The small, unobtrusive tick called Ixodes scapularis received worldwide medical attention almost 20 years ago, when it was found to play a crucial role in the transmission of Lyme disease. Although the ticks themselves pose no great threat to humans, they carry the disease-causing spirochete Borrelia burgdorferi and provide the route of human infection through their bites.

But their culpability doesn’t end there. In the inaugural issue of the journal Vector Borne and Zoonotic Diseases, Durland Fish, Ph.D., and several colleagues describe a spirochete from I. scapularis that closely resembles B. burgdorferi but does not match it in highly sensitive DNA tests carried out by polymerase chain reaction. The new species of Borrelia was discovered in I. scapularis nymphs that had previously been fed on mice known to be free of B. burgdorferi infection. The yet-unnamed spirochete may infect humans as well, since “all the other organisms that this tick transmits to mice can also infect people,” according to Fish, an associate professor of epidemiology. It is not known what symptoms, if any, such an infection would cause in humans.

“Our sampling of I. scapularis … from the field suggests that this novel Borrelia is widely distributed in nature,” the study’s authors wrote, and its prevalence is “surprisingly high in proportion to the total number of Borrelia species found in these ticks.” They concluded, “These data suggest that a significant proportion of spirochete-positive ticks previously thought to be B. burgdorferi by microscopy is instead this novel Borrelia.” Infection with B. burgdorferi is responsible for more than 15,000 cases of Lyme disease each year. Infection with the new Borrelia organism cannot be found by current Lyme-disease diagnostics—Yale scientists are now working on a specific test—but it might well respond to the same treatments as Lyme disease.