Before going outdoors in the summer, some people may don long-sleeve shirts and long pants to avoid being bitten by ticks carrying Lyme disease. But Yale researchers have discovered a new tickborne infection for which to watch out.

A Yale study published today in The New England Journal of Medicine found that 18 people in New England and New York had contracted the disease — the first time the still unnamed disease has been diagnosed in the United States. It is caused by the bacteria Borrelia miyamotoi, a relative of the bacteria causing Lyme disease. The germ is carried by the same ticks that transmit Lyme disease and is thus likely to be found in the same geographic regions.

“It’s always significant when you find an infectious disease that affects people,” said primary author Peter Krause, senior research scientist at the School of Public Health. “The question is, is this going to be a disease that infects a lot of people?”

Preliminary data show thousands of cases of the illness in the United States every year. Krause found that in a sample of 584 healthy people living in areas where Lyme disease is prevalent, 1 percent had a history of infection by the new disease, while almost 7 percent had a history of Lyme disease. There are 30,000 reported cases of Lyme disease in the United States annually, and this 7-to-1 ratio suggests there are approximately 4,000 annual cases of this other disease — though unreported and asymptomatic cases may raise the actual number of infections tenfold.

Though the B. miyamotoi bacteria was first discovered in ticks nearly two decades ago, scientists did not detect it in humans until 2011.

“Infections have been occurring under our nose and were being called atypical Lyme disease,” said Sam Telford, senior author of another study on the disease appearing in today’s issue of the Journal. “No one was looking very hard for B. miyamotoi, and it doesn’t come up in standard Lyme tests. Patients were treated for Lyme, and the drugs worked fine. So why probe any deeper?”

Telford’s study suggests that B. miyamotoi is an “under-recognized cause of disease” in regions in which Lyme disease is prevalent. The study describes an 80-year-old woman who was infected by the bacteria and developed symptoms associated with dementia. It suggests that B. miyamotoi and other poorly characterized germs may contribute to the neurodegeneration typically attributed to dementia or aging. Telford added that there have been some sick patients whose symptoms were attributed to Lyme disease despite testing negative for its bacteria, and researchers will need to explore whether these symptoms were instead caused by B. miyamotoi.

Both diseases cause similar symptoms such as aches and rashes, but the new disease may also cause relapsing fevers. Telford called the disease “yet another reason — although Lyme alone is enough — [that] we should take every precaution to prevent tick bites and even better, to reduce the environmental contamination with ticks by managing deer herds and habitat.” Still, Krause said none of the 50 infections encountered so far was untreatable. Krause said scientists now need to better characterize the disease’s “health burden” — how frequently the infection occurs and how severe its symptoms are — as well as understand how the infection arises and how to best diagnose and treat it.

Named after Lyme, Conn. where it was first identified, Lyme disease within the United States mostly occurs in the Northeastern states, Wisconsin, Minnesota and California.