The Dog Cancer Survival Guide (29 page)

Purebred dogs who have been spayed or neutered are
twice as likely
to develop osteosarcoma (bone cancer), and the risk is even higher for Rottweilers. One study showed that both male and female Rotties, who were surgically sterilized before the age of one year, had an approximately one in four risk for developing osteosarcoma during their lifetime, while intact Rotties were much less likely to develop the disease.

Spaying has also been shown to increase the risk of lymphoma, and other studies show that castration increases the risk for developing transitional cell carcinoma (bladder cancer) by three to four times. Spaying and neutering also increases the risk of prostate cancers and hemangiosarcoma of the heart.

Osteosarcoma, transitional cell carcinoma, hemangiosarcoma of the heart and lymphoma are generally more aggressive than mammary cancer; one in four of which is cured with surgery alone. These factors should be considered when we decide whether to spay and neuter dogs.

The prevailing opinion in the veterinary community is that sterilization is good, and keeping dogs intact is bad. There are some good reasons for this, one of which is the huge population of unwanted dogs in this country. Sterilization also helps to reduce uterine infections, and, in some cases, unwanted behaviors like humping and marking. It can also reduce aggression in some dogs. It’s also worth mentioning that spays and neuters are a profit center in many vet practices, mine included.

Because the increased cancer risks from spaying and neutering are serious, guardians should consider the big picture when evaluating the timing of these procedures. Most dogs reach sexual maturity at about twenty-four months, approximately at the fourth heat in females. At this point in their development, dogs have received the protective benefit of adult sexual hormones and are at a decreased risk for the cancers mentioned above. If you choose to spay or neuter your dog, my general recommendation is to spay females sometime between the third and fourth heats – which will have the added benefit of reducing the risk of mammary cancer – and neuter males sometime between the ages of eighteen and twenty-four months.

There are other health issues related to spaying and neutering that are beyond the scope of this book, and all of these factors should be considered. There may be a reason why you should spay earlier in life – as always, this decision should be made on a case-by-case basis, in consultation with your vet.

The sun is essential to life. In addition to providing light, warmth, and energy, there is new evidence that sun exposure in the right amounts – just fifteen to thirty minutes per week – can actually protect us from cancer by stimulating the body to generate the active form of vitamin D.

Excessive sunlight exposure, however, can increase cancer risks in some dogs, particularly dogs with light skin pigmentation. The two dog cancers associated with sun exposure are skin hemangiosarcoma and squamous cell carcinoma. Oddly, canine melanoma is not associated with sunlight, as it is in humans.

I see a great deal of skin hemangiosarcoma in Hawaii, where we get lots of equatorial sun all year round, and mostly in breeds that are already prone to develop it. A colleague from South Africa, Dr. Crewe, sees much more squamous cell carcinomas than skin hemangiosarcoma. She believes that this is because the ozone layer is so much thinner in the southern hemisphere than it is at the equator, allowing stronger rays to pass through and affect the skin.

It is probably rare that sunlight directly
causes
cancer, but it can tip the scales in favor of cancer, when other risk factors are also present. If your dog has a cancer linked to sun exposure, using sunscreen (making sure to keep it out of her eyes and nose) and keeping your dog indoors, especially during midday hours, is a good idea. Dr. Crewe even recommends a Lycra full-body suit, made just for dogs (also available at
www.DogCancerShop.com
).

Viruses can cause growths on the body: warts, for example. Usually benign, these growths can be contagious, or transmitted through physical contact.

One type of benign growth that you may see on your dog is called a papilloma. These are small, fleshy, wart-like growths that often show up on young dogs. They are mainly a cosmetic issue, although if they are large and occur in the mouth, they can create problems.

There are also venereal tumors, some of which can spread during mating. These tumors can interfere with fertility if they’re in the wrong location. Sometimes dogs chew or scrape them, which can lead to bleeding or infection.

Very rarely, a venereal tumor can be cancerous. Because these tumors are suspected to be caused by a virus, this cancer, known as canine transmissible venereal tumor (TVT), may be transmitted to other dogs through physical contact, such as copulation.

Mothers, throughout the millennia, have implored their children to get a good night’s sleep, and those children have often ignored this excellent advice, risking their health as a result. Perhaps because we’re unconscious during sleep, we often don’t realize just how many essential body processes happen during those hours; many of them can’t happen unless we’re asleep with the lights out.

One of those crucial processes is the manufacturing of melatonin, a hormone that, among many other helpful body activities, fights cancer. One way it does this is by blocking a dietary fat, called linoleic acid, from entering cancer cells (linoleic acid stimulates some cancer cells to grow).

