Mycobacterium leprae, in a case of Hansen disease – Lepromatous type

Femela M

Consultant – Pathology, Radial road, Kovilambakkam, Chennai

Introduction

Leprosy, also known as Hansen disease, is a chronic infectious disease, caused by a type of bacteria, Mycobacterium leprae. The disease predominantly affects the skin and peripheral nerves, the mucosa of the upper respiratory tract and the eyes. Left untreated, the disease may cause progressive and permanent disabilities. The bacteria are transmitted via droplets from the nose and mouth during close and frequent contact with untreated cases. Leprosy is curable with multidrug therapy (MDT). Leprosy is reported from all the six WHO regions; the majority of annual new case detections are from South-East Asia Region.1

M.leprae is an obligate intracellular organism. M. leprae parasitizes histiocytes (skin macrophages) and Schwann cells in the peripheral nerves. The WHO has designated leprosy as a neglected tropical disease (NTD). The Global Leprosy Strategy for the years 2021–2030 entitled “Towards zero leprosy” has begun. Leprosy has been classified into five types using the Ridley-Jopling classification: tuberculoid (TT), borderline tuberculoid (BT), mid-borderline (BB), borderline lepromatous (BL) and lepromatous (LL). Due to its higher lipid content, M. leprae does not become discoloured by acid-alcohol with the Ziehl-Neelsen stain, a red stain that contains fuchsin. Thus, it appears as characteristic acid-alcohol-resistant bacilli. A large number of bacilli are observed within foamy histiocytes with LL lesions in Fite-Faraco stain. The M. leprae cell wall includes more mycolic acid than that of M. tuberculosis (the ratio of mycolic acid to PGN is 21:10 versus 16:10).2

Fig (1): Microphotograph showing lepra bacilli (Mycobacterium leprae) in a case of Hansen disease – Lepromatous type (Fite-Faraco stain, ×1000)

In lepromatous leprosy, macrophages may be distended with large groups of lepra bacilli (globi); bacteria are present in large numbers in cutaneous nerves and in endothelium and media of small and large vessels.

The BI is an index of the bacillary load in the patient. This is expressed on a semi-logarithmic scale as given below.

BI Interpretation

1+ 1 to 10 bacilli per 100 high power (oil immersion) fields

2+ 1 to 10 bacilli per 10 high power fields

3+ 1 to 10 bacilli per high power field

4+ 10 to 100 bacilli per high power field

5+ 100 to 1000 bacilli per high power field

6+ >1000 bacilli per high power field

Reference may be made to the term ‘globi’ in some reports. These are clumps of bacilli and are generally found in LL.3

References

Kauverian Bookshelf

Applied Medical Statistics

Journals

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