Veterinary researchers in the U.S. say they’ve discovered a new disease-causing, tickborne bacteria in dogs. So far, only a handful of cases, with one possibly linked to a dog’s death, have been documented since 2018. But it’s likely that cases of this mystery pathogen have gone unnoticed for some time, and the researchers warn that it could pose a threat to dogs and potentially people as well.
The discovery was detailed in this month’s issue of Emerging Infectious Diseases, a journal run by the Centers for Disease Control and Prevention.
According to the report, the first case was found in a 10-year-old male neutered mixed-breed dog in Tennessee in 2018. The dog had become feverish, lethargic, and unwilling to eat, while his blood contained low levels of platelets. His owner remembered removing a tick from the dog in the previous two weeks. Blood samples revealed the dog carried Rickettsia bacteria, a group of bacteria known to cause a variety of illnesses in many animals, including Rocky Mountain spotted fever in people and dogs. Thankfully, antibiotics seemed to treat the illness with no problems.
The next case involved a 9-year-old male neutered Boston terrier diagnosed with similar symptoms as well as joint problems in May 2019. As before, the dog tested positive for Rickettsia bacteria, and antibiotics along with other medications got him back to health within five months.
Lastly, there was a 9-year-old male neutered terrier mixed-breed from Oklahoma diagnosed with similar symptoms in August 2019. Like the others, he tested positive for these bacteria. But unfortunately, despite treatment, the dog developed severe kidney disease and was ultimately euthanized. Later on, it became apparent the dog had been seen by a vet a year earlier, when he experienced fever and fatigue following a tick bite. Though the dog did test positive for antibodies to Rickettsia bacteria at the time, the actual infection wasn’t detected then and the antibiotics he was given appeared to clear up his symptoms initially. It’s likely that some other underlying illness involving his immune system, not just the infection, contributed to his severe kidney problems and death, the authors wrote.
In all three cases, the dogs carried antibodies that responded to Rickettsia rickettsii, a bacteria that’s the usual cause of spotted fever. But the actual bacteria collected from these dogs didn’t genetically match any species of Rickettsia known to scientists. So although these dogs all had spotted fever, the culprit is likely novel. Genetic analysis of the bacteria suggests that it’s closely related to two other species of Rickettsia known to cause illness in people.
“Thus, we report a previously unknown and unique Rickettsia [species] with clinical significance for dogs and potentially humans,” the authors wrote.
The findings shouldn’t be a surprise. Over the past two decades, scientists have discovered several new disease-causing germs that can be spread by ticks and insects in the U.S. Because these dogs had antibodies that cross-reacted to the most common cause of spotted fever, it’s possible this undiscovered species has been flying under the radar and making dogs sick for a long time, with vets mistakenly blaming cases on known bacteria.
The same could be true for people, too. More people have been testing positive for antibodies to R. rickettsii bacteria over time, and some scientists have worried that there are other potential disease-causing species that are being mistaken for R. rickettsii because doctors often only test for antibodies and/or don’t use tests that can identify a specific bacteria species.
“For me, knowing this pathogen makes dogs sick makes it more likely that it could make people sick, too,” said study author Barbara Qurollo, a research associate professor at North Carolina State University College of Veterinary Medicine, in an email. “Dogs are good sentinels for tickborne disease and can develop similar diseases as people.”
Qurollo is also the co-director of the university’s Vector Borne Disease Diagnostic Lab. And it was through the lab, which routinely tests blood samples from sick animals sent in by veterinarians, that she and her co-authors were to come across the mystery germ in the first place.
Rocky Mountain spotted fever in humans is the deadliest known tickborne disease, with a fatality rate as high as 25% if left untreated. Luckily, with prompt antibiotic treatment, the fatality rate is much lower, somewhere less than 1%. Judging from these canine cases, it seems the new variety is just as treatable.
Around 4,000 to 6,000 cases of spotted fever are reported annually, according to the CDC, with a majority of cases in North Carolina, Tennessee, Oklahoma, Arkansas, and Missouri. Unfortunately, illnesses spread by ticks and insects in general have been climbing in the U.S., partly due to warmer climates that have allowed disease-carrying bugs to spread farther into the country and survive and breed for longer.
Aside from the three dogs described in this current study, Qurollo and her team has since found four other dogs in 2020 who seem to have come down with the newly discovered bacteria. They all developed symptoms, but those that were treated with the standard antibiotic treatment for spotted fever recovered.
Despite the discovery of these cases, though, it usually takes time and more research to confirm that a new germ is causing illness. To that end, studies are underway to identify the species of ticks that can spread this novel bacteria, the animal hosts that it hides in, and just how commonly it’s infecting dogs and possibly people. Quollo and her team are also working on trying to grow the bacteria in the lab so that they can create a specific diagnostic test for it—no easy feat since these bacteria can only survive for long inside the cells that line blood vessels.
“We also have collaborations with physicians and plan to collaborate with the public health infrastructure to test human samples,” she added.
This article has been updated with comments and further information from one of the study’s authors.