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Cancer in our pets is a very broad and complicated subject. Its causes are no better understood than cancer in humans and its potential treatments are no different. So this article only scratched the surface of an emense landscape. I am not a cancer specialist; but like all veterinarians in general practice, I deal with the tumors in dogs and cats on a day-to-day basis. The great majority of these are older pets. Anything credible you read online about cancer in humans should apply equally well to cancer in your pet. But your veterinarian has considerably fewer advanced medication options. That is because monoclonal antibody (mAb) drug therapy is rapidly replacing the older, more toxic chemotherapy medications veterinarians and physicians have relied upon. Mabs home in precisely on the cancer processes one wants to destroy. They do so quite effectively. I believe that Mabs and CRISPR gene editing are the greatest breakthroughs in medicine that have occurred since the discovery of DNA. (ref1, ref2) But each mAb medication must be taylor-made for the species it is given to. in 2017, Zoetis (a spinoff of Pfizer) purchase Nexvet, an Irish company that developed a rapid and economical way to quickly make mAbs designed for humans acceptable to dogs or cats. (ref) That should speed development of a host of new therapy options for our pets quite soon.
I have always tried to furnish my clients with sound, practical advice during difficult times. Many cutting-edge cancer therapies offered by veterinary oncologists, universities and hub veterinary centers undoubtedly do extend the lives of pets. But because something can be done does not necessarily mean it should be done. I sometimes question the quality of these brief periods that can sometimes be added to your pet’s life and the emotional costs of them to both of you. I also have doubts whether many of these therapies are truly performed for the good of the pet. I sense that they are often performed for the peace of mind of an owner who is unable to bear the grief or accept the fact that all life must come to an end. So so often, it is long before we want it to. Please feel no guilt or shame if you accept that.
How Likely Is It That My Pet Will Develop Cancer ?
The likelihood that your dog or cat will develop cancer during its lifetime varies from place to place and breed to breed – but not by that much. In the UK for example, the likelihood that your dog will develope a tumor increases sharply after the age of 6 years. But once your dog reaches the age of 10, that likelihood begins to decrease. In 130,684 dogs that were followed there, about 2,671 cases of cancer appeared per year. Skin tumors led the list, followed by tumors of the digestive system, breasts, immune system, glands and mouth, in that order. Other studies in California dogs and dogs in Ontario ranged from 1134 - 4817 tumors per 100,000 dogs. Cat in the Ontario study had a much lower incidence of cancer than dogs, 748 per 100,000 cats. However, considerably more of the tumors on cats were the much more serious, malignant kind. A problem with that Canadian study was that it did not adjust the incidence for age. (Cats that come to veterinarians tend to be young ones; and those cats are the least likely to have cancers other than those caused by feline leukemia and feline AIDS) You can read those studies here and here.
Why Do Cancers Occur ?
Your pet’s cells are forever growing and replacing themselves, and growth gone awry is the basis of most cancer. Normal cell growth and replacement fills a bodily need. When cells grow for any reason other than the good of the body we call them cancerous or tumorous.
Cell growth is strictly controlled by instructions written into the DNA code of every cell in the body. In tumorous cells an error has occurred in this script, allowing the cells to grow out of control and evade the immune system policemen in charge of preventing it. (ref) When these errors are minor, the cells still look and act a lot like normal ones. We call tumors formed from these cells benign. When the errors are major we call the tumors malignant.
The Characteristics of Cancerous Cells :
Small snippets of tumor tissue are called biopsies. Examination of biopsies allows veterinary pathologists to determine if the growth is benign or malignant, the type of cells involved and what the best treatment might be. (Your vet will submit those samples in a way similar to this.)
A fibrous capsule often covers benign tumors and relatively few of the tumor cells are actively growing. Some common benign tumors of dogs are the lipomas or fatty tumors that form under the skin of dog’s (usually when they are too chubby) . Another type are the cauliflower-shaped papillomas that form within the skin and protrude out as "warts". Under a microscope, the cells of these tumors look very much like normal tissue. The borders of these tumors are usually regular and well-defined making them easy to remove surgically. Skin tumors on cats tend to be much more worrisome. Some turn out to be the dangerous fibrosarcomas associated with prior over-vaccination.
Before your vet removes skin tumors he/she will clip its fur coat off close to the skin and make the area as free of bacteria as possible. I usually send owners home with a surgical marker to mark the position of even the smallest tumors. The one the client brought be pet in to have removed may be largest, but it is uncommon for there to be just one papilloma or lipoma on your pet. It is exasperating for pet owners and veterinarians alike to find others a few weeks or months after the initial surgery.
