Independent New Hampshire Journalism · Northfield, NH

New Hampshire Reported 44 Lyme Disease Cases in 1992 and 1,810 in 2025. The Rules for Counting Them Changed Along the Way.

Measured per resident, New Hampshire ranked among the three highest states in six years of the 2010s, whether or not probable cases are counted. When a new national case definition took effect in 2022, the state’s count fell below its 2017–2019 average while the national count rose by more than two-thirds. And in 2023, Sullivan County’s reported rate was more than six times Coos County’s.

In 1992, the first year in the federal government’s public Lyme disease files, New Hampshire reported 44 cases to the Centers for Disease Control and Prevention. In 2025, the state Department of Health and Human Services counted 1,810, in a year-to-date tally it described as complete as of Jan. 29, 2026. That is about 41 times as many, and it is the largest annual count in either record.

The two numbers do not measure quite the same thing. The national rules for what counts as a case of Lyme disease have changed five times since the disease became nationally notifiable in 1991, according to the CDC, the latest of them in 2022. The newest rules changed who gets counted in New Hampshire, and the state’s own surveillance was incomplete in 2020 and 2021. Read with those limits in mind, the record still shows reported cases climbing from dozens a year to more than a thousand within about 15 years, and staying above a thousand in most years since.

The count, year by year

The CDC publishes Lyme disease case counts by state and county in three public-use files, split at 2008 and 2022 because the counting rules changed in those years. In the files, New Hampshire reported fewer than 50 cases a year through 1999. The count passed 100 in 2001, 500 in 2006 and 1,000 in 2008, the first year the national rules let states report “probable” cases alongside confirmed ones.

The state’s table and the CDC’s files disagree by a few cases in three of the four years they share. The department explains why in a note under its table: its counts are assigned to the date closest to the onset of illness, its counts for 2020 through 2024 were last updated on Feb. 21, 2025, and they “may not match data published annually by the Centers for Disease Control and Prevention.” The CDC’s public-use files also withhold some details to protect privacy, and a small number of cases nationally carry no state at all.

The sharp drops in 2014 and 2015 are in the federal files without explanation. The files are counts, and they do not say why a state’s reports rose or fell in a given year. Their documentation does note that before 2022, “several states with high Lyme disease incidence had initiated alternative methods of surveillance and those data were not reportable to CDC.” It does not name the states. The 2020 and 2021 drops come with a footnote. The state’s table says plainly that “Lyme disease surveillance is incomplete for 2020 and 2021,” and it warns that the pandemic’s effects on health care and disease reporting mean data from 2020 through 2023 “must be interpreted with caution.” The CDC says 2019 and 2020 data from some jurisdictions are incomplete for the same reason.

Why 2022 broke the series

The 2022 definition comes from a position statement of the Council of State and Territorial Epidemiologists, numbered 21-ID-05, and the CDC posts it as the national surveillance standard. Under earlier definitions, counting a case depended on clinical information, which health departments gathered by investigating each report. The 2022 definition split the country in two.

Jurisdictions with an average of at least 10 confirmed cases per 100,000 residents for three consecutive years became “high-incidence” jurisdictions. When the definition was adopted in spring 2021, there were 16 of them, and New Hampshire was one, along with the other five New England states. In those places, a case is counted on laboratory evidence alone. The definition has no “confirmed” category for high-incidence states at all: a case that meets the confirmatory laboratory criteria is “probable,” and one that meets only the presumptive laboratory criteria is “suspect.” That is why every New Hampshire case in the CDC’s 2022 and 2023 files is listed as probable.

The definition’s authors were explicit that it is for counting, not diagnosing. It “is intended solely for public health surveillance purposes and does not recommend diagnostic criteria for clinical partners to utilize in diagnosing patients with potential Lyme Disease,” the CDC’s posting of it says.

Nationally, the change pushed counts up. A CDC report in the agency’s Morbidity and Mortality Weekly Report in February 2024 found 62,551 cases reported in 2022, compared with an average of 37,118 a year in 2017 through 2019, an increase of 68.5 percent. Cases in the high-incidence jurisdictions as a group rose 72.9 percent. The authors concluded that the increase “likely reflects changes in surveillance methods rather than change in disease risk.”

