Leptospirosis Incidence in Hawaii (2026)
Based on the most recent high-quality data from Hawaii, leptospirosis remains highly endemic with incidence rates ranging from 17.74 to 30.90 cases per 100,000 population at the county level, with localized district-level rates reaching as high as 244.78 cases per 100,000 population—making Hawaii the state with the highest leptospirosis incidence in the United States. 1
Current Epidemiological Landscape
The 2025 analysis of confirmed leptospirosis cases in Hawaii from 2012-2024 provides the most contemporary data available 1. This represents a continuation of Hawaii's historical pattern as the leading state for leptospirosis in the US, a distinction it has maintained since national surveillance began.
Geographic Distribution
Island-specific patterns show marked variation:
- Kaua'i and Hawai'i Island: Highest county-level rates (17.74-30.90 per 100,000)
- Northeastern Maui: Highest district-level rate in the entire state at 244.78 per 100,000 1
- Northeastern coastal regions: Consistently elevated rates across all islands, correlating directly with higher annual rainfall 1
This geographic clustering represents critical information for risk assessment—the windward (northeastern) coasts of each island demonstrate substantially higher transmission risk than leeward areas.
Temporal Trends
A significant epidemiological shift has occurred over the past two decades:
The seasonal pattern has changed from predominantly summer occurrence to winter predominance 2. This shift coincides with changing rainfall patterns and may reflect evolving environmental conditions supporting Leptospira survival and transmission.
Between 2014-2020, national data showed an average annual increase of 13 cases (R² = 0.69), with cases peaking in early fall corresponding to warmer weather and hurricane season 3. However, Hawaii's endemic pattern differs from this national trend, maintaining year-round transmission with winter peaks.
Serogroup Evolution
The dominant infecting serogroups have shifted dramatically:
- Current dominance: Australis and Icterohaemorrhagiae 1
- Historical pattern: Icterohaemorrhagiae was previously dominant 2
Primary infecting serovars identified (2012-2024):
- Bratislava
- Icterohaemorrhagiae
- Georgia
- Canicola (localized to Maui and Hawai'i Island only)
- Australis 1
This serovar distribution has important implications for animal reservoir identification and targeted prevention strategies.
Clinical Severity
The disease burden in Hawaii remains substantial:
From the 1999-2008 Hawaii surveillance data, 57% of case reports were laboratory confirmed 2. National data from 2014-2020 showed that among cases with outcome data, 85% required hospitalization and 10% died 3. These mortality and morbidity figures underscore the serious nature of leptospirosis despite its relatively low public profile.
Exposure Patterns
Recreational and avocational exposures have become predominant:
Historical occupational exposures have plateaued while recreational exposures increased over the past 20 years 2. This shift reflects changing demographics and tourism patterns in Hawaii. Freshwater exposure (swimming, kayaking, hiking through streams) and contact with mud represent the primary risk activities 3.
Comparison to Historical Baseline
The mean annual incidence rate from 1974-1998 was 1.29 per 100,000 population statewide 4. Current county-level rates of 17.74-30.90 per 100,000 represent a substantial increase, though methodological differences in case ascertainment and laboratory confirmation may partially account for this apparent rise.
Clinical Implications for 2026
For healthcare providers in Hawaii, leptospirosis should be considered in any febrile patient with:
- Recent freshwater exposure (within 2-30 days, typically 7-12 days) 5
- Outdoor recreational activities in northeastern coastal regions
- Myalgia (particularly calf pain), conjunctival suffusion, headache
- Exposure during or after heavy rainfall periods
- Winter season presentation (current predominant pattern)
The biphasic illness pattern remains characteristic: initial bacteremic phase with flu-like symptoms (4-7 days), followed 1-3 days later by an immune phase with potential hepatorenal syndrome and hemorrhage 5.
Laboratory confirmation requires:
- IgM titer ≥1:320 (suggestive)
- Convalescent serology >10 days after symptom onset for IgM ELISA and microscopic agglutination test (MAT) 5
Treatment should be initiated on clinical suspicion given the high local incidence and serious outcomes, using penicillin or tetracycline antibiotics during the bacteremic phase 5. Severe cases may require renal or liver support despite antibiotic therapy, as severe disease is largely immunologically mediated 5.