Epidemiology and Infection

Environmental mycobacteria in northern Malawi: implications for the epidemiology of tuberculosis and leprosy

P. E. M.  FINE  a1 c1, S.  FLOYD  a1, J. L.  STANFORD  a2, P.  NKHOSA  a3, A.  KASUNGA  a3, S.  CHAGULUKA  a3, D. K.  WARNDORFF  a1 a3, P. A.  JENKINS  a4, M.  YATES  a5 and J. M.  PONNIGHAUS  a3 a6
a1 Department of Infectious and Tropical Diseases, London School of Hygiene and Tropical Medicine, Keppel Street, London WC1E 7HT
a2 Department of Medical Microbiology, Royal Free and University College Medical School, Windeyer Institute of Medical Sciences, 46 Cleveland Street, London W1P 6DB
a3 Karonga Prevention Study, PO Box 46, Chilumba, Karonga District, Malawi
a4 Bryn Glas, Llangcitho, nr Tregaron, Cardiganshire SY25 6TR, UK
a5 Mycobacterial Reference Laboratory, Kings College Hospital (Dulwich), East Dulwich Grove, London SE22 8QF
a6 Pappelweg 08548 Syrau Germany


More than 36000 individuals living in rural Malawi were skin tested with antigens derived from 12 different species of environmental mycobacteria. Most were simultaneously tested with RT23 tuberculin, and all were followed up for both tuberculosis and leprosy incidence. Skin test results indicated widespread sensitivity to the environmental antigens, in particular to Mycobacterium scrofulaceum, M. intracellulare and one strain of M. fortuitum. Individuals with evidence of exposure to ‘fast growers’ (i.e. with induration to antigens from fast growers which exceeded their sensitivity to tuberculin), but not those exposed to ‘slow growers’, were at reduced risk of contracting both tuberculosis and leprosy, compared to individuals whose indurations to the environmental antigen were less than that to tuberculin. This evidence for cross protection from natural exposure to certain environmental mycobacteria may explain geographic distributions of mycobacterial disease and has important implications for the mechanisms and measurement of protection by mycobacterial vaccines.

(Accepted November 29 2000)

c1 Author for correspondence.