Came across this today while looking into some TB stuff, an online (open access) paper on the connection between leprosy and TB: Co-infection of Mycobacterium tuberculosis and Mycobacterium leprae in human archaeological samples: a possible explanation for the historical decline of leprosy
Here’s the abstract:
Both leprosy and tuberculosis were prevalent in Europe during the first millennium but thereafter leprosy declined. It is not known why this occurred, but one suggestion is that cross-immunity protected tuberculosis patients from leprosy. To investigate any relationship between the two diseases, selected archaeological samples, dating from the Roman period to the thirteenth century, were examined for both Mycobacterium leprae and Mycobacterium tuberculosis DNA, using PCR. The work was carried out and verified in geographically separate and independent laboratories. Several specimens with palaeopathological signs of leprosy were found to contain DNA from both pathogens, indicating that these diseases coexisted in the past. We suggest that the immunological changes found in multi-bacillary leprosy, in association with the socio-economic impact on those suffering from the disease, led to increased mortality from tuberculosis and therefore to the historical decline in leprosy.
A few years ago some researchers proposed that leprosy was largely eradicated from Europe because of TB infection (here’s that article, I may have even blogged about it at the time). THe two bacteria are related both of the genus Mycobacterium which means they have a cell wall containing mycolic acid, which is a waxy substance that makes it resistant to the usual complement of antibiotics, and can also resist being engulfed by phagocytic cells from the body’ own immune system.
The basic idea of TB pushing out leprosy is that both leprosy and TB co-occurred for some time, but that, due to a slightly greater reproductive rate by TB, it eventually pushed out leprosy. It could do this because TB appears to confer immunity to leprosy. That is, if one becomes infected with the TB bacterium and one develops a successful immune response to it, that immunity to TB also confers immunity to leprosy as well.
This paper reaches a somewhat different conclusion. They think that instead of cross-immunity, those who contracted leprosy in the first place became more susceptible to TB infections, either by acquiring new infections or reactivating dormant infections. Since TB kills more quickly, those with leprosy would succumb more quickly than they otherwise would have, and therefore be unable to spread the disease around.
I don’t have any deep thoughts on this to share, I’ve only just started poking around this topic. I tend to favor the cross-immunity theory, if only because it makes more sense to me quantitatively: the TB death rate is only about 50% when left untreated, and the paper only cites numbers of around 20ish% of coinfections documented historically (though one would assume a higher death rate among those already weakened by leprosy). That seems to me more likely to reduce but not eliminate the disease, whereas we see it was almost completely eradicated.
So, I dunno. We have a TB unit here that I may try to hook up with and maybe go down that road. Kind of a fascinating topic since it (TB and leprosy) have both fascinating histories archaeologically and are current threats as well.