The Stealthy, Tumor-Linked Pathogen That Hides in Cells

Analysis by Dr. Karen Shaw Becker

bartonella bacteria in dogs

Story at-a-glance -

  • A recent study indicates that bartonella bacteria is prevalent in tumor and non-tumor tissue samples taken from dogs with hemangiosarcoma, which indicates it could be a factor in the disease
  • Hemangiosarcoma is a cancer of the blood vessels and in dogs is typically diagnosed when the disease is far advanced, resulting in a relatively low survival rate
  • The nature of bartonella bacteria allows the pathogen to hide from both antibiotics and a dog’s immune system, which can lead to chronic infection, prolonged immune suppression, and increased risk for other infections and diseases like cancer
  • The NC State study demonstrates that bartonella DNA isn’t present in the blood of dogs with hemangiosarcoma, but only in tissue samples, which indicates blood tests alone may not find the infection
  • At this time, further studies are needed, along with more sensitive diagnostic testing

Results of a recently published study by researchers at North Carolina State University found that bartonella bacteria is extremely prevalent in the tumors and tissues of dogs with hemangiosarcoma. Interestingly, Bartonella was not prevalent in the dogs’ blood samples, even though hemangiosarcoma is a cancer of the blood vessels.

According to an NC State news release, this work “further supports the connection between persistent infection and some types of cancer and adds to the evidence that Bartonella can remain and thrive, undetected, within tissue.”1

To better understand the implications of these study results, let’s first discuss hemangiosarcoma and Bartonellosis.

Hemangiosarcoma: A Stealth Cancer

Of all the types of cancers that strike companion animals, hemangiosarcoma (HSA) is one of the sneakiest. Sadly, more often than not, a dog suddenly develops internal hemorrhaging and HSA is diagnosed as a result of the crisis. By then, the disease has progressed to an advanced stage, resulting in a one-year survival rate of only 12 to 20%.

Hemangiosarcoma is almost exclusively a disease of dogs, though it does occasionally occur in cats. Dogs of any breed, gender, and age can develop this type of cancer, but it’s most commonly diagnosed in dogs between the ages of 6 and 13. Predisposed breeds include German Shepherds, Labrador and Golden Retrievers, Boxers, Doberman Pinschers, and English Setters.2

Cancer can occur in any number of different organs in the body, for example, the intestines, liver, and bones. In hemangiosarcoma, the tumors develop in the endothelial cells that line the blood vessels of the circulatory system. Hemangio is the Greek word for blood vessel; sarcoma means a malignancy of connective tissues.

Because the tumors involve blood vessels, they’re often filled with blood and when they rupture, they can trigger massive internal or external bleeding. Hemangiosarcoma can occur wherever there are blood vessels in the body, but most often develops in the spleen, heart, liver, skin, and soft tissue. HSA is responsible for two-thirds of all heart or splenic tumors in dogs.

This type of cancer is aggressive and highly metastatic, meaning it frequently spreads to other organs, including the brain, lungs, spleen, heart, kidneys, skeletal muscle and bone.

Because dogs most often get the visceral form of HSA, frequently no signs of disease are present in the early stages. Even dogs with large tumors will show no symptoms early on. Once the tumors invade surrounding normal tissue and spread to other parts of the body, they can develop small ruptures that allow blood to escape into the abdomen, chest, the sac around the heart, or right below the skin (subcutaneous).

This blood loss causes some dogs to show intermittent symptoms of lethargy and weakness, but usually the signs are so subtle they go unnoticed or are attributed to another less serious cause. Other subtle signs can include a decrease in appetite, mild anemia, and slight elevation of liver enzymes.

When the tumors metastasize, they aggressively invade the lungs, liver and/or intestines. As I touched on earlier, often dogs with HSA die abruptly when a tumor ruptures, causing severe hemorrhaging.

Signs of a life-threatening hemorrhage include weakness, a pale color to the tongue, panting, rapid heartbeat, weak pulse, a distended abdomen, and collapse. In dermal and hypodermal hemangiosarcoma, a mass can often be felt in or under the skin. It may become ulcerated and bleed.

