Leprosy is a fascinating disease for many reasons. Historical, because, well, it’s leprosy. Genetic, because the bacterium is apparently derived from a single clone that infected humans some 4000 years ago,1 and that has undergone “massive gene decay” in the process of becoming an obligate pathogen:
Thus, since diverging from the last common mycobacterial ancestor, the leprosy bacillus may have lost more than 2,000 genes. 2
Immunological, because as mycobacteria, leprosy and tuberculosis may have an entire branch of the immune system dedicated to their control and destruction. Epidemiological, because leprosy is one of the very few diseases that has the potential for elimination without vaccines. And now let’s add phylogeography and anthropology to the list, with a paper that offers a detailed analysis of leprosy’s migration through humanity. 3
This was done by genetic analysis, tracking through sub-types of leprosy in various areas, both modern and ancient — the latter being “obtained from leprosy graveyards in Croatia, Denmark, Egypt, England, Hungary and Turkey“, and allowing the authors to determine the strains of leprosy that circulated as much as 1500 years ago. Their conclusions (building on and extending earlier work):
- The progenitor of leprosy arose in East Africa
- New strains then spread into Asia, through two different routes: One northern route, and one southern
- The Southern route into Asia was probably the Silk Road: “the trade route between Europe and Asia known as the Silk Road appears likely to have been a means of transport and disease transmission“. They point out that this is the opposite path of the Black Plague, which likely spread from Asia to Europe along the Silk Road.
- Another strain of leprosy moved from East Africa westward into the Middle East and Europe
- This strain in turn spawned strains that are found in West Africa and countries linked to West Africa by the slave trade. (Compare to the phylogeography of hepatitis C, among other diseases spread by slavery)
- Leprosy in North America came from relatively recent European immigrants, rather than coming along with the original Bering Strait peoples.
|“Pillars are located on the country of origin of the M. leprae sample … The gray arrows indicate the migration routes of humans, with the estimated time of migration in years shown. The red dots indicate the location of the Silk Road in the first century.” 3 (Click for a larger version)|
One interesting conclusion is that the genome decay of M. leprae is much older than humans (occurring over a million years ago, whereas humans are only a few hundred thousand years old), even though the genetic evidence says the present bacteria were clonal just a few thousand years ago. They suggest that
Alternatively, the genome decay could well be ancient, but M. leprae may only recently have become a human pathogen. For instance, it is conceivable that an ancestral form of M. leprae infected an invertebrate host such as an insect, which later acted as a vector for transmitting the bacillus to humans. 3
- Monot, M. (2005). On the Origin of Leprosy Science, 308 (5724), 1040-1042 DOI: 10.1126/science/1109759[↩]
- Cole, S., Eiglmeier, K., Parkhill, J., James, K., Thomson, N., Wheeler, P., Honoré, N., Garnier, T., Churcher, C., Harris, D., Mungall, K., Basham, D., Brown, D., Chillingworth, T., Connor, R., Davies, R., Devlin, K., Duthoy, S., Feltwell, T., Fraser, A., Hamlin, N., Holroyd, S., Hornsby, T., Jagels, K., Lacroix, C., Maclean, J., Moule, S., Murphy, L., Oliver, K., Quail, M., Rajandream, M., Rutherford, K., Rutter, S., Seeger, K., Simon, S., Simmonds, M., Skelton, J., Squares, R., Squares, S., Stevens, K., Taylor, K., Whitehead, S., Woodward, J., & Barrell, B. (2001). Massive gene decay in the leprosy bacillus Nature, 409 (6823), 1007-1011 DOI: 10.1038/35059006[↩]
- Monot, M., Honoré, N., Garnier, T., Zidane, N., Sherafi, D., Paniz-Mondolfi, A., Matsuoka, M., Taylor, G., Donoghue, H., Bouwman, A., Mays, S., Watson, C., Lockwood, D., Khamispour, A., Dowlati, Y., Jianping, S., Rea, T., Vera-Cabrera, L., Stefani, M., Banu, S., Macdonald, M., Sapkota, B., Spencer, J., Thomas, J., Harshman, K., Singh, P., Busso, P., Gattiker, A., Rougemont, J., Brennan, P., & Cole, S. (2009). Comparative genomic and phylogeographic analysis of Mycobacterium leprae Nature Genetics, 41 (12), 1282-1289 DOI: 10.1038/ng.477 [↩][↩][↩]