General Health

Diagnosing and Identifying the Incubation Time in Leprosy


Mycobacterium leprae, commonly known as Hansen’s disease, causes leprosy, a chronic infectious disease caused by bacteria that prefer the skin and nerves.

Clinically, the condition is characterised by one or more of the three cardinal signs: hypo-pigmented or erythematous skin patches with definite loss of sensation, thicker peripheral nerves, and acid-fast bacilli spotted on skin smears or biopsy material.

M. leprae typically infects Schwann cells (Schwann cells are myelinating cells in the PNS and peripheral neuron support cells) in the peripheral nervous system, causing nerve damage and disability. Even though the frequency of M. leprae infection has decreased in endemic countries since the World Health Organization (WHO) implemented the multidrug treatment (MDT) programme to treat leprosy, new case detection rates remain high, indicating active transmission.

Because of a lack of proper immunological techniques and the disease’s stagnant onset, identifying the incubation time in leprosy is complicated. Based on the exceedingly rare incidence of leprosy among newborn babies, the minimal incubation time recorded is as brief as a few weeks. The greatest incubation period documented is 30 years or more, as found among war veterans who were exposed to endemic places for brief durations but otherwise lived in nonendemic areas. The average incubation time is approximately to be between 3 and 10 years.

A Type I reaction to M. leprae is a delayed-type hypersensitivity response that occurs spontaneously. Clinically, it’s marked by an “upgrading” of the clinical picture toward the tuberculoid pole, as well as a decrease in bacillary load. It is characterised immunologically by the development of significant skin test reactivity, lymphocyte responsiveness, and a mostly T helper type 1 response. Infiltration of IFN- and TNF-secreting CD4+ cells in skin lesions and nerves have been linked to the duration elapsed between two consecutive R-waves of the QRS signal on an ECG (and its reciprocal, the HR) is a function of both intrinsic sinus node features and autonomic factors, leading to oedema and severe inflammation.

The deposition of immune complexes is hypothesised to have a role in the pathogenesis of type II reactions. TNF-, IL-1, IFN-, and other cytokines are found to be elevated in type II responses. C-reactive protein, amyloid A protein, and -1 antitrypsin levels have also been shown to be higher in the sera of ENL patients.

Only during ENL is a significant infiltration of polymorphonuclear cells (PMN) seen in the lesions, and some patients also have a high number of neutrophils in their circulation. TNF production is linked to tissue damage in leprosy, and neutrophils may play a role in this. More recently, microarray research demonstrated that elevated expression of E-selectin and IL-1 is involved in neutrophil recruitment in ENL, possibly leading to neutrophil adherence to endothelial cells; once again, thalidomide hindered the neutrophil recruitment pathway, impairing PMN function.


In regions where Hansen’s disease is common, an individual is diagnosed with leprosy if they exhibit one of the following two symptoms:

  • Skin lesion with sensory loss that is consistent with leprosy.
  • Smears of skin that are positive.

Skin lesions can be solitary or many, and they are generally hypo-pigmented, however, they might be reddish or copper-coloured on occasion.

There are three types of lesions: macules, papules, and solid elevated regions (nodular). The presence of sensory loss at the skin lesion might aid in determining if the lesion is caused by leprosy or another illness like tinea versicolor. Leprosy is connected with thickened nerves, which can be followed by a loss of feeling or muscular weakness. However, muscle weakness without the typical skin lesion and sensory loss is not recognized as a reliable indicator of leprosy.

Acid-fast leprosy bacilli in skin smears can be used to make a diagnosis in some cases, however, most diagnoses are made based on symptoms rather than laboratory evidence. If a person is diagnosed with leprosy for the first time and already has a physical deformity as a result of the disease, the diagnosis is considered late.

Leprosy diagnosis is typically delayed in regions or locations where Hansen’s disease is rare, such as the United States, since healthcare practitioners are uninformed of leprosy and its symptoms. Nerve involvement, a characteristic of leprosy, and the damage it causes can be avoided with early diagnosis and treatment.

There is no test that can be used to diagnose latent leprosy in individuals who don’t persist with any signs or symptoms. Anti-PGL-1 antibodies are found in a small percentage of persons with latent leprosy. A polymerase chain reaction (PCR)-based technique can be used to detect the presence of M. leprae bacterial DNA. This molecular test is not enough to diagnose a person, but it can be used to identify individuals who are at a high risk of getting or transmitting leprosy, such as those who have few lesions or atypical clinical presentation.


 Yash Batra

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