Leptospirosis 

Overview 

Leptospirosis is a zoonotic (carried by animals) infection caused by spiral-shaped bacteria called spirochetes. Infection most commonly occurs when water, soil or mud contaminated with the urine of an infected animal (typically livestock, dogs and rodents) comes in contact with human skin, particularly damaged skin or mucous membranes. The clinical course of leptospirosis can range from a self-resolving illness of fever, headache and body aches to a serious and sometimes fatal illness. In tropical settings, it can present very similarly to other illnesses such as dengue or chikungunya, making it difficult to diagnose without laboratory support.  

Military personnel may be at high risk of infection when exposed to fresh water during training exercises and deployments in endemic areas. Risk is particularly high in Jungle Warfare Training Centers, floodwaters and after hurricanes or monsoon events. While leptospirosis can occur worldwide, the bacteria persist longer in warmer water and infections are more common in tropical and subtropical regions.
Prevention measures center on avoiding skin exposure to or swallowing of potentially contaminated freshwater. Risk is highest in situations where skin injury is common (e.g., training or operational environments) and there is immersion. Careful field hygiene can minimize rodent sources of infection. U.S. military personnel operating in wet, tropical environments may also be provided with prophylactic antibiotics for risk mitigation when the threat of leptospirosis is high. While antibiotic prophylaxis doesn’t completely prevent infection, evidence suggests that it significantly reduces the severity of disease (including the risk of hospitalization and death), leading to better health outcomes and preserving force readiness.

Policy 

• NAVMED P-5010-8 Navy Entomology and Pest Control Technology 

• NAVMED P-5010-9 Preventive Medicine for Ground Forces 

• NAVMED P-5010-10 Sanitary Control and Surveillance of Field Water Supplies  

• BUMEDINST 6230.15B Immunizations and Chemoprophylaxis for the Prevention of Infectious Disease 

Clinicians 

Clinical Presentation & Case Recognition:  

Clinicians should suspect leptospirosis in personnel presenting with acute febrile illness in the context where personnel may have been exposed to contaminated fresh water.  Key symptoms observed in a 2014 outbreak among U.S. Marines included: 

• Fever/Chills/Myalgia 

• Headache 

• Scleral icterus 

• Nausea and abdominal pain 

The disease may occur in two phases: 

• The septicemic first phase, in which people may have an acute presentation of fever, chills, headache, muscle aches, vomiting/diarrhea. This phase typically lasts ~7 days. Symptoms may resolve after this phase or may improve somewhat before progressing. 

• The second, immune phase, of illness is more severe and may progress to renal failure, liver failure, hemorrhagic symptoms (especially pulmonary), aseptic meningitis and shock. 

Because the symptoms are somewhat non-specific and overlap with several viral illnesses that may coexist in the same region, it is important to keep leptospirosis in the differential of febrile presentations. While scleral icterus and jaundice can be present in dengue, they are notably more common in leptospirosis, occurring in ~30% of cases. During an outbreak in Bangladesh when both dengue and leptospirosis were circulating, untreated leptospirosis had a case-fatality rate (CFR) of 5%, whereas the CFR for dengue was 1.2%. Empiric treatment with antibiotics should be initiated immediately if leptospirosis is suspected, without waiting for laboratory confirmation. Doxycycline is the first line drug for treating leptospirosis. 

Diagnosis: 

While treatment should not be delayed pending results, the following tests can be used for confirmation: 

• PCR: 

• First Week of Illness: PCR of whole blood and serum. Be aware that PCR of blood will likely become negative after the first 5 days or so.  

• After First Week: PCR of urine or CSF  

• Serology: IgM and paired acute/convalescent IgG to detect antibody increase 

• Microscopic Agglutination Test, or MAT, is the reference standard that can be used for serovar determination during outbreaks 

Leptospirosis testing IS part of the Biofire Global Fever Panel, which also tests for dengue, chikungunya and malaria spp. However, because of the transient nature of the bacteremia, a negative result may not exclude disease. 

More detailed information on treatment and diagnosis may be found on the CDC’s Leptospirosis-Fact Sheet for Clinicians.  