New Hampshire went the other way. Its 2022 count of 1,085 was 28 percent below its 2017–2019 average of 1,506, and its rate fell from 109.4 to 78.8 cases per 100,000 residents, the report’s table shows. Only three other high-incidence jurisdictions reported fewer cases in 2022 than their 2017–2019 averages: Delaware, the District of Columbia and Pennsylvania. The report does not explain New Hampshire’s decline. In 2023 the state’s count rose to 1,573, above its 2017–2019 average and below its 2019 count of 1,710.

Against the other states

Measured per resident, New Hampshire ranked among the three highest states in six of the 10 years from 2010 through 2019: 2010, 2011, 2012, 2013, 2018 and 2019. Granite State Report divided each state’s case count in the CDC files by the Census Bureau’s population estimate for the same year, and ran the ranking two ways: once counting confirmed and probable cases together, and once counting confirmed cases alone, the measure the 2022 definition uses to decide which states are high-incidence. The same six years come out either way.

In 2012, New Hampshire’s rate was the highest of any state or the District of Columbia on both counts. In 2010 it was the highest on the combined count and second, behind Delaware, on confirmed cases alone. Some of the margins are thin. On the combined count, New Hampshire finished a close second to Vermont in 2011, and in 2018 New Hampshire, Rhode Island and Maine finished within a single case per 100,000 residents of one another.

Under the 2022 rules, New Hampshire ranked seventh in 2022, behind Rhode Island, Vermont, Maine, West Virginia, Wisconsin and New York. In 2023 it was in a virtual tie with New York for seventh, a tenth of a case per 100,000 apart, behind Rhode Island, Vermont, Maine, West Virginia, Massachusetts and Pennsylvania.

County by county

Inside the state, the most cases come from the most populous counties, but the highest rates do not. In 2023, Hillsborough County reported 410 cases, more than any other county, at a rate of about 96 per 100,000 residents. Sullivan County reported 89, a rate of about 202 per 100,000, the highest in the state. Coos County, the northernmost, reported 10, about 32 per 100,000. Sullivan’s rate was about 6.3 times Coos’s.

Every county reported more cases in 2023 than in 2022. Three counts more than doubled: Belknap’s, from 30 to 79; Merrimack’s, from 107 to 227; and Carroll’s, from 26 to 53. County of residence is where the patient lives. It is not necessarily where the tick bite happened.

What the counts leave out

A reported case is a case that reached a health department and met the rules. The CDC says more than 80,000 Lyme disease cases are reported to it each year by states and jurisdictions, while an estimate based on insurance claims suggests about 476,000 people may be diagnosed and treated for Lyme disease each year in the United States. The agency cautions that the claims estimate may run high, because billing codes do not always reflect a confirmed diagnosis. The 2024 CDC report put the limits more directly: “surveillance for Lyme disease is subject to under- and overreporting.”

The CDC’s public-use files describe the same limits in their documentation. They warn that each state’s count depends on its “abilities to capture and classify cases, which is dependent upon budget and personnel,” and that “a sudden or marked change in reported cases does not necessarily represent a true change in disease incidence.” The state’s table adds that its numbers “may represent an underestimate of the true absolute number and incidence rate of cases in the state.”

So the record supports some conclusions and not others. Reported Lyme disease in New Hampshire rose from dozens of cases a year in the 1990s to more than a thousand in most years since 2008. Per resident, the state has spent long stretches near the top of the country. The year-to-year swings, including the 2022 drop and the 2025 high, may mix real changes in illness with changes in how cases are found and counted, and the files alone cannot separate the two.

Granite State Report’s field guide to the state’s ticks, how to tell them apart and how to avoid bites is in “A Guide to Ticks in New Hampshire.”

— Granite State Report

Fact check

Editor’s note. Case counts for 1992 through 2023 are drawn from the CDC’s public-use Lyme disease files. Counts for 2024 and 2025 are drawn from the New Hampshire Department of Health and Human Services’ table of reportable infectious diseases for 2020 through 2025. Case-definition details are drawn from the CDC’s posting of the 2022 definition and its Morbidity and Mortality Weekly Report of Feb. 15, 2024. Population figures are U.S. Census Bureau estimates. Sums, rates, rankings and ratios the agencies did not state are Granite State Report arithmetic. Nothing here is medical advice.

Corrections: Granite State Report corrects verified errors promptly and appends a dated note identifying what changed and when. The corrections policy and the corrections register are public, and correction notes are never removed.