Clinical Characteristics and Factors Associated with a Cluster of Severe Leptospirosis Cases — Chicago, Illinois, 2025
- Presentation Day/Time: Friday, April 24, 1:45 PM
- Presenter: Jennifer Trebbin, MPH, MS, Chicago Department of Public Health
The Issue
- Most leptospirosis infections are mild or asymptomatic, but up to 15% can cause severe symptoms. Leptospira bacteria are transmitted to humans through contact with urine of infected animals, including rodents. During 2015–2024, a median of two leptospirosis cases per year were reported to the Chicago Department of Public Health (CDPH), but four cases were identified in close geographic proximity during July–August 2025.
What We Did
- We investigated to describe clinical severity and explore contextual factors that could have contributed to this cluster.
What We Found
- All four case-patients were males who lived within an approximately 2.4-mile radius and had multiple epidemiologic risk factors for disease, including direct or indirect contact with rodent waste. All patients had multi-system organ involvement and three of the four patients had altered mental status, indicating neurologic involvement. Neurologic involvement is generally reported in 10–15% of leptospirosis cases. All patients were hospitalized with severe infections and one patient died.
What This Means
- Clinicians should consider leptospirosis in the differential diagnosis in urban settings. Ongoing rodent control efforts in Chicago remain essential for reducing transmission risk. Public guidance should emphasize protective measures during gardening, yard work, and freshwater-related outdoor activities.
