Leptospirosis (Leptospira interrogans)
2025 Case Definition

Background

Leptospirosis is a zoonotic disease identified globally, with most cases occurring in tropical climates. Human infection may occur following direct contact with urine or other body fluids from an infected animal, or indirectly through contact with contaminated water, soil, or food. Leptospira bacteria may enter the body through mucous membranes or abraded skin.^{1, 2}

Clinical Criteria

An illness characterized by one or more of the following: fever, headache, chills, myalgia, vomiting, nausea, diarrhea, abdominal pain, conjunctival suffusion, renal insufficiency, jaundice, respiratory insufficiency, meningitis, or rash. Symptoms may be biphasic.

Laboratory Criteria

Confirmatory Laboratory Evidence:

• Isolation of Leptospira from a clinical specimen, OR
• Fourfold or greater increase in Leptospira agglutination titer between acute and convalescent phase serum specimens studied at the same laboratory, OR
• Demonstration of Leptospira in tissue by direct immunofluorescence, OR
• Leptospira agglutination titer of ≥800 by Microscopic Agglutination Test (MAT) in one or more serum specimens, OR
• Detection of pathogenic (P1 clade) or intermediate (P2 clade) Leptospira DNA (e.g., by PCR) from a clinical specimen.

Presumptive Laboratory Evidence:

• Leptospira agglutination titer of ≥200 but < 800 by Microscopic Agglutination Test (MAT) in one or more serum specimens, OR
• Demonstration of anti-Leptospira antibodies in a clinical specimen by indirect immunofluorescence, OR
• Demonstration of Leptospira in a clinical specimen by darkfield microscopy, OR
• Detection of IgM antibodies against Leptospira in an acute phase serum specimen.

Note: The categorical labels used here to stratify laboratory evidence are intended to support the standardization of case classifications for public health surveillance. The categorical labels should not be used to interpret the utility or validity of any laboratory test methodology.

Epidemiologic Linkage

Involvement in an exposure event (e.g., adventure race, triathlon, flooding, occupational exposure) with associated laboratory-confirmed cases of leptospirosis.

Criteria to Distinguish a New Case from an Existing Case

A new case should be enumerated when:

• A person previously enumerated as a probable or confirmed case with new onset of symptoms that meets the criteria for a confirmed or probable case, after consultation with CDC leptospirosis SMEs.

Probable

• Meets clinical criteria AND meets presumptive laboratory evidence, OR
• Meets clinical criteria AND meets epidemiologic linkage criteria.

Confirmed

• Meets confirmatory laboratory evidence.

• Leptospirosis (Leptospira interrogans) | 2013 Case Definition
• Leptospirosis (Leptospira interrogans) | 1997 Case Definition