding. If found in time, removal of the spleen can cure most.
Radiotherapy of the abdomen can have nasty side-effects and is rarely done. Chemotherapy is of limited use in most major organ tumours. Surgery is the best option, but not all cases are suitable.Go to top of page
Tumours in rabbits.
Most lumps on rabbits are abscesses (but these can be as difficult as cancers to deal with!).
The commonest tumours of rabbits are in the uterus of the old doe and in the mammary gland. Uterine cancers are usually advanced when found and hysterectomy will usually provide only temporary relief for the rabbit.
Both uterine and mammary tumours are very well prevented by spaying (neutering) of does.Go to top of page
Tumours in small furries.
Small rodents are particularly prone to cancer. Mammary, skin and thyroid tumours are common. It has been suggested that one reason is the use of wood shavings or sawdust as bedding: if the wood has been treated with carcinogenic wood preservatives before milling, these chemicals build up in the pets.
Many tumours are surgically removeable, but some will be malignant and may already have spread. Surgery will usually give a few months of control, which in a cage pet can be a significant percentage increase in life.Go to top of page