Zoonosis: A New Species of Rickettsia Bacteria in Dogs

Researchers at North Carolina State University have identified a new species of Rickettsia bacteria that may cause serious diseases in dogs and humans.   In 2018 and 2019, three dogs in the United States, from three different states (Tennessee, Illinois, and Oklahoma), developed fever and blood abnormalities suggestive of spotted fever. Their blood samples were seropositive for Rickettsia rickettsii, the bacteria responsible for this disease. However, further genetic and phylogenetic analysis showed that the dogs were infected with a new species of Rickettsia, closely related to the suspected pathogenic bacteria that also infect humans. Rickettsia bacteria are classified into four groups. Among these classifications is the best-known group, responsible for tick-borne spotted fever. This group contains the most identified species. More than 25 species of Rickettsia belong to this group, including Rickettsia rickettsii—one of the most virulent and dangerous—which causes Rocky Mountain spotted fever (RMSF). In dogs, as in humans, it causes fever, lethargy, weight loss, and symptoms related to vascular inflammation, such as swelling, skin rashes, and pain. In humans, Rickettsia parkeri, R. philipii (Rickettsia 364D), and R. rickettsii can cause RMF. However, until recently, R. rickettsii was the only Rickettsia bacteria in the group known to cause clinical symptoms in dogs. It was therefore only natural that the initial instinct was to diagnose the three dogs—which presented with similar symptoms and tested positive for R. rickettsii via serology—with FPMR. However, serological tests for Rickettsia are not always 100% specific, and antibodies targeted against one species may cross-react with another. Further genetic testing via PCR was therefore requested, and the results showed that the DNA of the bacteria in question is actually only 95% identical to that of R. rickettsii. The pathogenic organism in question is an unidentified species, also belonging to the Rickettsia group responsible for spotted fever. It remains unnamed for the time being, pending further testing that would allow for better characterization of the bacteria and identification of its geographic range—a slow but essential process. This year, the new Rickettsia species has already been detected in four other dogs living in the southeastern and Midwestern United States. Researchers are urging veterinarians to be more vigilant and are asking them, whenever possible, to collect and preserve ticks found on dogs exhibiting symptoms in order to determine which tick species carry the new bacteria. Researchers are also concerned about the zoonotic nature of the disease, which has the potential to infect humans. Dogs serve as excellent sentinels for tick-borne diseases. They are much more exposed to these insects, and ticks often remain attached to them longer than to humans, which increases the risk of infection. So far, the new bacteria has not been detected in humans, but that does not mean the risk does not exist. Veterinarians, researchers, and doctors would like to collaborate on a “One Health” approach to this new pathogenic agent in order to better understand and control it.