Every veterinarian has techniques they are most comfortable with. I personally do not like to take dogs into deep anethesia for the removal of these superficial tumors. It is possible to just heavily tranquilize the pet and infiltrate the area surrounding the tumor with a combination of local anethetic ( xylocaine [ Novocaine]) and epinephrine just before and during the procedure. This both numbs the area and stops loss of blood. Then the tumors can be easily removed with a scalpel or electrosurgical unit , the incision sutured and a drain added if need be. I generally freeze or cauterize off small papillomas. I have never had a tumor of this type regrow. Lipomas or benign fatty tumors are present just under the skin. They are considerably more common in dogs than in cats. I have only seen them on chubby dogs. They are not a threat to your pet's health. If the dog looses sufficient weight, these tumors willshrink on their own. Because they are invariably encapsulated and have little blood supply, they are quite easy to remove. Lipomas that have grown around nerves or large blood vessels are considerably more difficult to remove (and it may be wiser to leave them alone as long as possible.) But even with these, the outcomes are usually good if sufficient care is taken to preserve the nerves and blood vessels in the area of the lipoma. Your veterinarian can do a simple microscopic test in the office to confirm that the tumor is a simple lipoma.
Tumors that arise from glandular and surface cells are called carcinomas. Tumors that arise from muscle, bone and fibrous connective tissues of the body are called sarcomas. When cancers are found in their original location they are called primary tumors. When they have moved (spread) to a new location in the body they are called metastatic tumors. Only malignant tumors have the capacity to move to new locations. Because of this and their invasiveness, they are the life-threatening ones.
Cancers that move often become trapped in the sieve-like structures of the lungs, liver, bone marrow and kidneys. When they do, the symptoms that we see are due to the physical destruction of these organs as the tumor grows and crowds out healthy cells. Not anything toxic the tumor cells produe (see the tests). Metastatic tumors are usually highly vascular. That is, they are rich in blood vessels to supply the nutrients that the fast-growing tumor cells require.
Cancers have many causes or risk factors. Agents that increase the likelihood of cancer are called carcinogens. Some of the risk factors are written within the genetic code you pet was born with which made it particularly susceptible to one form of cancer or another.
Boxers and the giant breeds of dogs are renown for their predisposition to certain tumors. Other risk factors, such as cancer-causing or oncogenic chemicals, may be found in the pet’s environment or diet (formaldehyde, chlorine-containing compounds, nitrites, etc.). They are the same products you should avoid to preserve your own health.
Some of these chemicals cause the cells genetic code (DNA) to mutate and so are called mutagens. Physical agents (radiation, asbestos, etc.) can also cause cancer through chronic irritation and inflammation. Certain tumor-causing viruses have also been found to cause cancer in animals. Often cancer results from the combined effects of genetics, physical and chemical carcinogens. Your pet's immune system plays an important role in detecting and eliminating new cancers.
Any factor or disease that causes immunosuppression increases the risk of tumors occurring in your pet. Feline immunodeficiency disease (feline AIDS) and feline leukemia both of which are caused by retrovirus, are conditions leading to a variety of tumors in cats. Hormones that cause body organs to proliferate excessively, like hyperthyroidism, can also progress to cancer.
Breast or mammary tumors in dogs are quite common and occur almost exclusively in older unspayed females. This is because of the twice-a-year hormone rises that unspayed female dogs experience, associated with their estrus or heat cycles. Luckily, breast tumors in dogs are much less dangerous than breast tumors in people. The ones that occur in dogs rarely metastasize or threaten your dog's life.
As in people, the earlier we detect and remove the more dangerous cancers from pets the more successful we are. Skin tumors are rather easy to diagnose - and some of them do need immediate removal. But tumors within the body often only show up as weight loss, low-grade fever, weakness, and lethargy. By the time these cancers are large enough to detect they can be in advanced stages and very difficult to treat successfully.
X-rays (radiographs) are often the first choice in diagnosing internal tumors in dogs and cats. Many tumors are bulky and distort the shapes of the organs they reside in making them readily apparent on x-rays. Many can also be seen using ultrasound equipment. Large veterinary facilities and universities have more sophisticated CAT-scan and MRI imaging equipment to use when simpler techniques fail or when difficult surgery is contemplated. Since smaller veterinary hospitals do not have this equipment, they often make more use of biopsies and exploratory surgery to diagnose and treat cancer. When the abdomen is opened and all the organs inspected, small tumors or those that were not visible on radiographs for one reason or another are sometimes obvious. When they are not, biopsies of the major organs , sent to a pathologist, often discover the tumor. Pathology reports also reveal the aggressiveness of tumors and the likelihood that they have already moved or will move and threaten your pet's life.
