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Could Beneficial Bacteria Counteract Cancer in Dogs?

By Dr. Bill Bernard, DVM, ACVIM

Cancer in dogs has long been a frustrating ordeal, leaving owner to struggle for answers to questions regarding causes and treatments. In this article, we will look at the basic types of canine cancer as well as a promising study that might be helpful in the fight.

Cancer refers to a group of related conditions in which cells undergo uncontrolled growth. Uncontrolled growth is known as malignancy.

Fifty percent of dogs over the age of 10 develop cancer at some point. These tumors vary in the type of cells involved and in their location in the body. A study in purebred dogs in the UK identified that 27% of the deaths that occurred were a result of cancer. A similar Danish study reported 14%. A post mortem study of 2000 dogs reported that 23 % to 45 % of dogs over 10 years of age died as a result of cancer. A list of the more common and brief description is to follow:

Lymphoma or Lymphosarcoma is among the most common type of tumor seen in dogs. Swollen lymph nodes are a common clinical sign of lymphoma.

Hemangiosarcoma is a tumor that develops from cells that line blood vessels (endothelial cells). Hemangiosarcoma develops slowly over time and is often times not painful to the dog. Signs usually do not show up until late in the disease when the dog suffers from internal bleeding due to the tumor rupturing.

Osteosarcoma is the most common type of primary bone tumor in the dog. It most frequently affects the long bones (front and rear limbs), but it can be found in any bone including the skull or ribs.

Mast cell tumors. Mast cells are immune cells found throughout the body that play an important role in allergic reactions. Most mast cell tumors are found on the skin and may be detected by a sudden swelling or growth.

Melanoma or malignant melanoma is a tumor made of pigmented or dark skin cells that can be found anywhere on the dog’s body. Squamous cell carcinomas can develop on the skin and inside the mouths of dogs.

Mammary carcinomas. Tumors of the mammary glands are the most common tumor seen in unspayed female dogs. 40 to 50 percent of these tumors are malignant meaning they have spread to other locations.

Apocrine gland carcinomas (anal sac cancer) are also known as carcinomas of the anal sac in the dog. Approximately 50% of these tumors are diagnosed by your veterinarian during a routine rectal exam and show no apparent clinical signs.

Transitional cell carcinoma is the most common tumor of the lower urinary system (bladder and urethra) in the dog. This tumor is considered locally invasive and is moderately to highly likely to metastasize to another area.

Soft tissue sarcomas are a group of several different types of tumors that share similar characteristics. These tumors are made of connective tissue and are located either within the skin or in tissues just below the skin.

What are the causes of Cancer in the dog.

In animals and humans, cancer is a complex process involving numerous factors. Cancer may be initiated by nutritional, hereditary, environmental and/or stress influences that result in an abnormal replication of cells that goes unchallenged by the hosts immune system.

Genetic Factors

The fact that certain cancers are more common in certain breeds of dogs is highly suggestive of a hereditary/genetic component. Statistics strongly support this assumption. For example, giant breeds of dogs are 200 times more likely to develop osteosarcoma (bone cancer) than the toy breeds.

Environmental Factors

A large majority of human cancers are related to risk factors present either in the environment or in the diet. It is likely that these same risk factors play a major role in the development of cancers in pets. Because pets live in the same environment as their owners, pets are exposed to many of the same environmental hazards that have been identified as risk factors for humans. Each pet may vary in their sensitivity to carcinogens.

Known carcinogens that may be related to cancer in dogs include ultraviolet radiation from long exposure to the sun. Other environmental risk factors may include: second-hand tobacco smoke; a variety of herbicides, insecticides, and pesticides commonly used in agriculture; and the air pollution and smog common in many urban areas. Nickel, uranium, benzene, radon, vinyl chloride, cadmium and asbestos are all common substances that have been identified as carcinogens.

Stress is widely know to have an adverse effect on the immune system. This alone could be a huge factor. Stress is something working dogs deals with every time they get in a cruiser.


Viruses can cause cancers. A wart or oral papilloma that develops in the dog’s mouth is caused by a virus. Another viral induced genital cancer is canine transmissible venereal tumor that is transmitted during sexual intercourse. The cancerous cells are transferred during contact between a healthy animal and an affected animal.


It has been noted that cancer occurs more frequently in older animals. Why aging causes cancer is not completely understood. One possibility is that aging cells undergo deterioration of organelles (interior machinery). This aging machinery results in a lack of control of cell processes. In combination with this, a weakening/ageing of the immune system may influence the body’s ability to control mutated cells. This lack of control may allow these abnormal cells to multiply without control.