Case Reporting 

Leptospirosis is a nationally notifiable disease and must also be reported within DoW.  BUMEDINST 6220.12D  requires that all cases of leptospirosis be reported to Preventive Medicine authorities in the Disease Reporting System internet (DRSi). Case classification details are fully described in the Armed Forces Medical Reportable Events Guide. DCPH-A has also put together a Flowchart for Case Definition. Detailed information on reporting in DRSi can be found on NMCFHPC’s Medical Surveillance and Reporting page.    

Notify local Preventive Medicine and/or your cognizant Navy Environmental and Preventive Medicine Unit (NEPMU) so that measures can be taken to mitigate the risk of local transmission.   

Public Health 

Transmission:

Image from Sykes, JE "Role of Diagnostics in epidemiology, Management, Surveillance, and Control of Leptospirosis. Pathogens 2022, 11(4), 395.

Infection usually occurs after dermal exposure to fresh water or soil contaminated with urine from infected animals, typically livestock and rodents. Risk is increased by abraded or damaged skin. While most leptospirosis infections occur via dermal exposure, ingestion of contaminated water is also a recognized route of transmission.  This risk is particularly relevant in field environments where personnel may inadvertently swallow untreated water, particularly in high-exposure training environments where it has been identified as a key risk factor in at least two outbreaks. 
Leptospirosis often occurs as part of an outbreak (e.g. multiple members of a unit who had the same exposure) but can also present as an isolated case. Incubation period is between 2-30 days, typically 1-2 weeks after exposure. Leptospirosis is not transmitted person-to-person, and
isolation of patients is not required. 

Control Measures:  

Vaccines: There is no human vaccine for leptospirosis licensed by the FDA in the United States, although military working dogs are vaccinated against leptospirosis annually. Human vaccines are available in some countries, and vaccines are also widely available for livestock. There are multiple leptospirosis serovars, however, and limited cross-protection between them limits the effectiveness of currently available vaccines. Nonetheless, routine livestock vaccination can reduce the burden of leptospirosis in agricultural areas, particularly in conjunction with rodent control measures.   

Exposure control: The most effective prevention strategy is avoiding exposure to untreated fresh water. Minimize time spent in contaminated water or soil, especially after heavy rainfall. Use protective clothing, especially waterproof boots and gloves to reduce contact. Recreational exposure from swimming in and around waterfalls is a common source of leptospirosis infection and should be discouraged. 

When exposure to potentially contaminated water or soil is required for operational reasons: 

• Minimize duration of exposure and avoid submersion of head or face 

• Cover cuts, abrasions and open wounds with occlusive dressings before exposure  

• Avoid swallowing or intentionally ingesting untreated water  

• Wash exposed skin thoroughly with soap and water after contact  

Environmental Controls: Optimize camp hygiene and implement rodent control measures in endemic areas. Commanders and Preventive Medicine personnel should ensure compliance with field sanitation standards in accordance with NAVMED P-5010-9 Preventive Medicine for Ground Forces. 

Drinking Water Safety: Personnel must consume only potable water that has been approved, treated, and monitored in accordance with NAVMED P-5010-10 Sanitary Control and Surveillance of Field Water Supplies. Untreated surface water (e.g., rivers, streams, floodwaters, standing water) should not be consumed.  When approved potable water is not available, water must be treated using authorized field-expedient methods (e.g., boiling, approved chemical disinfection, or military-issued water purification systems) prior to consumption.  Personnel should be specifically instructed to avoid inadvertent ingestion of water during training operations. 

