Leptospirosis is a zoonotic illness caused by spirochetes of the genus Leptospira. Traditional classification held 1 pathologic species (Leptospira interrogans) with separate strains. However, genetic classification has identified at least 19 species (13 pathogenic). Numerous synonyms for the disease were named prior to understanding of the causative agent: Weil disease (used most commonly for the severe icteric form of the disease), swineherd’s disease, swamp fever, field fever (Europe), nanukayami, autumnal fever (Japan), canefield fever (Australia), and Fort Bragg fever (United States).
PATHOGENESIS AND EPIDEMIOLOGY
Humans become infected with leptospirosis via contact with animal urine, either directly or secondarily through contaminated soil or water. Leptospira species are very sensitive to acid and perish in solutions of low pH in a few hours; however, in alkaline or neutral medium, they persist for weeks. Infection can be acquired through cut or abraded skin or through respiratory or conjunctival epithelium with immersion. It is more frequent in summer and early fall and has a 3:1 predominance of males. Seventy percent of infections occur in individuals between 10 and 40 years of age.
Leptospirosis is the most common zoonosis in the world and is emerging as a major public health problem. Unfortunately, clinical leptospirosis is not always recognized, and countries with a high disease burden often lack notification systems. As such, it remains an underreported disease, and there are no reliable incidence figures globally. However, in 2009, the World Health Organization established a Leptospirosis Burden Epidemiology Reference Group (LERG). Modeling exercises in their annual reports estimate there are 873,000 cases worldwide and an annual mortality rate of 48,600 individuals.
Reservoir animals for leptospirosis include rodents, cattle, swine, dogs, horses, sheep, and goats. Reservoir animals retain the spirochete in their renal tubules and shed large numbers of these organisms in the urine for months after infection. Aside from contact with these animals, a strong association is also found with the use of rainwater catchment systems, farming, camping, and fishing.
Clinical manifestations vary somewhat with the infecting serogroup. The majority of Leptospira infections are subclinical or produce very mild symptoms that would not lead an individual to seek medical attention. Clinically significant leptospirosis is a biphasic disease that develops after a median incubation period of 1 week, with a range of 2 to 30 days. The initial phase, lasting 4 to 7 days, is the septicemic stage. There is sudden onset of fever, headache, myalgia, and gastrointestinal disturbances, such as abdominal pain, nausea, and vomiting. Physical examination usually reveals an acutely ill patient, who may be confused or delirious. Conjunctivitis, uveitis, pharyngeal infection, lymphadenopathy, hepatosplenomegaly, macular exanthem, proteinuria, and icterus may be seen. Up to two-thirds of patients develop abnormal radiographic findings in the lungs. Small nodular densities predominate, but a few patients have larger areas of consolidation. The patient may remain ...