Scientists Suggest Re-Thinking Which Flea-Borne Bacterium Causes Fevers in Humans

By Josh Lancette

In Southern California over the past five years, there have been nearly 500 cases of flea-borne rickettsioses, more commonly known as flea-borne typhus. Rickettsia felis has become the presumed bacterium causing the fevers, but scientists from the California Department of Public Health think the presumption might be unfounded. Instead, in an article recently published in the Journal of Medical Entomology, they propose that a different bacterium might be the culprit: Rickettsia typhi.

The cat flea, Ctenocephalides felis, is considered the main vector of both species of bacteria, R. felis and R. typhi, to humans in suburban communities of Texas and California. This flea is common throughout the United States and is frequently infected with R. felis. Due to its prevalence and the common interactions between cats and humans, as well as some reports implicating it as a human pathogen, R. felis has been the presumed bacterium causing illness in humans.

However, the authors of the paper have a different view. They think that given the prevalence of R. felis, there should more cases of rickettsioses, given the large human populations in potential contact. Furthermore, they believe that cases of rickettsioses wouldn’t be limited to a small region in Southern California if R. felis is to blame. They also mention that not a single R. felis isolate has been obtained from the blood or tissues of a presumed infected patient anywhere in the world.

Given this evidence, the authors suggest that R. felis is not the culprit behind human illness. Instead, they point to R. typhi.

“Evidence of only limited numbers of R. typhi-infected cat fleas in the environment may indicate a very rare infection and explain why so few cases of flea-borne rickettsioses are reported each year in Southern California relative to the population size,” write the authors.

In addition, the geographic limitation of illness to Southern California fits better with the R. typhi hypothesis, argue the authors.

“To our knowledge, R. typhi has never been detected in a cat flea outside of historically endemic areas of Southern California,” they write.

While the authors make their opinion in this classic whodunit known, they don’t proclaim to have solved the problem for good, mentioning that “further research is needed to unravel this long-standing epidemiological mystery.”

Photo caption: Female Ctenocephalides felis. Photo by Katja ZSM – Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=38959624.