Epidemiology

Leptospirosis is the most widespread zoonotic infection in the world. The number of cases annually is estimated at 1.03 million, with approximately 60,000 deaths.[21] Approximately 100-200 cases are identified annually in the US, with about 50% of cases reported in Puerto Rico; Hawaii has the second highest incidence.[22]

Infections and outbreaks occur in both rural and urban areas in tropical, subtropical, and temperate regions.[23][24] Peak incidence occurs during the rainy season in tropical areas and during the late summer in temperate regions.[1][7] Outbreaks are commonly associated with outdoor work activities, exposure to flood water, and contact with contaminated water.[24] Leptospirosis outbreaks associated with floods are likely to continue to increase due to extreme weather events and climate change.[25][26]

The main risk factor for leptospirosis is direct or indirect contact with urine of infected animals. Contact can occur as a result of occupational exposure or through contact with water or soil that is contaminated with leptospires. Therefore, groups at high risk for infection include rural subsistence farmers, urban slum dwellers, veterinarians and animal handlers, military personnel, and miners.[21][27] Travellers and athletes participating in water sports also represent a growing population at risk for infection: freshwater swimming, rafting, kayaking, canoeing, fishing, hunting, and trail biking are associated with leptospirosis infection, particularly after heavy rainfall or flooding.[28][29][30][31][32]

Uncommonly, infections have occurred after laboratory accidents, blood transfusion, and surgical transplants.[33] Human-to-human transmission is very rare.[34] Infection can be acquired by walking barefoot during activities of daily living.[35][36] Other risk factors for leptospirosis include poor living conditions, including dirt floors, proximity to sewage and open sewers, and lack of sanitation.[36]

There is a higher incidence of leptospirosis in men, although this most likely is a result of exposure-related bias, as outbreaks associated with athletic events have resulted in similar rates of infections among men and women.[29][31] Some literature reports that severe illness may be more common in men than in women.[37] No predominance in any particular age group has been observed.

Clinicians should be aware that co-infections can occur with leptospirosis, including dengue, malaria, rickettsia, scrub typhus, and HIV.[14][15][16][17][18]

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