Viruses That Cause Cancer

Viruses and Cancer

Cancer and its development are complex and multifactorial. There are many factors that can affect the risk of developing cancer – genetics, environmental factors such as carcinogens and radiation, nutrition and lifestyle habits, and viral infections.¹

Usually, we do not know why a particular dog gets a particular cancer.

While a few cancers have been suspected to be caused by viral infections, in larger mammals this is rare.⁶ In dogs, there have only been a few documented associations between viruses and tumor development.²

As a dog owner this is encouraging, because it means that most viral infections are not putting your dog at greater risk of cancer.

Below we outline the few viruses that have been associated with cancers seen in dogs, the risk of getting cancer following viral infection, and how to minimize these risks if possible.

How Do Viruses Cause Cancer?

Viruses can cause cancer and tumor formation by inserting themselves into an animal’s genes, causing mutations and what is termed as “chromosomal instability”.²

Viruses survive by invading cells and taking over the cell machinery to replicate and make more virus.³

Many viruses do this by inserting their own genetic material into the host cell (the cell being infected), which can affect the host cell’s genes – sometimes switching genes on and off that regulate and control their growth.⁶ This can push a cell towards becoming cancerous and potentially cause tumor formation.

In humans, about 10-15% of all cancers are linked to viral infections, including human papillomavirus, hepatitis viruses, and the Epstein-Barr virus.³ It is estimated that 1-8% of the human genome contains viral genes.^4,5

In canines, few studies have identified an association between viral integration into the dog genome and cancer incidence.

There are just a few viruses associated with cancer in dogs.

Canine Papillomaviruses (CPV)

Canine papillomaviruses (CPV) are viruses that target the squamous epithelial tissue, causing changes in the skin and mucus membranes.⁷ This virus is transmitted via direct contact with an infected individual/canine.⁸ There are at least 20 known canine papillomaviruses.⁹

CPV can cause skin lesions that are benign warts in dogs. These warts and lesions usually resolve over time due to the immune system effectively fighting off the viral infection. However, CPV infection rarely can result in a carcinoma.¹⁰

Canine Transmissible Venereal Tumor (CTVT)

One study found the presence of canine papillomavirus DNA in canine transmissible venereal tumor samples (CTVT).¹⁹

CTVT is a highly contagious cancer that is a sexually transmitted tumor through direct skin-to-skin contact from dog to dog, which results in cancer cells being transplanted from one dog to the other.¹¹ The most common cause of this type of cancer comes from direct contact with a dog with CTVT through sexual contact, licking, and biting.

More research is needed, but this finding suggests that canine papillomaviruses may be involved in CTVT cancer in dogs.

Epstein-Barr Virus Maybe a Link

The Epstein-Barr virus (EBV) is a virus known to cause cancer in humans,³ but it’s less clear at this point whether it can cause cancer in dogs.

A study in 2012 in canines found a possible link between an Epstein-Barr-like virus and lymphoma occurrence.¹² Epstein-Barr-like viral genetic material (DNA and RNA) and herpesvirus particles were found in the lymph nodes of canine lymphoma patients in this study.

It is important to note that the virus studied was similar to EBV, but not the same virus. However, another study in 2015 also looked at the association between an EBV-like virus and lymphoma incidence and found no evidence that this type of virus is directly involved in canine lymphoma.¹³

While we cannot exclude the presence of an EBV-like virus in the canine population, it is clear that more research is needed to determine if this could be one of the underlying causes for canine lymphomas.

Do These Viruses Always Cause Cancer?

While it is common for viral infections like HPV to turn cancerous in humans, the rate of incidence of viral infections that turn malignant is very low for canines.

One study from the Michigan State University Veterinary Diagnostic Laboratory found that the overall frequency of canine papillomavirus-associated lesions that turned malignant was only 3.6%.⁹

There are other rare and single case reports on canine papillomavirus progressing to invasive squamous cell carcinoma (SCC), however the large majority of SCC diagnoses are not connected to a canine papillomavirus infection. The largest connection of canine papillomavirus leading to SCC was observed in a study that was conducted on very immunocompromised dogs (their immune systems did not function properly),^14,15 which does not represent the general population of canines who have normal functioning immune systems.

Canine transmissible venereal tumor (CTVT) is found in dogs all over the world and is most associated with the presence of free-roaming dogs. To date only one study has shown a possible association between a papilloma viral infection and CTVT cancer occurrence. More studies are needed to determine if there is a plausible connection between viral infection with a canine papillomavirus that could lead to CTVT.

Based on these data, experts in the field have concluded that most cases of cancer in dogs do not arise from viral infections.

Risk Factors for Viruses and Cancer

One of the most common risk factors for viral infections that could eventually lead to cancer are dogs who are immunocompromised.⁷ A decrease in immune system function can be due to age (older dogs), disease, or use of immunosuppressant drugs.⁷ This immunodeficiency may play a role in the development of squamous cell carcinoma after canine papillomavirus infection.

Canine papillomavirus is usually self-limiting and resolves on its own, but immune suppression may cause a decrease in surveillance and eradication of the infection by the immune system, which could lead to development of squamous cell carcinoma.⁷

Certain breeds have been documented to have increased risk of getting dog warts (which are caused by canine papillomaviruses). These breeds include:

• Shar-Pei
• Cocker Spaniels
• Miniature Schnauzers
• Kerry Blue Terriers
• Pugs

This does not mean that these breeds are automatically more at risk for cancer, but because of the rare connection between canine papillomavirus and cancer, this should be taken into consideration.

Aside from one study no further connection has been made about the connection between canine papillomavirus infection and canine transmissible venereal tumor occurrence. Beyond a viral correlation for risk, CTVT is a large problem in countries where the mating of dogs is not under control and good spay/neuter programs are not in place.¹⁷

Dogs most at risk of contracting CTVT are those that are free-roaming and sexually active, usually in tropical and sub-tropical regions.¹⁸

How to Reduce Risk

The best way to reduce the risk of cancer incidence, regardless of possible cause, is annual vet checkups and at home vet examinations (checking to see if your dog has any lumps, lesions, or abnormalities).

It is also important to keep in mind your dog’s general health and well-being through a good diet and regular exercise. Additionally, giving your dog all of the core recommended vaccines can help keep them healthy and protected against the deadliest illnesses your dog can encounter.

It has been shown that spaying or neutering your dog reduces risk for canine transmissible venereal tumor.¹⁶ In addition, it is advised to monitor your dog when they are in social situations with other canines to make sure that sexual interactions as well as biting and licking are not occurring. Finally, it is strongly recommended to not let your dog be ‘free roaming,’ as encountering possibly infected dogs, or wild dogs, is higher.

It is important to remember that there are only a few studies that have been conducted looking at viral infections and their contribution to cancer occurrence in dogs. Currently there is no direct evidence that a viral infection will cause cancer.

There is the possibility that cancer CAN occur after a viral infection, but the incidence is very rare. Therefore, even though your dog may get infected with one or more of these viruses, it does not mean they will automatically progress to a cancer diagnosis.