**2.** *Mycobacterium ulcerans* **infection**

The time of infection or exposure to *M. ulcerans* to showing clinical symptoms of BU ranges from 1 week to 9 months [44]. The median incubation period is 4.5 months [44]. Non-ulcerative and ulcerative are the two main forms of the disease. The non-ulcerative form manifests as either painless nodule, swelling, plaque or oedema [45].

Opened necrotic lesion on the skin describes the ulcerative form and is categorised into 3 forms. Category 1 ulcers are single lesion on the skin and 5 cm in diameter [46]. Ulcers with size 5 to 15 cm is characterised as category 2 [46]. Multiples ulcers on different part of the body and/or ulcers greater than 15 cm is clinically described as category 3 [46]. African cohort usually report category 3 and 2 ulcers whereas category 1 are common in Australia [46]. This has been attributed to differences in geographical strains and virulence factor of *M. ulcerans* [47]. Access to adequate medical services in rural endemic regions in Africa and late presentation of condition to medical centres is another reason [30, 48].

Rook *et al*., described 3 phases of immune response to *M. ulcerans* in murine model in 1975 [49]. The first phase involves leucocyte migration and delayed

hypersensitivity response at the site of infection [49]. This response is usually cellmediated and involves proliferation of T-cells, monocytes and macrophages. In the next phase, as *M. ulcerans* cells multiple at the site of infection, there is subsequent reduction in the migration of inflammatory biomarkers at this site [49]. Though, there appears to be reactive cells in the lymph nodes [49]. The last phase is marked by depletion of T-cells [49]. This is attributed to excessive exposure to the bacillus toxins (mycolactone) [49]. Studies in tuberculosis and BU patients also confirms that individuals could have a negative skin-test while their lymph tissues is filled with enormous responsive cells [50, 51]. These 3 phases describes the phenomena of "sensitization" and "desensitisation" in mycobacteria infection. The latter being the non-appearances of cell-mediated response to a previously encountered antigen [49, 52].
