Viability and drug susceptibility testing of M. leprae using mouse footpad in 37 relapse cases of leprosy

Mycobacteria leprae isolates obtained from 37 referral relapse cases of leprosy (37 skin and 10 nerve biopsy samples) received during the years 1994-2001, were tested for viability and drug sensitivity in the mouse footpad. A significant M. leprae yield in the footpads of control mice was obtained, with 32/47 (68%) isolates (from 26 cases) thus confirming viability. Of the 28 isolates successfully drug tested, 6 (21%) were resistant to one or more drugs. All except one, were multidrug treated cases (5/24 = 21%). One of the isolates was resistant to all three drugs, i.e., dapsone (di-aminodiphenyl sulphone, DDS), rifampin (RFP), and clofazimine (CLF). Two were resistant to two drugs, i.e., DDS and RFP, and each of the others were mono resistant to DDS, RFP, or CLF. Notably, one of the isolates that showed combined resistance to DDS and RFP was derived from a borderline tuberculoid case. Also, in one case skin and nerve showed that discordance viz: M. leprae derived from skin were resistant to RFP, while those derived from nerve tested sensitive to all three drugs, indicating tissue related difference.