Chemoprophylaxis: In operational settings with high risk of leptospirosis, chemoprophylaxis is generally recommended. The standard prophylactic regimen is doxycycline 200 mg orally once weekly, started 1-2 days before exposure and continued throughout the period of exposure. This approach was described in a seminal paper in 1984 (Takafuji) after a large leptospirosis outbreak occurred among U.S. Army personnel among students at the Jungle Operations Training Center in Panama. Subsequent evidence for the effectiveness of this regimen in preventing infection is variable. Analysis of a large 2014 leptospirosis outbreak among U.S. Marines at the Jungle Warfare Training Center in Okinawa showed that taking pre- or post-exposure doxycycline made no statistical difference in attack rate (Dierks), although a review of the inpatient cases from that outbreak found that personnel who took their prescribed doxycycline prophylaxis had significantly less severe outcomes (Hall). Those not taking prophylaxis were more likely to be admitted to the ICU, require surgery, and experienced a more severe infection overall. A Cochrane review from 2024 to evaluate the benefits and harms of antibiotic prophylaxis for human leptospirosis concluded that antibiotics probably do not reduce the chance of developing leptospirosis infection and may cause non-serious adverse events. The evidence was very limited, however, and the variability of study designs, populations, and endpoints makes meta-analysis unreliable. The authors themselves cite very low certainty for their results (Win). Because doxycycline remains an effective treatment for leptospirosis, it is likely that the timing of doxycycline administration in relation to the exposure adversely impacted its effectiveness in preventing infection.  

Extending chemoprophylaxis administration until a week after training completion provides better coverage into the incubation period of leptospirosis and will likely reduce both infections and serious outcomes. Future research into alternative chemoprophylaxis regimens (e.g., daily 100 mg doxycycline as is used for antimalarial prophylaxis or weekly azithromycin) is both operationally relevant and necessary to prevent future DoW outbreaks. 

Web Resources 

CDC 2026 Yellow Book  

CDC Main Leptospirosis Site 

CDC Fact Sheet 

DHA Case Investigation Worksheet 

Epi and Selected Publications 

Dierks J, Servies T, Do T. A Study on the Leptospirosis Outbreak Among US Marine Trainees in Okinawa, Japan. Mil Med. 2018 Mar 1;183(3-4):e208-e212. doi: 10.1093/milmed/usx013. PMID: 29514334. Outbreak report of 81 cases from JWTC; Doxy chemoprophylaxis (pre or post exposure) had no impact on attack rate among the cohort of 239. 

Hall MT, Do TA, Shusko MP. The value of pre-exposure prophylaxis: A case series of US Marines infected with leptospirosis. Travel Med Infect Dis. 2023 Mar-Apr;52:102523. doi: 10.1016/j.tmaid.2022.102523. Epub 2022 Dec 22. PMID: 36566953. Review of hospitalizations that concludes that while doxycycline prophylaxis did not prevent infection, it was significantly correlated with less severe health outcomes. 

LaRocque, Regina C et al. Leptospirosis during Dengue Outbreak, Bangladesh. Vol.11, no5, EmerInfectDis 11(5):766-769 (2005). Comparison of dengue and untreated lepto in the setting of concurrent transmission.  

Petersen K, Maranich A. Antibiotic Chemoprophylaxis for Leptospirosis: Previous Shortcomings and Future Needs. Trop Med Infect Dis. 2024 Jul 2;9(7):148. doi: 10.3390/tropicalmed9070148. PMID: 39058190; PMCID: PMC11281621. Review of state of play and research needs. 

Sykes JE, Reagan KL, Nally JE, Galloway RL, Haake DA. Role of Diagnostics in Epidemiology, Management, Surveillance, and Control of Leptospirosis. Pathogens. 2022; 11(4):395. https://doi.org/10.3390/pathogens11040395. In depth discussion of diagnostics and vaccines.

Takafuji ET, Kirkpatrick JW, Miller RN, Karwacki JJ, Kelley PW, Gray MR, McNeill KM, Timboe HL, Kane RE, Sanchez JL. An Efficacy Trial of Doxycycline Chemoprophylaxis Against Leptospirosis. N Engl J Med. 1984 Feb 23;310(8):497-500. doi: 10.1056/NEJM198402233100805. PMID: 6363930. This landmark study in Panama demonstrated 95% efficacy for doxycycline, which became the basis for current military prophylactic policy.

Win TZ, Perinpanathan T, Mukadi P, et al. "Antibiotic Prophylaxis for Leptospirosis." Cochrane Database of Systematic Reviews (2024). This recent review found that evidence for the effectiveness of prophylactic antibiotics in preventing leptospirosis is of very low certainty.