Global Leprosy Programme

Basic information on leprosy

Leprosy is a chronic infectious disease caused by Mycobacterium leprae. It usually affects the skin and peripheral nerves, but has a wide range of clinical manifestations.

Classification of leprosy cases

Leprosy is a classic "spectral disease", being manifest in a variety of clinical forms related to the type and strength of the immune response. A strong cellular immune response is effective in curtailing the multiplication of M. leprae and is thus associated with PB disease. A weak cellular response allows bacilli to replicate freely in the body, leading to MB disease forms. (WHO Expert Committee on Leprosy: Eighth report)

A simple clinical rule is now used to divide patients into these two groups. The number of individual skin lesions is counted (this means that the whole body must be examined, including more private parts, to make an accurate count):

  • PB cases have up to five skin lesions in total.
  • MB cases have six or more skin lesions.

If a skin smear is done and is positive, the patient must be classified as MB, whatever the number of skin lesions. If the smear is negative, the classification is decided by the number of skin lesions. Other factors like nerve involvement may be considered at the referral level for classifying the disease.

The risk of nerve damage is greater in MB patients. Therefore, classification is helpful in assessing future risk (section 6.1) and in guiding patient care.

Disability due to leprosy

Among communicable diseases, leprosy is a leading cause of permanent physical disabilities. Timely diagnosis and treatment of cases, before nerve damage has occurred, is the most effective way of preventing disabilities due to leprosy; effective management of leprosy complications, including reactions and neuritis, can prevent or minimize the development of further disabilities. The disease and its associated deformities are responsible for social stigma and discrimination against patients and their families in many societies.