There are 15 different tick-borne diseases, including the topic of this article, Bartonella. The ability of ticks to harbor and transmit Bartonella has been heartily debated for years because of the lack of direct, conclusive evidence. This blog works to dispel this debate by sharing existing research in order to outline why both patients and medical care providers should consider Bartonella as part of the differential diagnosis.
Although Lyme disease (Borrelia burgdorferi) is the most well-known disease spread by ticks, there are many other tick-borne infections. In fact, there are 15 different tick-borne diseases, including the topic of this article, Bartonella.
Bartonella bacteria have been found throughout the world, currently with 37 different identified species. Bartonella is most often associated with transmission by an infected cat scratch or bite, as made famous through the song “Cat Scratch Fever” by Ted Nugent. Bartonella can also be transmitted by other animal bites as well as fleas, sandflies, ants, mosquitos, chiggers, and other insects. There is also evidence indicating that Bartonella may be transferred through a contaminated blood transfusion and even maternal-fetal transmission. Clearly, there are numerous ways to acquire this infection. However, the ability of ticks to harbor and transmit Bartonella has been heartily debated for years because of the lack of direct, conclusive evidence.
First, to determine whether ticks harbor Bartonella infection, scientists have tested ticks for various Bartonella species. For example, in a study conducted in 2001 (1), researchers analyzed Ixodes pacificus ticks in California and found that 19.2% of the ticks collected tested PCR positive for Bartonella. “These findings indicate that I. pacificus ticks may play an important role in Bartonella transmission among animals and humans.”
A 2004 study (2) conducted in New Jersey demonstrated Bartonella species DNA in questing Ixodes scapularis ticks using PCR analysis. A large percentage (34.5% of 107) of ticks in this study tested positive for DNA of Bartonella species.
A 2008 review (3) provided numerous citations demonstrating various types of ticks harbor Bartonella species based on PCR evidence. The authors concluded that “case studies and serological or molecular surveys involving humans, cats, and canines provide indirect evidence supporting transmission of Bartonella species by ticks.”
Literature suggests that transmission of Bartonella from a tick bite is possible
Dr. Joseph Burrascano
There is emerging evidence suggesting that in addition to harboring Bartonella, ticks can also transmit the infection to mammals. For example, according to a study published in 2011 (4), the transmission of Bartonella from tick to host was demonstrated in a mouse model. Representing the first in vivo demonstration of a Bartonella sp. transmission by ticks, researchers concluded that “Bartonellosis should be now included in the differential diagnosis for patients exposed to tick bites.”
In addition to this initial discovery, there are myriad scientific studies that also support the fact that ticks can harbor and transmit Bartonella to their hosts. Indeed, Bartonella has been detected in humans and animals after known tick bites by a variety of testing methods, including polymerase chain reaction (PCR), serology, or culture. According to a 2014 analysis (5) of 3,000 Lyme disease patients, 28.3% were found to be positive for Bartonella, suggesting that Bartonella is a tick-borne co-infection to Lyme disease.
In a German study conducted in 2021 (6), researchers found Ixodes ricinus ticks transmitted Bartonella grahamii, Bartonella henselae, and Bartonella schoenbuchensis in a bovine model. In the study, ticks artificially fed bovine blood spiked with Bartonella species were then tested with a transmission rate of up to 65.3%. Significantly, Bartonella DNA was also found in 22% of eggs laid by previously infected females. The “transovarial and transstadial transmission of bartonellae suggest that I. ricinus could be a potential vector.”
Columbia University takes the position that “the evidence for ticks as vectors of Bartonella organisms is circumstantial but fairly strong. Laboratory studies have found that Ixodes ticks can be competent vectors of B. henselae.” Conversely, the Infectious Diseases Society of America (IDSA) asserts that there is “no convincing evidence that Bartonella infections can be transmitted to humans by a tick bite.” While the CDC agrees that “ticks may carry some species of Bartonella bacteria,” there is “currently no causal evidence that ticks can transmit Bartonella infection to people through their bites.”
If ticks can acquire Bartonella bacteria from infected blood and transmit the infection to mammals, it is reasonable to presume that ticks can likewise transmit Bartonella to humans. While more research is needed, there is sufficient evidence to strongly suggest that not only do ticks harbor Bartonella, but they can also transmit it to the human hosts they bite. Although we still do not have a definitive answer, it is our responsibility to review and share published research on the topic to help patients make informed decisions.
Based upon ample, credible scientific studies, Project Lyme recommends both patients and medical care providers consider Bartonella as part of the differential diagnosis, particularly in patients with a clinical presentation consistent with Bartonella.
If bitten by a tick, Project Lyme also suggests patients have it tested for pathogens, including Bartonella. You can find a list of all locations that test ticks on our website here. Note that not all labs test ticks for Bartonella. IGeneX, Tick Report, and Clongen are three laboratories that can test for this infection.
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