The most common cancers in dogs are those in or near the skin. Skin cancers make up over half the total number of cancers that occur in dogs they are considerably less common (but more worrysome) in cats :
Among the most common skin cancers that veterinarians encounter are papillomas. These are small, cauliflower-like, viral-produced tumors that proliferate as a dog ages. They are common on the muzzel, trunk and extremities of dogs with graying hair. The vast majority are not malignant and cause no damage beyond being easily nicked or worried into bleeding by the pet as it grooms. When they form on the edge of the eye lid, they do need to be removed as they tend to irritate the cornea. The vast majority are easily removed by your vet with a tranquilizer and the aid of a bit of local anethetic.
The next most common cancer in dogs are lipomas. They are soft, and often have the consistency of a baggy filled with water. They are almost never life-threatening. They often occur multiply just under a dog’s skin. They are much rarer in cats and ferrets but quite common in parrot-like birds. They will shrink, but not entirely disappear, if you put your pet on a lower calorie diet.
Mast Cell Tumors
The next most common skin cancers in dogs are mastocytomas (or mast cell tumors). These distinctive tumors are oval, firm and slightly raised. Sometimes their center is brown or bluish. Mastocytomas are usually only locally invasive (malignant) and do not readily metastasize to other organs. Cancerous cells project outward from the tumor into what appears to be normal skin. So when they are removed, a considerably larger diameter than the visible tumor needs to be excised to be sure that all tumorous cells are removed. The biggest problem occurs when mast cell tumors are on the extremities (legs) where insufficient extra skin may be left to close the incision after the tumor is removed. Your dog's individual genetics play a part in its susceptibility to mast cell tumors as well. (ref)
Skin tumors on cats are considerably more likely to be malignant than those on dogs. Your vet need to removed them as rapidly as they are discovered and always sent the removed tissue to a veterinary pathologist for evaluation. In an unfortunately high a number, these tumors have already metastasized to other locations before they were discovered.
Fibrosarcoma In Cats
The most common cause of these tumors is thought to be repeated vaccinations with vaccines. You can read about that problem here. This is a heartbreaking condition because it often occurs in relatively young cats and it was usually preventable. It can sometimes be treated successfully with surgery - but the surgery must be very aggressive, removing a large amount of tissue surrounding the tumor - possibly even bone. However, surgery alone is only successful if every single tumor cell is removed or destroyed and this is quite difficult to do. The larger the mass and the longer it has been there, the less likely that surgery will be successful. Up to 70% of these tumors re-grow after surgery. Chances of your pet's survival increase if the surgery is followed by combinations of radiation therapy and immunotherapy. When you do decide to have your cat vaccinated, insist that the shot be given low on a rear leg or, better yet, in the tail. If a tumor occurs there, that portion of the pet's body can be sacrificed preserving its life.
Mammary Gland Tumors
Mammary gland tumors have a high incidence in older unspayed dogs. They often begin to develop between six and ten years of age and are caused by the hormone, progesterone, associated with estrus, and reproduction. The most common form is the fibrous and hard mixed mammary carcinoma. They form most frequently in the posterior breasts nearest the rear legs. Often the breasts involved give small amounts of milk or milk-like fluid. Most are well encapsulated and easy to remove. They are usually not highly malignant and most of the time vets remove them them long before they have metastasized. Of special concern are tumors that are ulcerated (bleeding) and which have infiltrated the skin. These might be malignant. Also worrying are mammary tumors that involve the lymph nodes of the groin and those at the base of the foreleg. It is not uncommon to sent sections from the same tumor of this type to two different pathologists and receive differing opinions as to the tumor's degree of malignancy. This is because determination of malignancy is a subjective process. Spaying females before their third heat cycle can help reduce the likelihood of these tumors.
