Geographic Distribution of Lyme Disease in the United States
Lyme disease is highly concentrated in specific states, with the northeastern, mid-Atlantic, and upper north-central regions accounting for approximately 90-93% of all U.S. cases. 1, 2, 3
Highest Risk States
Connecticut has the highest reported state incidence of Lyme disease in the United States, with 70 cases per 100,000 population documented in 1997. 4, 2 The disease is not uniformly distributed across the country but rather shows marked geographic clustering:
- Ten states account for 93% of all reported cases: Connecticut, Delaware, Massachusetts, Maryland, Minnesota, New Jersey, New York, Pennsylvania, Rhode Island, and Wisconsin 3
- Approximately 90% of cases occur in roughly 140 counties concentrated along the northeastern and mid-Atlantic seaboard 2
- The highest county-level incidence was 600 cases per 100,000 population in Nantucket County, Massachusetts 4
Regional Patterns
Northeastern United States
The northeastern region represents the epicenter of Lyme disease transmission, where optimal ecological conditions exist for the primary vector tick Ixodes scapularis and its reservoir hosts. 1, 2 In highly endemic northeastern areas like Connecticut, 15-30% of blacklegged tick nymphs carry Borrelia burgdorferi, creating substantial transmission risk. 2
North-Central States
Minnesota and Wisconsin represent the second major endemic focus, with both Borrelia burgdorferi sensu stricto and the more recently identified Borrelia mayonii reported in 10 counties across these states. 1, 5
Western United States
California has limited Lyme disease transmission despite the presence of vector ticks, with cases primarily occurring in northern and north-coastal regions. 1, 5 The western vector Ixodes pacificus is far less efficient at disease transmission than the northeastern I. scapularis. 2
Emerging Geographic Trends
The geographic distribution of Lyme disease is actively expanding due to ecological and environmental changes. 1 Recent surveillance data demonstrates:
- North Carolina has experienced rapid emergence of Lyme disease in northwestern regions along the Appalachian Mountains, with some zip codes now reporting incidence rates similar to historically high-incidence northeastern states 6
- The spatial distribution of cases has become increasingly clustered over time, with Moran's I increasing from 0.012 (p=0.127) in 2010-2012 to 0.403 (p<0.0001) in 2019-2020 in North Carolina 6
- Annual case counts increased 101% from 9,908 cases in 1992 to 19,931 cases in 2006 nationally 3
Within-State Variation
Risk varies dramatically not only between states but within counties and townships. 4 Even in high-incidence states, the probability of Lyme disease can vary by as much as 30-fold based on specific residential location. 7 In Maryland, for example, the probability of Lyme disease is greatest north and west of Baltimore, with substantial local variation. 7
Critical Pitfall to Avoid
Clinicians must avoid assuming all tick bites carry equal Lyme disease risk, as geographic variation is profound. 2 Connecticut has 20-30 times higher risk than southern states like Texas based on available data. 2 Additionally, a rash similar to erythema migrans but NOT caused by B. burgdorferi has been described in southern states like Texas, associated with Amblyomma americanum tick bites, which can lead to misdiagnosis. 2
Practical Recommendations
For accurate risk assessment, detailed information regarding the distribution of Lyme disease risk within specific areas is best obtained from state and local public health authorities rather than relying solely on state-level data. 4