Leprosy (Hansen's Disease)
2025 Case Definition

Background

Leprosy, or Hansen’s disease (HD) is a chronic bacterial disease of the skin and peripheral nerves caused by bacteria in the Mycobacterium leprae complex, comprising M. leprae and M. lepromatosis. Routes of transmission are uncertain but believed to be by respiratory secretions through close, prolonged contact with untreated patients with leprosy or from prolonged or frequent direct contact with infected armadillos or their environment ^1,2. During 2013–2022, 124 to 216 cases per year were reported in the United States (U.S.) ³, most in people with exposure outside the U.S., although endemic leprosy is found in some states. Leprosy can be cured with early diagnosis and treatment*; in the U.S., case detection, treatment, and contact management have been major control strategies. Ongoing public health surveillance is needed to facilitate case detection and control efforts, which might include post-exposure prophylaxis ⁴, typically arranged in coordination with public health agencies. Current surveillance case definitions do not include leprosy cases with rare presentations; this may inhibit implementation of control strategies for those cases.

* Note that leprosy cases may be classified into types for treatment purposes using Ridley Jopling or WHO classifications ⁵. These Ridley Jopling and WHO classifications are not surveillance case definitions.

Clinical Criteria

A clinically compatible illness characterized by:

• Any of the following skin lesions
  • an ill-defined hypopigmented or erythematous macule or patch
  • a few well-demarcated, hypopigmented or erythematous skin lesions with reduced sensation
  • multiple diffuse erythematous papules and nodules on arms and legs, sparing the torso
  • an infiltration of skin, progressing to thickened skin, possibly with reduced sensation
  • diffuse infiltration of the skin and neuropathy (e.g., "glove and stocking") (representing diffuse leprosy)

OR 

• The absence of skin lesions and thickening of a peripheral nerve trunk with pain or tenderness of the nerve (representing primary neural leprosy).

Laboratory Criteria

Confirmatory Laboratory Evidence:

• Detection of acid-fast bacilli in a nerve by the Fite-Faraco method, OR
• Detection of acid-fast bacilli in skin by the Fite-Faraco method, without growth of mycobacteria on culture** (if done), OR
• Detection of M. leprae or M. lepromatosis in skin or a nerve by a nucleic acid detection test.***

Supportive Laboratory Evidence:

• Detection of non-sarcoid non-caseating granuloma with peripheral nerve involvement, without growth of mycobacteria on culture**** (if done)

** 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.

*** If acid-fast bacilli are detected in skin only, mycobacterial culture negativity is highly recommended to rule out infection with mycobacteria other than those in the M. leprae complex. To rule out M. haemophilum, hemin or iron-citrate containing medium would be needed. To rule out M. xenopi or M. marinum, incubation at 42 and 30 degrees centigrade, respectively, would be needed.

**** Note that a negative nucleic acid test on a tissue specimen does not rule out Mycobacterium leprae or Mycobacterium lepromatosis as the cause of illness.

Epidemiologic Linkage

• Prolonged close contact ^6,7 with an untreated person with new or recurring leprosy, OR
• Residency or repeated travel in a region with higher endemicity (prevalence >1 case per 10,000 population or new case detection rate ≥ 50 per million population per year) for leprosy ⁸, OR
• Prolonged or frequent, direct contact† with armadillos, especially nine-banded armadillos, or soil in the environment in which they live.

† Prolonged or frequent direct contact refers to activities such as raising, maintaining, butchering, hunting, field dressing, or consuming armadillos. It does not refer to brief, cursory, or sporadic touching such as might occur with a visitor to a petting zoo.

Criteria to Distinguish a New Case from an Existing Case

For surveillance purposes, a new case of leprosy should be enumerated by public health based on the following criteria:

• A person should be enumerated as a case if not previously enumerated as a case, OR
• A person was previously enumerated as a case, followed by adequate treatment with current, standard, multidrug therapeutic regimen and newly meets the criteria for a confirmed or probable case, OR
• A person was previously enumerated as a case, but genetic sequencing results are distinctly different in a new positive specimen from a previous positive specimen, OR
• A person was previously enumerated as a case, but the M. leprae complex species identified (e.g., M. leprae vs. M. lepromatosis) in a new positive specimen is different than identified in a previous specimen in the same person.

Suspect

• Meets clinical criteria for a clinically compatible illness with skin lesions AND meets epidemiologic linkage criteria, OR
• Meets clinical criteria for a clinically compatible illness with skin lesions AND meets supportive laboratory evidence.

Probable

• Meets clinical criteria for primary neural leprosy AND meets epidemiologic linkage criteria.

Confirmed

• Meets clinical criteria AND confirmatory laboratory evidence.

• Hansen's Disease / Leprosy (Mycobacterium leprae) | 2013 Case Definition
• Hansen's Disease / Leprosy (Mycobacterium leprae) | 1997 Case Definition