Oldest infectious disease of humans

D. Balasubramanian

Source - http://www.thehindu.com/sci-tech/health/oldest-infectious-disease-of-humans/article5167806.ece


Mycobacterium tuberculosis (MTB) may well be the oldest pathogen to have infected humankind.  AP

Modern humans (or homo sapiens) emerged out of the “hominid” group almost two million years ago, and began wandering out of Africa about 70,000 years ago to populate the world. How healthy were these people? What kind of illnesses affected them? Do we carry these afflictions to this date?

Questions such as these form the main research themes for a group of scientists who call themselves paleopathologists — paleo for ancient and pathology to define and describe the kind of illness. One such paleopathologist, Dr. Charlotte Roberts of Durham University, U.K. has written the book “The Archaeology of Disease”, where she argues that analysis of the DNA found in ancient human samples would help in understanding the origin and history of diseases that have affected us since antiquity.

Dr. Garth Sundem writes in his lucid essay “10 oldest known diseases” that in such studies, one should distinguish between diseases caused by external agencies (addiction, poisoning, infection) and age-dependent bodily dysfunctions (arthritis, epilepsy and such “conditions”) which are innate natural process of systemic malfunctions. The clue to zone in on the most ancient infection comes from both a study of bone abnormalities (seen in excavated bodies and mummies) and from analysis of all the DNA present in them. He points out that, contrary to the oft-quoted statement, dead men do tell tales.

Such a double analysis, plus information contained in ancient texts from across the world suggest the presence of ten diseases to be among the oldest to affect mankind. These are: tuberculosis (or TB), leprosy, cholera, smallpox, rabies, malaria, pneumonia, trachoma (chronic infection of the eyelid), influenza, measles and the black plaque. This list has been compiled by analysing information available from ancient texts and books such as the Vedas, the Bible, Greek history, oriental texts and oral history. The Rigveda (about 1500 BC) refers to TB and leprosy, the Egyptian “Ebers papyrus” (about 1500 BC) mentions leprosy, Thucydides of Greece (430 BC) mentions the plague, the Bible (Leviticcus 13.2) talks about leprosy and the Romans describe malaria. Aboriginal skeletons (800 BC) have shown skull lesions around the eyes, later suggested by circumstantial evidence as due to trachoma.

Sundem also refers to the analysis in Israel of the fossilized bones of a mother and child (estimated to be about 9000 years old) revealing the infection as due to TB, and also to a Turkish sample even older (50,000 years old!) again with the suggestion of TB affliction. It would thus seem that mycobacterium tuberculosis (MTB) may well be the oldest pathogen to have infected humankind.

MTB comes not as single strain but there are as many as 259 varieties that we know of today. Yet, DNA analysis of these strains has revealed not a great deal of diversity or heterogeneity, but very few mutations and nearly identical DNA sequences. Earlier work on such low level genetic variation, studied in 2005 by Dr. Veronique Vincent and colleagues at the Pasteur Institute, Paris, suggests that the present-day bacterium originated form a precursor or progenitor species — call it mycobacterium proto-tuberculosis, which could be as old as 3 million years. And the question is — when did this divergence from the single ancestor progenitor occur, how closely related in their DNA these 250 strains are and how sensitive or resistant each set of these strains is towards anti-tubercular drugs that we have today.

It is here that the recent paper by an international group led by Dr. Sebastien Gagneux of the Swiss Tropical and Public Health Institute, published in the 1 September 2013 issue of Nature Genetics is of value. The group analysed the DNA sequences of 259 TB strains from around the world, and showed that genetic diversity arose in them roughly around 70,000 years ago, concurrent with the outward migration from Africa of anatomically modern humans. When interviewed, Dr Gagneux pointed out that “the evolutionary path of humans and the TB bacteria show striking similarity. We see that diversity of MTB has increased markedly when human population expanded.”

In other words, what was dormant and restricted largely to the Rift Valley of Africa, where our far remote ancestors lived, became active and diverse as they started living in communities and passed on infection from person to person. And contrary to conventional wisdom, rather than getting infected from domesticated animals, we may even have passed on the TB germ to our pet animals.