Mycobacterium leprae

Background

About 1 million cases worldwide each year, but is rare in North America
- Number may be underestimated due to difficulties with reliable diagnosis
Most commonly occurs in Southeast Asia (especially India) and Brazil
Decreasing incidence over the past several decades, likely due to short-course multidrug therapy starting in 1982
Humans are thought to be the main reservoir, but it has been found in animals as well (particularly armadillos)
Transmitted most likely by respiratory droplets, though can also be transmitted by direct contact, transplacentally, through breast milk, and after animal exposure

Age, with peaks in adolescence and ≥30 years
Adult men (compared to adult women)
Duration of contact with an infected patient, and the burden of bacilli in the patient

The clinical spectrum of disease depends on the host immune response to infection
A robust Th1 CD4 response causes tuberculoid leprosy, which is paucibacillary and well-controlled
- Requires interferon-γ
A Th2 CD4 response causes lepromatous leprosy, which is multibacillary and more progressive

Following exposure, about 95% clear it spontaneously
For those who do not, there is an incubation period of 3-5 years (with wide range) that is usually followed by indeterminate leprosy
- Single, ill-defined, hypopigmented skin lesion
- About 75% sponetaneously resolve, with the other 25% progressing
Classic presentation is anaesthetic hypopigmented skin lesion with thickened nerves

Clinical spectrum can be classified based on the number of lesions and burden of mycobacteria
- Paucibacillary (PB) disease has 1 to 5 skin lesions, without bacilli on skin slit smear
- Multibacillary (MB) disease has more than 5 skin lesions, with or without nerve involvement or bacilli on slit-skin smear (regardless of number of lesions)
Can also be classified based on general clinical appearance
- Tuberculoid leprosy (TT) corresponds to paucibacillary
- Borderline tuberculoid leprosy (BT)
- Borderline leprosy (BB)
- Borerdline lepromatous leprosy (BL)
- Lepromatous leprosy (LL) corresponding to multibacillary disease

A cell-mediated hypersensitivity reaction that can develop in the course of treatment
Also known as a reversal reaction due to the apparent worsening of the lesion
Occurs most commonly in the borderline cases and may signal progression to the cell-mediated tuberculoid end of the clinical spectrum

A humorally-mediated hypersensitivity reaction that can develop in the course of treatment
Also known as erythema nodosum leprosum
Characterized by systemic illness and immune-complex deposition that appears as groups of tender subcutaneous nodules
May have other signs of vasculitis, including fevers, arthralgias, neuralgia, lymphadenopathy, orchitis, and dactylitis

Disease	Treatment
Paucibacillary	6 months of rifampin, dapsone, and clofazimine
Multibacillary	12 months of rifampin, dapsone, and clofazimine
Rifampin resistance	6 months of at least two second-line drugs with clofazimine, followed by 18 months of one second-line drug with clofazimine
Quinolone resistance	As for rifampin resistance, but without a fluoroquinolone

Disease	Treatment
Tuberculoid (TT & BT) (i.e. paucibacillary)	12 months of dapsone and rifampicin
Lepromatous (LL, BL, and BB) (i.e. multibacillary)	24 months of dapsone, rifampicin, and clofazimine

Reaction	Management
Reversal reaction limited to skin	monitor clinically
Reversal reaction involving nerves	corticosteroids with slow taper, and rifampin changed to monthly from daily
Other reversal reaction	corticosteroids based on clinical judgment
Erythema nodosum leprosum with neuritis	prednisone 1 mg/kg/day (40 to 60 mg/day), tapered over 2 to 4 weeks, possibly with thalidomide or clofazimine as a steroid-sparing agent
Mild erythema nodosum leprosum	thalidomide 50 to 100 mg nightly