Healthy cells in the body are constantly dividing. As an animal ages, it statistically becomes more likely that an error will occur in cell division resulting in uncontrollable multiplication. Age also results in more exposure to environmental carcinogens. Therefore, there are numerous reasons that age may lead to an increased rate of cancer in the dog. It is likely that these factors may act together, influencing alterations and lack of control of cell replication.


The link between cancer and diet is as mysterious as the disease itself. However, there is no doubt that nutrition plays a roll. Much research in humans has pointed toward certain foods and nutrients that may help prevent or, conversely, contribute to certain types of cancer. Lack of or lower levels of antioxidants have been correlated with cancer in humans. Antioxidants are typically low in commercial pet diets. Heat processing can destroy these important components of the diet. Processed foods are also high in many carbohydrates that lead to a “pro-inflammatory” metabolic state. This state of cellular and intracellular inflammation is thought to be pro-carcinogenic.


Two factors that dogs share with humans that have been correlated with increased risks of cancer are weight gain,obesity and life style. Our pets are often overweight/obese and /or consume diets high in processed foods.

Weight gain and lack of exercise are a frequent issue in our pet population. A study in humans found that a higher body mass index increases the risk of developing some of the most common cancers by as much as 10%. The proposed link was that body fat produces hormones and inflammatory proteins that can promote tumor cell growth.


Could beneficial bacteria help mitigate the predispotion to cancer in dogs?

Lactobacillus reuteri (L. reuteri) is a bacterium that is found in the gastrointestinal tract of virtually any animal species. It is one of the major probiotic (bacteria that benefit health) in humans and animals. L. reuteri has been extensively studied in humans. The bacteria lives along the surface of the GI tract were it secretes products to regulate the good bacterial population, killing pathogens such as salmonella, E Coli and Clostridial organisms. These killings are the result of the secretion of a metabolite called reuterin that is produced by L. reuteri. In addition, L. reuteri prevents an attachment of the “bad” bacteria through competitive exclusion.

It influences the development and modulation of immune responses in the GI tract and throughout the body. Studies have shown influence on aging, regulation of blood glucose and involvement with the gut –brain axis. L.reuteri is a critical organism for development, health and well-being.

A recent scientific publication from the Division of Comparative Medicine, Massachusetts Institute of Technology, Cambridge MA. (Lakritz,Poutahidis, Lefkowich et al). has shown that fermented foods containing probiotics suggest† health benefits including protection against cancer. This study used 2 differing models of cancer: 1) a group of mice that developed breast cancer when eating a “Westernized Chow and 2) a group of animals with a genetic predisposition for breast cancer.

The westernized diet mimicked typical “fast food”-style diets that were high in fat, starch and sugar and low in vitamins B and D. Ten mice were fed the Westernized diet and 10 mice were fed the control (normal ) diet. Mice fed the Westernized diet resulted in pre-neoplastic and neoplastic changes in the mammary glands. These changes were not shown in the normal diet fed mice. The study was repeated with the westernized diet fed mice receiving L. reuteri in their drinking water. In this study L. reuteri prevented the features of mammary carcinogenesis. The study was repeated in the mice that were genetically predisposed to breast cancer. The genetically predisposed mice did not develope cancer , The drinking water L. reuteri prevented mammary neoplasia in the genetic model of mammary neoplasia.

The researchers progressed to looking at the immune system of the mice to determine if this was where L. reuteri was having its influence on preventing of mammary cancer. They determined that L. reuteri was inducing a change in a specific cell involved in immunity, the lymphocyte. More specifically, L. reuteri triggered CD4+ CD25+ lymphocytes which are specifically anti-inflammatory.

There next step was to determine whether lymphocytes from the protected mice could confer protection to mice in the westernized diet or genetically predisposed group. Transferring the CD+4.CD+25 lymphocytes to the predisposed mice protected them from developing mammary cancer. This was further proof that the microbial induced protection was via lymphocytes of the immune system.

From this data these researchers demonstrate that host immune responses to Lactobacillus reuteri inhibit cancer progression in distal tissues such as mammary glands. This leads us to conclude that consuming fermentative microbes such as L. reuteri may offer a tractable public health approach to help counteract the accumulated dietary and genetic carcinogenic events integral in the Westernized diet and lifestyle.

Bill Bernard DVM, ACVIM is board certified in internal medicine and a graduate of University of California, Davis with residency at the University of Pennsylvania, New Bolton Center. He is the author of Equine Pediatric Medicine and numerous scientific papers.

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Posted by on Mar 20 2018. Filed under Current Articles, Featured. You can follow any responses to this entry through the RSS 2.0. Both comments and pings are currently closed.

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  • January 2019