Most US Lyme disease cases are urban, but incidence is climbing fastest in rural counties

Lyme disease is often imagined as a hazard of forests and rural fields, but a new analysis of two decades of United States surveillance data complicates that picture: most reported cases are in urban residents, even as the disease's incidence is rising fastest in the countryside [s1]. The study, published in Public Health Challenges on 6 November, examined trends in Lyme disease incidence and case burden across high-incidence US jurisdictions from 2000 to 2021, grouping counties by how urban or rural they are using the federal Rural–Urban Continuum Codes (RUCC) [s1].

The authors drew county-level counts of confirmed Lyme disease cases from national surveillance data and calculated incidence rates against census population projections, using mixed-effects models to estimate annual change; data from 2020 were excluded [s1]. The resulting split between where cases occur and where they are growing fastest is the study's central finding.

Where the cases are

More than 70% of confirmed Lyme disease cases reported each year from 2000 to 2021 occurred in people living in metropolitan counties [s1]. Between 42% and 53% of cases each year came from large metro counties with populations of at least one million, and between 60% and 80% of cases each year were in residents of counties in the top population-density quintile [s1]. In raw numbers, in other words, the burden of reported Lyme disease in the US is predominantly borne by urban and suburban populations — a reflection of where people live, and of the expanding wildland–urban interface where human housing meets tick habitat.

Incidence rates tell a different story. Every RUCC category had an average incidence greater than 10 confirmed cases per 100,000 population per year from 2001 to 2021 [s1]. But the highest county-level incidence in every year since 2006 was in nonmetro counties, and incidence was consistently lowest in the most densely populated counties from 2008 to 2021 [s1]. High population totals, not high per-person risk, are what make urban counties dominate the case counts.

Where it is rising fastest

Across the 20 years studied, Lyme disease incidence increased significantly in every county type [s1]. The annual increase exceeded 10% in all RUCC categories except the largest metro counties — those with populations of one million or more, which saw the smallest year-over-year increase [s1]. The steepest climbs were at the rural end of the spectrum: the largest estimated county-level increases were 15.8% and 15.6% per year in nonmetro counties not adjacent to a metro area, with populations fewer than 5,000 or of 20,000 or more, respectively [s1].

That combination — the bulk of cases in cities, the fastest growth in the most remote counties — is what the authors flag for public health planning. It suggests that messaging and prevention aimed only at classically "rural" risk settings may miss where most diagnoses actually occur, while surveillance and clinician awareness in rural areas need to keep pace with a risk that is rising there faster than anywhere else [s1].

What the analysis can and cannot show

This is an ecological study of surveillance data, and its limits run with the method. Confirmed-case surveillance reflects not only true infections but also testing patterns, clinician reporting and case-definition changes over a 20-year span, so apparent trends can be shaped by how diligently cases are captured as much as by underlying transmission. Counts are tied to county of residence, which is not necessarily where exposure occurred — an urban resident can acquire the infection on a weekend in the woods, which complicates any simple reading of "urban" cases. The analysis also covers only high-incidence jurisdictions, so it describes the established Lyme belt rather than the country as a whole [s1].

Within those bounds, the message is a useful corrective. The reflex to treat Lyme disease as a rural problem understates how much of its measured burden sits in and around cities, while the fastest proportional growth is happening in exactly the remote counties where awareness and testing capacity are often thinnest [s1]. The study's contribution is not a forecast but a map of where a steadily rising disease is concentrated now, and where it is moving [s1].

This article is informational and is not medical advice.