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Understanding Cancer

In my latest book, The Natural Vet's Guide to Preventing and Treating Cancer in Dogs, I discuss many of the causes of cancer. I also share a number of tips for preventing or at least minimizing cancer in pets, and discuss a number of treatments that can help the pet with cancer. In this article, I'm going to discuss genetics in the environment and how these factors contribute to cancer in pets.

In a sense, all cancer ultimately arises from some interaction between genetics and the environment. To put it very simply, the cause of cancer follows a predictable pattern. The genetic information (DNA) of a cell is damaged, either through the normal growth and reproductive phases of the cell, or through some environmental influence. Damaged cells either die, are repaired through normal cells repair mechanisms, or live on and continue to reproduce forming cancer. There are numerous genes which control the multiple functions of each cell. These genes control the growth and reproduction of the cell, as well as the ability of the cell to repair any damage done to its DNA. Ultimately, a cell becomes cancerous when the repair mechanisms fail. This allows the damaged DNA of the cell to cause the cell to do things a normal cell cannot do, such as invade surrounding tissue, develop its own blood supply, and spread throughout the body.

While the canine genome has finally been elucidated, there is still much to be learned. In people, inherited cancer syndromes account for less than 5% of all cancers. We don't really have good figures for what percent of pet cancers are definitely inherited.

There are several ways I like to look at the heritability of cancers in pets. First, certain cancers seem to be more common in certain breeds of dogs. For examples, I'm never surprised to make a diagnosis of lymphosarcoma in a Labrador or Golden retriever, as this seems to be a very common cancer in these breeds. Likewise, osteosarcoma, a very malignant cancer of the bones, is more commonly seen in older, larger breeds of dogs. Also, tumors of the spleen and liver (hemangiosarcoma) tend to occur more commonly in larger breeds of dogs. I expect in the future we may actually uncover a genetic basis for these cancers in these breeds of dogs.

Second, we've all seen certain cancers recur in related pets. Obviously there must be genetic factors involved when relatives of a certain pet develop the exact same cancer.

Finally, there is no doubt that environmental factors can exert a negative influence on the genetics of a pet, which can lead to cancer. As an example, exposure to certain herbicides seems to cause an increased risk of bladder cancer (transitional cell carcinoma) in Scottish Terriers, a breed which is well known for developing this type of tumor.

Turning our attention to environmental factors, there are certainly some proven as well as some suspected environmental causes of cancer.

Hormones can certainly influence the development of cancer. It is well known for example, that there is an increased risk of breast cancer in dogs and cats that are not spayed before they reach puberty and experience their first estrous or heat cycle. The risk of developing breast tumors in dogs is almost 0% if the dog is spayed before her first heat cycle. The risk increases to approximately 25% for dogs that are spayed after their second heat cycle. This means that dogs (and also cats) that are not spayed until after several heat cycles have a one in four chance of developing breast tumors, about 50% of which are malignant in dogs and about 85% of which are malignant in cats. Early spaying therefore can almost totally guarantee that the pet will not develop breast cancer.

Excessive inflammation at the site of an injection, most commonly from repeated and unnecessary vaccinations, can lead to the development of malignant sarcoma tumors in cats and very rarely in dogs and ferrets. Depending upon which study is used, the risk for development of a vaccine associated sarcoma is 1 in 1000 or 1 in 10,000 cats. This finding underscores the fact that vaccination of pets, while sometimes necessary and helpful, is not a benign event. The exact reason why some cats which receive vaccines developed sarcoma tumors while most do not develop these tumors is not known. Speculation centers on the fact that cats which develop tumors may be genetically predisposed to develop more inflammation at the site of the injection; persistent inflammation can lead to cell damage causing cancer. It is also possible that certain genes, such as the p53 gene which normally prevents cancer, may not function properly in cats genetically predisposed to develop vaccine associated sarcomas.

It is well known that tobacco smoke increases the risk of lung cancer in people. However, the data linking tobacco smoke and lung cancer in pets is controversial. There is week evidence that exposure to tobacco smoke may increase the risk of lung cancer in pets. However, there is greater evidence linking exposure to tobacco smoke and an increased incidence of malignant lymphoma and oral squamous cell carcinoma cancers in cats. Regardless of the proven link between tobacco smoke and cancer in pets, it is prudent to avoid exposing your pet to tobacco smoke.

Various pesticides, insecticides, herbicides have also been linked to cancer in pets. The use of 2, 4-D has been linked to an increased incidence of lymphoma in dogs. Transitional cell carcinoma of the bladder has been linked to dogs treated with topical insecticides and several lawn care products containing phenoxy herbicides. Cats that wear flea collars have shown a higher risk of oral squamous cell carcinoma when compared to cats that do not wear flea collars. It should be stated that while there are studies which revealed all of these findings, there have also been studies that failed to find a link between the various chemicals and cancers in pets. Still it is wise to limit your pet’s exposure to these chemicals as there is enough evidence to suggest the possibility of harm to your pets, even if definitive proof in every study that has been done is lacking.

As is true with people, there is an increased risk of developing skin cancer in pets exposed to sunlight. Light skin color and chronic sun exposure increase the risk of squamous cell carcinoma. Fortunately, this cancer is usually easily cured with complete surgical removal if diagnosed early. It should also be pointed out that sun exposure itself is not harmful. In people, the incidence of malignant melanoma is not linked to sun exposure and this cancer tends to occur in parts of the body are shielded from the sun. In people, lack of sun exposure has been linked to an epidemic of low vitamin D levels, which actually predisposes people to various cancers. The same may be true for pets.

Finally, while not commonly discussed, it should be pointed out that chronic exposure to chemotherapy drugs can increase the risk of secondary cancers. For this reason, I tend to recommend that pets which are "cured" of cancer and which live longer than expected not continue to receive maintenance chemotherapy for the rest of their lives. This area of cancer medicine is controversial as many oncologists prefer to keep a pet which has survived cancer on maintenance chemotherapy to prevent a relapse. There is no right or wrong approach to this problem, and the owner must make an individual decision based upon all of the information at hand.

While the field of cancer medicine has come a long way in just the last few years, there is still so much to be learned. You now have a greater understanding of how cancer forms in people and pets. We're beginning to understand how genetics and the environmental factors which can influence genetics may increase the development of cancer in certain people and pets as cancer cells and give up more of their secrets, the hope is that we will develop better interventional strategies to prevent and treat cancer in people and in pets. For now, minimizing harmful environmental influences on our pets, breeding only pets with outstanding health, and using a combination of conventional and complementary therapies when necessary to treat pets with cancer is the best we can do to win the war against this dreaded disease.




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