Publication of research findings into probable zoonotic leprosy in the southern United States in the New England Journal of Medicine on 28th April 2011 has attracted press coverage around the world, much of which could be confusing. To gain a clearer understanding of that study, members of the ILEP Technical Commission were asked to provide a commentary.  

The paper by Truman et al is an important contribution to our knowledge of the epidemiology of leprosy. It is clearly written and contains a wealth of information, attractively displayed – for example, there is a map of the geographical distribution of leprosy in the United States since 1894.

The paper presents data from DNA sequencing to show how different strains of leprosy are found in different patients, and that these relate to how the person became infected. Patients who have lived overseas in an endemic area tend to have a strain associated with that locality. A puzzling fact that has been known for some time is that a significant number of new leprosy cases diagnosed in the United States have never been outside the country – often this is about one third of patients diagnosed in any year. The study now being reported shows that almost all of these cases were infected with the exact strain that is found in the wild armadillo population in the southern States.

Another interesting chart in the paper is a ‘family tree’ showing how the different strains of M leprae are related to each other, clearly showing how the strain in armadillos (and in most people infected within the US) is a very distinct branch, not found anywhere else.

This is strong evidence that some people are infected with leprosy by armadillos, but the exact mechanism of disease transmission remains obscure. Only half of new cases who were questioned about it said they had had any direct contact with armadillos.

There are several questions that remain and a number of challenges going forward.  It is not yet clear how important leprosy in armadillos may be in spreading leprosy in Central and South America, where the incidence in humans is much higher, particularly in Brazil. Armadillos only occur in America (North and South) and there is no suggestion that an animal reservoir plays a significant role in leprosy transmission in the rest of the world, but it will be possible to use the same techniques to examine transmission elsewhere in the same detail. This is a beginning and the authors of this paper, amongst others, are helping with this analysis in endemic areas.

A theoretical development from these findings is that a proven animal reservoir makes eradication of a disease more or less impossible. However, this should not deter us from seeking new tools with which to achieve better control of leprosy and to reduce transmission to humans as far as possible.

Dr Paul Saunderson, Medical/Research Director, American Leprosy Missions; member of the ILEP Technical Commission
Professor Cairns Smith, Professor of Public Health, Chair of Section for Population Health of the University of Aberdeen; Chair of the ILEP Technical Commission 

Free abstract of research paper Probable Zoonotic Leprosy in the Southern United States: http://www.nejm.org/doi/pdf/10.1056/NEJMoa1010536   

Categories: Technical information