Cattle, swine, sheep, dogs, raccoons, and other animals serve as the disease reservoir. Infected animals may develop overt illness or become asymptomatic carriers, shedding leptospires in the urine for many months. Leptospires may enter the human host directly through abraded skin, the conjunctivae, or oral mucous membranes. About 50 to 100 cases of leptospirosis were reported annually in the United States until 1995, when cases were no longer required to be reported. However, it is likely that many cases, including subclinical infections, go unrecognized.
Leptospirosis occurs sporadically or as a common-source outbreak.
It was once regarded primarily as an occupational disease, particularly among farmers, workers in rice paddies and sugarcane fields, veterinarians, sewer workers, and slaughterhouse workers. In the United States, however, leptospirosis is now most common among children, adolescents, and homemakers. It is contracted during recreational activities such as fishing, hunting, and swimming or wading in contaminated ponds, participating in other water sports, or exposure to rat urine in urban or rural settings. Infections occur in most areas of the United States, but more cases have been reported in southern and western regions, usually during the summer and autumn months. After the entry of leptospires through mucous membranes or abraded skin, the organisms are disseminated via the blood.