Borrelia burgerdorferi bacteria as seen with a scanning electron microscope. Claudia Molins/CDC

NEW YORK (AP) — The geographic areas where Lyme disease is a bigger danger have grown dramatically, according to a new government study published Wednesday.

US cases remain concentrated in the Northeast and upper Midwest. But now more areas in those regions are considered high risk.

"The risk is expanding, in all directions," said the lead author, Kiersten Kugeler of the Centers for Disease Control (CDC).

There are now 260 counties where the number of Lyme disease cases is at least twice what's expected, given the size of each county's population. That's up from 130 a decade earlier, the report shows.

Lyme disease is most common in wooded suburban and far suburban counties. Scientists aren't sure why high-risk areas are expanding, but it likely has something to do with development and other changes that cause animals (e.g. deer, mice, and squirrels) and ticks that carry the bacteria to move, Kugeler said.

Overall, 17 states have high-risk counties. The entire state of Connecticut, where the illness was first identified in 1975, has been high-risk for decades. Now, high-risk zones encompass nearly all of Massachusetts and New Hampshire and more than half of Maine and Vermont.

CDC CDC Other states that saw expansion of high-risk areas include Virginia, Pennsylvania and New York along the Eastern seaboard, and Iowa, Michigan and Minnesota in the Midwest.

Here's an animation showing how the problem has grown:

The disease is transmitted through the bites of infected deer ticks, which can be about the size of a poppy seed.

Symptoms include a fever, headache and fatigue and sometimes a telltale rash that looks like a bull's-eye on the tick bite. Those infected can develop severe, rheumatoid arthritis-like joint and muscle pain. Fatigue and neurological disorders — such as numbness, tingling, weakness, and cognitive impairment — can set in too.

Most people recover after a course of antibiotics. If left untreated, infections can lead to brain inflammation or heart problems. At least a handful of such cases have proven fatal.

About 30,000 US cases are reported each year, but experts say there actually are as many as 10 times more. Up to 440,000 cases might have occured in 2008, for example.

What's more, a recent study suggests 36% of patients treated for Lyme disease experience lingering symptoms — which some argue are chronic — and continue to seek medical help within the year following treatment.

Nymph blacklegged ticks (right) are tiny compared to adults (left) but are the most common vectors for Lyme disease. Getty Images Some counties have dropped off the high-risk list, including those in Virginia, Georgia, Missouri and North Carolina where significant clusters were reported in the 1990s. Scientists now think those were a different condition caused by a different tick's bite, Kugeler said.

Some experts argue a vaccine could be effective in preventing cases to begin with.

SmithKline Beecham (now GlaxoSmithKline) developed a vaccine for Lyme disease, called LYMErix, and started selling doses in 1998. It was 80 percent effective after two booster shots.

But several forces conspired to lead the manufacturer to stop selling it in 2002 — not the least of which were outspoken antivaccine groups and their lawyers. Plaintiffs in one case claimed the vaccine caused Lyme disease-like symptoms in some people. GlaxoSmithKline eventually settled out of court, but an official follow-up study found nothing abnormal about the vaccine.

Another manufacturer had developed a second promising vaccine at the time, but it never pursued licensing following their competitor's lawsuits. At least one new vaccine is in development, one expert recently told Business Insider, but it's not yet ready for a public debut.

Until then, vigilant prevention and quick treatment with antibiotics will have to do until what some researchers call a public-health fiasco can be turned around.

"Primary prevention is something where we're clearly losing the battle on," Dr. Paul Mead, chief of epidemiology and surveillance for the CDC's Lyme disease program, told Business Insider in June. "A safe and effective vaccine could help us turn the tide."

The recent article was published online in a CDC journal, Emerging Infectious Diseases.