Veterinary oncologist Erin Bannink and integrative veterinarian Steve Marsden are conducting a clinical trial using Chinese herbs to improve the health and lifespan of dogs diagnosed with splenic HSA.3

Bartonella: A Stealth Pathogen

Bartonella bacteria cause bartonellosis, an infectious zoonotic disease that can be transmitted from animals to humans and vice versa. In humans, the infection is also known as cat scratch disease or cat scratch fever, even though it’s not always acquired through a cat's scratch or bite.

Bartonella bacteria can be transmitted to dogs via fleas, ticks, flies, biting flies, sandflies, mites and lice, however, it doesn’t appear canines are a natural host for the pathogen. It is not yet known whether dogs, like cats, can transmit the infection to humans.4

The following is from the Companion Vector-Borne Diseases web page on canine bartonellosis, and provides a good description of the stealth nature of the bacteria:

“Bartonella spp. infect erythrocytes [a type of red blood cell], endothelial cells [found in the inside lining of blood vessels, lymph vessels, and the heart] and macrophages [a type of white blood cell], often leading to persistent blood-borne infections.

Location within erythrocytes and also within vascular endothelial cells is believed to protect Bartonellae from antimicrobial agents. Immune system avoidance via intracellular location, frequent genetic rearrangements, and alteration of outer membrane proteins, is also considered important.

In dogs, [bartonella] causes chronic infections by establishing intracellular infection in erythrocytes and endothelial cells, thereby escaping the acquired humoral and cell-mediated immune defenses of the host. Infection … might induce a degree of chronic immunosuppression that could predispose dogs to infections with other pathogenic agents, resulting in a wide array of clinical manifestations in naturally-infected dogs.”5

In a nutshell, the specific manner in which bartonella invades a dog’s body seems to allow the bacteria to hide from both antibiotics and the immune system, thereby causing chronic infection, which in turn can create prolonged suppression of the immune system, making the dog more susceptible to other types of infections and diseases.

Clinical signs of bartonellosis depend on the strain of bacteria present and can range from no symptoms at all to severe disease and sudden death. There are also large variations in the duration of illness. In symptomatic dogs, clinical signs include:

Endocarditis (inflammation of heart valves)

Polyarthritis (an immune-mediated disease featuring joint inflammation)

Myocarditis (inflammation of the heart muscle)


Hepatitis (inflammation of the liver)

Weight loss

Uveitis, choroiditis (inflammatory conditions of the eye)

Epistaxis (severe bleeding from the nose)

Blood Tests to Detect Canine Bartonellosis Appear to Be an Insufficient Diagnostic Tool

According to NC State study co-author Ed Breitschwerdt, a professor at the College of Veterinary Medicine, as quoted in the university news release:

"There are clear precedents for the involvement of bacterial infections in tumor development. Given the established links between chronic inflammation and cancer, we wanted to determine whether chronic infection of blood vessels due to bacteria could be a contributing cause of this cancer."6

The NC State researchers evaluated samples from 110 dogs with hemangiosarcoma from across the U.S. They looked at tumor tissue, non-tumor tissue and blood samples, and screened each for babesia, mycoplasma and bartonella —three types of bacteria specifically associated with blood infections.

Babesia wasn’t detected in any of the dogs and mycoplasma was found in only 5. However, Bartonella was found in 80 of the dogs. Bartonella DNA was amplified and sequenced and was identified in 34% of tumor tissue and 63% of non-tumor tissue, but not in any blood samples.

"Research in recent years has confirmed that persistent infection with or inflammation caused by stealth pathogens is a risk factor for developing cancer later in life," Breitschwerdt told ScienceDaily.

"With the exception of Helicobacter pylori, the emphasis on evaluating the relationship between infection and cancer has focused on viruses. But intracellular bacterial pathogens such as Bartonella may also play an important and previously uninvestigated role.

Bartonella is a stealth pathogen — it can 'hide' in the cells that line blood vessel walls, which is part of what makes it so difficult to detect. This work adds more evidence to the connection between infection and cancer risk, and demonstrates that molecular testing of whole blood samples does not rule out the tissue presence of this pathogen.”7

Breitschwerdt says further studies are needed to determine if bartonellosis is in fact a cause of hemangiosarcoma in dogs. Toward that end, he and his colleagues are focused on creating more sensitive diagnostic testing.