Geographic Distribution of Lyme Disease
Lyme disease is most commonly found in the northeastern and upper north-central regions of the United States, with approximately 90% of cases reported from about 140 counties along the northeastern and mid-Atlantic seaboard and in the upper north-central region. 1, 2
Global Distribution
Lyme disease is endemic in several regions across:
- United States (primarily Northeast and Upper Midwest)
- Canada
- Temperate regions of Eurasia 1
United States Distribution
High-Risk Regions
- Northeast: Connecticut, Delaware, Massachusetts, Maryland, New Jersey, New York, Pennsylvania, Rhode Island
- Upper Midwest: Minnesota, Wisconsin 1, 3
These 10 states account for 93% of all reported Lyme disease cases in the United States 3.
Regional Differences in Causative Agents
- In the United States: Borrelia burgdorferi sensu stricto is the primary causative agent 1, 2
- In Europe and Asia: B. garinii and B. afzelii are the dominant genospecies 1, 4, 5
Tick Vectors by Region
Lyme disease is transmitted by ticks of the Ixodes ricinus complex:
- Eastern United States: Ixodes scapularis (black-legged or deer tick)
- Western United States: Ixodes pacificus (western black-legged tick) 1, 2
Infection Rates in Ticks
- In highly endemic areas of the Northeast and Upper Midwest: 15-30% of I. scapularis nymphs carry B. burgdorferi
- In Western United States: Up to 14% of I. pacificus nymphs are infected
- In Southern United States: Only 0-3% of I. scapularis ticks are infected 1
Risk Factors and Populations
The highest risk populations include:
- Children aged 5-15 years
- Adults aged over 50 years 6
- People living in residential areas surrounded by woods or overgrown brush
- Those participating in outdoor recreational activities in tick habitats
- Individuals in outdoor occupations (landscaping, forestry, wildlife management) 1
Epidemiological Trends
- Lyme disease cases more than doubled between 1992-2006, from 9,908 to 19,931 cases 3
- The disease continues to spread geographically due to:
- Expansion of deer populations (principal hosts for adult ticks)
- Reforestation in previously developed areas
- Climate change effects on tick habitats 1
Southern Tick-Associated Rash Illness (STARI)
A condition that mimics the erythema migrans rash of Lyme disease occurs in the southern United States but is not caused by B. burgdorferi infection. This condition is suspected to be associated with bites from Amblyomma americanum ticks 1, 2.
Clinical Implications
Understanding the geographic distribution of Lyme disease is crucial for:
- Assessing pre-test probability when evaluating patients with suspected Lyme disease
- Making appropriate decisions about prophylactic treatment after tick bites
- Implementing targeted prevention strategies in high-risk areas
Common Pitfalls to Avoid
- Assuming Lyme disease in non-endemic regions without travel history to endemic areas
- Overlooking the possibility of other tick-borne diseases that may co-occur with Lyme disease
- Failing to recognize that prophylactic antibiotics are generally not necessary after I. pacificus bites in the western United States due to lower infection rates 2