By Dr Walter Tarello, Pet Connection Veterinary Clinic

Many people believe that cancer is a man-made unnatural condition caused by bad habits and polluted environments. So, in our culture there is a strong tendency to feel guilty and blame ourselves not only for our illness but also for the sickness of our pets. That’s why understanding canine cancer is so important.

When we go to survey the rate and type of cancers in many animals and pets, we realise that wherever cells divide and are replaced, DNA replicates and tissues grow, there will be cancer. When DNA replication contains errors, as it happens sometimes, the out-of-control growth we call cancer can develop. From this standpoint, unfortunately, cancer is natural as birth, reproduction as death.    

A prevailing theory attributes the growth of tumors to progressive difficulties that aging immune system has to face in distinguishing its own cells from foreign cells. As such cells in any organism are constantly replaced. Red cells live four months, while bladder cells are totally replaced in two months, intestine cells are substituted every few days. The hairs, like cancers, are growing constantly but orderly. Making a new cell requires copying every single one of the roughly three billion small molecules called nucleotides lying in a cell’s DNA.

Also read: Understanding canine epilepsy

What is cancer?

Occasionally, 1 over 10,000 nucleotides is mistaken. The molecule involved can be doubled or placed in the wrong place or left out. These are so called mutations that most of the times are repaired and fixed within the original cell. Occasionally a mutation is introduced in the new cell that often goes ahead normally even with the misprint. Sometimes these mistakes harm the cell’s function. Normal cells are provided with suicide codesthat trigger self-destruction (apoptosis) when the cell is damaged beyond any possible repair, or it is too old to carry on its work. Mutation in the genes that direct the normal cell-suicide-programme can cause the cell to stay alive, immortalised, forever, replicating tens, hundreds and thousands of times, producing a legion of immortal cells that grow out of control following abnormal DNA instructions. This is cancer.        

A tumor that tends to remain in the same location we call it benign whereas a cancer that travel far from its original location, replicating in a new site named metastasis, is generally considered malignant. This is due to the tendency to spread and the often-fatal outcomes.  

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Cancer has many faces and causes

Why do pets with access to best medical care, kept with plenty of food in leisurely conditions grow cancers? In the bare fact of developing a cancer pets resemble all other animals and humans. A cancer is just the consequence of an error in the genetic code promoted by many factors, agents, conditions and exposure to toxic agents. Cancer has many faces and causes.

Animal species and breeds differ in their repair mechanisms and rate of aging since genetic susceptibility to a number of cancers vary greatly between individual, breeds, species and size as well.

For unknown reasons beagles and dachshunds get cancer less often than others may be due to a not yet unveiled genetic protection.

Boxers constitute the canine breed more exposed to the risk of skin cancers, such as mast-cell or brain tumors. This is something that pet parents should be aware of.

While German Shepherds are prone to develop a kind of heritable renal tumor like the way German families from the Black Forest region are susceptible to kidney cancer. The genetic mutation that causes renal cancer in German Shepherds is similar to the one that causes the Birt-Hogg-Dube’ syndrome in people, characterised by increased risk for renal cancer too.

Salukis are among the oldest breeds known to humankind and therefore they keep at the purest level the original canine DNA. However, the genes that codify for their lean, elegant physical appearance also bring with them a high risk of developing hemangiosarcoma, an aggressive tumor that affects heart, spleen and liver. Sometimes a good physical feature comes with the hidden genetic predisposition to some tumors because hereditary traits are not isolated from each other. Importantly breeding for desirable characteristics involuntarily can select and transmit other mutations some of which can cause cancer.

Osteosarcoma, a bone cancer which strikes tall human teenagers during their growth (for instance, cyclist Lance Armstrong) tends to affect many long-limbed and giant-breed dogs. My beloved Borzoi Antonov was killed by an osteosarcoma aged 11 years. Amputation was not capable to stop the spread. This cancer is also common in Saint Bernard, Great Dane and Doberman.

Mouth cancer is common in Labrador retriever. Chow-Chows have high rate of melanoma. As in people, many canine cancers become resistant to therapy. In both species they can recur after successful surgery or chemotherapy. 

A revolutionary ongoing project helps in understanding canine cancer

In 2012 a revolutionary project called the Canine Lifetime Health Project began to enroll hundreds of Golden Retrievers for a longitudinal study that will follow each dog for the rest of their life, focusing on cancer in a breed that has a 60% risk of dying from tumors. The study will last two decades and will measure everything in the life of dogs involved from food to radiation exposure, from genetic codes to pollutants in their environment. It’s expected that the study may yield radically new information about canine cancer – where it comes from, how it spread and possibly how to stop it.     

Human tumors are remarkably like those of dogs, so observing and treating cancer in companion animals may help to develop correct cancer therapies in humans. It is called comparative oncology. Studying naturally occurring tumors in pets who share our homes (with their catalog of toxic substances), our habits (secondhand tobacco smoke, for instance), our food (chemicals) and exposed to electromagnetic fields will help to develop new therapeutic strategies for humans as well.       

Comparing human and canine melanoma scientists have recently discovered that the two conditions are virtually identical. Malignant melanomas often show up in the mouth and under fingernails. In both humans and dogs, the cancer metastasizes to the same spots favouring adrenal glands, lungs, brain, heart and liver, resisting chemotherapy.

A vaccine for canine cancer?

Collaboration between veterinarians and physicians led to the release in 2009 of a melanoma canine vaccine (Oncept, Merial). The drug was tested on 350 pet dogs with spontaneous occurrence of melanoma. Therapy involves injecting human DNA in the muscles of affected dogs. It is technically called xenogenetic plasmid DNA vaccination. When the foreign proteins (human DNA) circulate in the blood of a dog, the immune system thinks there is an alien invader and reacts attacking his own (malignant) cells breaking the tolerance towards the tumor. The vaccination prolonged life so well that half the dogs that got the injections exceeded their career-shortened life expectancies.

Now the vaccine is available to veterinary oncologists worldwide and its success is inspiring work on a similar vaccine for melanoma in humans, in which not dogs, but mice will provide the foreign protein.