Leptospirosis is a zoonotic disease caused by gram-negative Leptospira bacteria. Direct transmission to humans occurs when broken skin and mucous membranes come into contact with the urine of infected animals such as rodents. The early phase of the disease is mild and characterized by nonspecific symptoms (e.g., fever, headache, and myalgia). In most cases, symptoms resolve spontaneously after a week. However, in 10% of cases, the disease progresses rapidly to a severe form (icterohemorrhagic leptospirosis, or Weil disease), which typically presents with a triad of jaundice, bleeding manifestations, and acute kidney injury. Diagnosis is based on patient history, clinical findings, and laboratory tests. Treatment consists of antibiotics and supportive care.
- Leptospirosis is the most common zoonotic disease worldwide and is most common in the tropics
- Low incidence in the US (100–200 cases per year): Half of these cases are reported in Hawaii.
Epidemiological data refers to the US, unless otherwise specified.
- Pathogen: Leptospira (especially L. interrogans); , a genus of gram-negative spirochete with hook-shaped ends
- Route of infection
The incubation time is 2–30 days. The disease has a mild form, which is characterized by nonspecific symptoms that generally resolve spontaneously after a week. In 10% of cases, the disease progresses rapidly to a severe form (icterohemorrhagic leptospirosis, or Weil disease). 
Mild (anicteric) leptospirosis 
Clinical manifestations during the early phase are due to bacteremia.
- High fever, headache
- Diarrhea, vomiting
- Conjunctival suffusion: bilateral diffuse reddening of the conjunctivae without exudates
- Myalgias (especially in the calves and lower back)
- Possibly aseptic meningitis → worsening headache and photophobia
Severe leptospirosis (Weil disease, icterohemorrhagic leptospirosis) 
Clinical features are due to systemic spread and multiorgan involvement.
- Hepatitis; → hepatomegaly, jaundice, acute liver failure
- (interstitial nephritis, acute tubular necrosis) → oliguria, hematuria
- Anemia, azotemia
- Hemorrhagic diathesis
- Cardiac abnormalities (e.g. myocarditis, pericarditis, arrhythmia, conductivity impairment)
- Dark-field microscopy of urine or blood; samples (the thin Leptospira spirochetes cannot be visualized by light microscopy)
- Serological tests
- PCR: detect leptospiral DNA in bodily fluids
- Complete blood count
- Kidney function tests: elevated BUN in Weil disease
- Liver function tests: ↑ AST/ALT
- For mild leptospirosis: doxycycline, azithromycin, or aminopenicillins (ampicillin, amoxicillin)
- For severe leptospirosis
If leptospirosis is suspected based on a patient's clinical features and history, empiric therapy should be started right away.