Burden, Epidemiology, and Clinical Characteristics of Leptospirosis within the United States Military Health System

Leptospirosis is a global zoonosis with varying severity that remains underreported due to diagnostic limitations. We analyzed the burden, epidemiology, and clinical characteristics of leptospirosis diagnoses in the Military Health System, which includes care to more than 1.3 million active-duty service members and 6.5 million retirees and dependents worldwide. Potential diagnoses were identified by leptospirosis-specific International Classification of Diseases (ICD)-9 and ICD-10 codes or laboratory tests between January 1, 2013 and December 31, 2021. Upon chart review they were classified per U.S. CDC case definitions. Of 296 identified potential leptospirosis cases, there were 11 confirmed, 134 probable, and 53 possible cases. The majority of cases were young adult males. Japan, Guam, and Hawaii were the most common areas of exposure. Most cases were associated with freshwater exposures, both from recreational and occupational exposures (including a significant number associated with jungle warfare training in Okinawa). Confirmed cases were statistically more likely to be diagnosed in Hawaii. This study from a worldwide distributed unified health system reaffirms the global distribution and wide range of clinical severity of leptospirosis. It highlights the relatively few confirmed diagnoses in the setting of diagnostic limitations even in a population already presumed to be at elevated risk. Strategies and improved diagnostics are needed to shift clinician practice to pursue confirmatory testing, particularly in cases with a high suspicion or high-risk of exposure. Recent improvements in availability and performance of various testing methodologies (multiplex polymerase chain reaction based-assays and serological testing) may further define the epidemiology of this global disease.