There are numerous studies showing that melatonin can lessen the risk of cancer development and progression. In one, higher rates of breast cancer were found in women working the night shift, and in another, higher rates of breast, uterine and prostate cancer were found in men and women with low melatonin levels.

Correcting a melatonin deficiency in cancer patients has been shown to lessen cancer cachexia (weight loss), reduce metastasis, lessen the side effects of chemotherapy and radiation treatments, help with sleeplessness, and even extend life expectancy.

Melatonin is generated by the pineal gland, a pea-sized gland located between the eyes, deep in the skull. The pineal gland does not generate melatonin in the presence of light, especially blue light, which is why complete darkness at bedtime is essential. And just turning out the lights and blocking out light from outside is not enough. Televisions and computer screens throw off a lot of blue light, so reducing their use in the evening can help to generate more melatonin.

Our dogs generally sleep when we sleep. Making sure that your dog has a dark place to rest, and goes there early in the evening (perhaps by going to bed yourself), can boost his melatonin levels and give him an edge on cancer. Some dogs may also tolerate melatonin supplements (the doses are on
page 185
).

I’ve given vaccines to thousands of dogs over the course of my career, so when I first heard that vaccines could be linked to cancer development in dogs, I was absolutely sure that was not true. Even the word “vaccinosis” sounded made up to me, as if it were created by someone with an axe to grind, someone who wanted to vilify vets who vaccinate dogs.

It turns out that vaccines may, at least theoretically, be linked to cancer development in dogs, as is shown in studies conducted in rodents and humans. To understand how, let’s quickly look at how the immune system works.

There is a well-known soft tissue sarcoma tumor associated with vaccines and other injections that develops in cats at the injection site, but this syndrome does not occur in dogs.

To defend the body against pathogens (viruses, bacteria and other microorganisms) and against damaged or mutated body cells (cancer), the immune system deploys several layers of defense.

Barriers
physically protect the body by blocking pathogens. An example is the skin, which almost completely seals the body off from the outside. Another barrier is mucus, which entangles pathogens in slime so they can’t enter the body. Another is the lungs, which can initiate coughing or sneezing to eject pathogens. There are also barriers inside the body, which help defend cells from microbes that succeed in reaching them. For example, cells have outer membranes, which are like a skin and have chemical barriers, which can neutralize or block pathogens.

The
innate immune system
activates if viruses, bacteria, or other microbes make it through barriers and enter the body. This system takes immediate and massive action to deal with problems. One example of many innate immune responses is inflammation. If a tissue is damaged, it sends chemical signals to alert the immune system, which immediately responds with extra blood flow to the area, and an increase in the concentration of immune cells, which can destroy foreign cells and repair tissue, among other tasks. Natural killer cells are also part of the innate immune system. These cells can recognize and target cancer cells and cells that have been infected by a virus.

The
adaptive immune system
is more sophisticated than the innate system. This part of the immune system has a “memory.”When certain cells from this part of the immune system encounter a pathogen, they develop a response (antibody) that is specific to that pathogen. Once they have destroyed it, the system can “remember” it in the future, and respond to it much more quickly than the first time around. This process is called humoral immunity.

In general, the purpose of a vaccine is to stimulate immunity. When an injectable vaccine is used, a tiny amount of a pathogen (a modified virus, for example) is injected: not enough to cause a full-blown infection, just enough for the cells involved in humoral immunity to learn about it and develop a method of destroying it. If the body is exposed to the real virus later in life, the adaptive immune system “remembers” it and eliminates it before it can do any damage.

Vaccines are very useful, because they use the immune system’s natural actions to protect the body. When everything works perfectly, it is very unlikely that the vaccinated disease will ever develop. Vaccines are a miracle of science, and they have prevented many very serious diseases. However, after decades of using vaccines to immunize humans, especially infants, against specific diseases, the picture is not entirely rosy.

According to researchers, early vaccinations in human infants stimulate humoral immunity at the expense of another function of the adaptive immune system, called cell-mediated immunity. Cellmediated immunity includes cells such as natural killer cells and cytotoxic T-cells, both of which target cancer cells for destruction. As fewer natural killer cells and cytotoxic T-cells are produced, more pre-cancerous cells go undetected, which could mean cancer has more opportunities to develop. By immunizing our infants at a young age, we may be making a trade-off: increased humoral immunity against specific diseases that come from outside the body in exchange for decreased cancer-killing cell-mediated immunity.

This is called a polarity shift, and studies in mice have found that once an immature immune system has made this shift, the effect tends to last well into adulthood. So, a choice made in childhood may raise the risk of cancer development later in life. We don’t yet know whether infant vaccines actually increase the overall cancer rates – but the question is certainly worth closer examination.