Lymphatic tumors are tumors of certain white blood cells called lymphocytes. They are classified as lymphosarcomas, lymphomas and lymphoid leukemias. These tumors are quite common in cats, dogs and ferrets. In cats, these tumors can occur under the immunosuppressive effect of the feline AIDS and Feline Leukemia virus. Cats are also quite susceptible to a form of lymphoma that invades their intestinal walls (intestinal lymphoma). They are the second and third most common tumors in ferrets and dogs respectively occurring most commonly in Golden and Labrador retrievers and Doberman pinchers. In ferrets and dogs these cancers appear spontaneously. The tumors appear as sold growths which begin in the lymph nodes or bone marrow or as individual cells freely circulating in the blood, in which case they are called leukemia. Animals as young as four years may develop these tumors. These animals are often presented to veterinarians with painless, enlarged lymph nodes over the whole body; but occasionally it is a single lymph node that is enlarged. Some of the dogs have an increased number of abnormally large lymphocytes in their blood stream but most do not. At this early stage the pets do not appear to be ill. Other animals, particularly cats, develop this form of cancer in the walls of their intestines, which leads to diarrhea and weight loss. A biopsy of one of the enlarged superficial lymph nodes confirms the diagnosis when those are the ones affected. This type of tumor in dogs responds well to chemotherapy. It does not respond that well in cats. The drugs commonly used to treat lymphomas are vincristine, L-asparaginase, cyclophosphomide, doxorubicin, and prednisolone. With this treatment, three-quarters of the dogs caught early in the disease live an additional six month or more. Without treatment their average future life span is about four months.
Tumors Of The Mouth, Lips and Tongue
Tumors of the mouth, lips and tongue are relatively common in dogs and cats. These tumors often bleed by the time they are noticed. A large percentage of them are highly malignant – especially in cats. A variety of tumors form here. They include squamous cell carcinomas, adenocarcinomas, fibrosarcomas and melanomas. Dogs that develop these tumors are generally six to ten years of age. A big problem in dealing with these tumors is that they often surround important structures of the mouth and are therefore next to impossible to remove in their entirety. But most of these tumors can be surgically removed (debulked) and the dog or cat then treated with radiation. This procedure is particularly stressful to cats that may have to be force-fed or fed intravenously. As i mentioned earlier, I am not inclined to suggest radical surgical procedure to most clients but I do make them aware that these procedures exist. There is some evidence that the viruses responsible for papilloma might be involved in the formation of some of these oral tumors in dogs as well. Chemotherapy has not been very rewarding in these cases. Before considering chemotherapy for your pet, ask your veterinarian what the average increased in life expectancy after the procedure might be. Specialists tend to be highly optimistic about the benefits of their specialty so take their numbers with a grain of salt and really try to pin them down. (ref)
Bone cancers or osteosarcomas occur frequently in large and giant breeds of dogs. They tend to form at the growth plates near the ends of the long bones of the legs. These dogs are often brought to veterinarians initially because of lameness. X-rays of these tumors are highly distinctive and usually easy to diagnose. Because they often metastasize to the lungs, vets include a chest film of every dog radiograph for this problem. Not all cases are so advanced that tumors in the lungs can be detected. That does not mean they are not there. When the tumor has moved, later in the disease these dogs may have a cough. Neutered male dogs have a higher risk of this disease, as do dogs with a previous injury to the leg involved. The best treatment for these tumors when they occur on a leg is the removal of that leg. A chemotherapeutic drug called Cisplatin along with radiation treatment is thought to be helpful in cases where the tumor is inoperable because of location. Most dogs do quite well with only one rear leg. But I rarely suggest that a front leg be removed from a large dog because of the extent of disability that produces.
Keeping Your Pet Happy
Our pets are blessed by the Creator in that they do not fear the future or dwell on their illnesses. Unlike us, they do not worry about the passage of time or where they will spend eternity. So if they even consider it, they face death with considerable peace. Veterinary medicine has made tremendous strides in delaying the death of pets. You can read an article that explains strategies your veterinarian can use to do that here. But would your pet want you to do those things? My feelings are that it probably would not. Will you be doing those things for the pet's happiness or for your happiness? Your pet will remain happy if it has something good to eat, a loving home and the pleasure of your company. Pet's key off of their owners emotions. So if you are sad, they will be sad - if you are at peace and accepting, they will be at peace too.
Be wary about purchasing products online that claim to cure cancer. They do not work. I have never understood how anyone could be so cruel and cold-hearted as to sell them. That goes for any Google ads that might sneek onto my website as well. They are placed there by a robot and - as you know- robots treat the honest and the dishonest equally.
Feeding Your Pet When It Has Cancer
Nutritional support is very important for all cancer patients. This is especially true for cats because minimal cancer-induced stress can make them disinterested in eating. Cancer cachexia is a form of malnutrition that affects many pets with cancer - especially those in which the disease is widespread in their bodies. Offering flavorful, highly digestible and energy-dense diets can reverse some of the signs of cancer, prolong their lives and increase their happiness. The best diets for pets with cancer are rich in fat and protein and low in carbohydrates. They are also the ones you make yourself. Read about them here. Stimulate them to eat with savory home-cooked aromas, pet them, hug them, encourage them, love them.