Syracuse, N.Y. -- Lyme disease, once largely a problem for Long Island and the Lower Hudson Valley, continues its march northward and westward, according to state Department of Health Data.

An increasing number of cases of Lyme, caused by a bacteria spread through the bite of deer ticks, are coming from areas such as Central New York and the Adirondacks.

"What I call the leading edge is a line between Syracuse and Rochester," said Bryon Backenson, the health department's deputy director of communicable disease control. "Syracuse is seeing a higher incidence of cases than Rochester is, but with every year it moves further and further."

From 2013 to 2015, Onondaga County had 355 new cases of Lyme disease, while Monroe County had 272. Because Monroe County is more populous, however, Onondaga County had double the rate of Lyme diseases cases per 100,000 people.

The disease was first identified in Old Lyme, Connecticut, in the 1970s, and has been moving northward ever since. The Hudson Valley and Catskills still have the highest rates of cases. In 2016, Columbia County had a rate of 635 cases per 100,000 people.

By contrast, the disease is much more rare in Western New York. Columbia County's rate is nearly 80 times greater than Allegany County's a rate of just 8.4 cases per 100,000 people.

Summer is the season when Lyme disease cases soar. Not only are people outdoors more and thus have more contact with the deer ticks that carry the Lyme bacteria, but nymphs emerging this time of year account for about 80 percent of Lyme cases. The nymphs, about the size of a poppy seed, are smaller than adult ticks, so they're harder to see and remove before they pass on the bacteria through their bite.

The steady movement of Lyme disease across the state is due to many factors, Backenson said. Deer and rodents that are the hosts of the deer, or black-legged, tick, move about and carry ticks with them.So do birds, which have much larger ranges than deer and rodents do.

Climate change might also be fueling the expansion of Lyme disease, Backenson said.

"It's hard to pin down, but the milder winters we have had over the past decade is probably helping with the northward and westward movement of deer ticks," he said. "Ticks bed down for the winter, and depending on the harshness or mildness of the winter, more might survive."

One of the most important factors in the risk of contracting Lyme disease is simply the sheer number of ticks. Another is the percentage of ticks that carry the Lyme bacteria, which has also been rising. About half of all adult deer ticks, and about 25 percent of the nymphs, in New York carry the bacteria, according to health department research.

Below is a list of New York state counties, the number of new cases of Lyme disease reported in 2016, and the rates per 100,000 people. Counties designated with an asterisk had fewer than 10 cases, which means the rates may not be accurate. Because of problems with data collection in 2016, the chart uses 2015 data for Onondaga County.

New York state Lyme disease cases and rates for 2016

County Cases Rate per 100,000 Albany 153 49.5 Allegany* 4 8.4 Broome 294 149.6 Cattaraugus 12 15.4 Cayuga* 7 8.9 Chautauqua* 4 3.1 Chemung 94 108 Chenango 44 90.1 Clinton 30 36.9 Columbia 391 635.7 Cortland 25 51.6 Delaware 55 119.4 Dutchess 386 130.5 Erie 53 5.7 Essex 75 194.9 Franklin 15 29.6 Fulton* 6 11.1 Genesee* 6 10.2 Greene 297 623.6 Hamilton* 2 42.4 Herkimer 25 39.6 Jefferson 53 45.1 Lewis 16 59.4 Livingston* 3 4.6 Madison 29 40.4 Monroe 109 14.5 Montgomery 14 28.2 Nassau 154 11.3 Niagara* 0 0 Oneida 33 14.2 Onondaga** 127 27.1 Ontario 21 19.2 Orange 531 140.6 Orleans* 2 4.8 Oswego 76 63.3 Otsego 106 174.8 Putnam 344 347.3 Rensselaer 497 310.1 Rockland 165 50.6 St. Lawrence 135 121.6 Saratoga 245 108.3 Schenectady 117 75.7 Schoharie 30 95.8 Schuyler 20 110 Seneca 12 34.5 Steuben 68 69.7 Suffolk 644 42.9 Sullivan 101 134.9 Tioga 65 131.4 Tompkins 129 122.9 Ulster 351 194.9 Warren 54 83.5 Washington 85 136.6 Wayne 17 18.6 Westchester 343 35.1 Wyoming* 3 7.3 Yates 16 63.9 New York City 946 11.1 Statewide Totals 7,543 38.1 Outside NYC 6,597 58.7

Source: New York state Department of Health