On the origins of smallpox - where and when did variola virus emerge?

2011 may be the year where the last known officially acknowledged stocks of the deadly smallpox virus, variola are destroyed - a virus that claimed over 500 million lives in the 20th century alone. The extensive collection of 'live' virus and DNA stocks totalling over 500 isolates/strains, which are held between the US Centres for Disease Control and the Russian State Research Centre of Virology and Biotechnology may be ordered to be eliminated following World Health Organisation (WHO) recommendations soon to be announced.

Although the impending fate of this pathogen has been covered elsewhere by Vincent Racaniello and Steven Salzberg I have been led to ponder its beginnings, at least in humans: where and when, over the course of human history did variola virus emerge  and have we always suffered from it? What confuses the matter further is that there are two clinical forms of smallpox - major (30% mortality) and minor, including both African and Alastrim minor (<1% mortality)- do these viruses have the same evolutionary history and if so, when and where did they diverge? Luckily, we can now study the origins of infectious diseases through both molecular and historical records.

A depiction of Shapona the west-African Yoruba god of smallpox. Courtesy James Gathany (photo), CDC/ Global Health Odyssey.

Conflicting historical records

It has been very confusing trying to make sense of the historical records of suspected smallpox cases as there are significant gaps in documentation and many conflicting reports. Smallpox-like skin lesions have been observed on Egyptian mummies dating from as far back as 1580 B.C yet there is no mention of the disease at all in the Old or New testaments nor even the Hippocratic texts. There was some mention of a smallpox-like disease in China and India as early as 1500 B.C but the only unmistakable description can be found from the 4th century A.D in China.  Interestingly there was no mention of smallpox in the American continents nor in sub-Saharan Africa prior to European exploration. But as shown in the picture above, smallpox has shaped west-African culture. So, did smallpox originate in Asia and spread to Egypt around 1,500 B.C? Or, is smallpox a relatively recent human disease, emerging around the 4th century A.D in Asia?

Molecular data shed light on variola evolution

A 2007 study using genomic data from the CDC's variola collections - the same ones that may soon be destroyed, added a phylogenetic perspective to the origins of smallpox and how it spread worldwide. Through studying single nucleotide polymorphisms (SNPs) from 47 variola genome isolates from geographically distant areas and collected between the 1940s and '60s they examined the genetic relatedness between isolates and were able to estimate the time since they shared a last common ancestor. They combined this DNA evidence with the above historical records to generate an idea as to where, when and how smallpox originated and spread throughout human populations.

Variola genome phylogeny

Abstract: Human disease likely attributable to variola virus (VARV), the etiologic agent of smallpox, has been reported in human populations for >2,000 years. VARV is unique among orthopoxviruses in that it is an exclusively human pathogen. Because VARV has a large, slowly evolving DNA genome, we were able to construct a robust phylogeny of VARV by analyzing concatenated single nucleotide polymorphisms (SNPs) from genome sequences of 47 VARV isolates with broad geographic distributions. Our results show two primary VARV clades, which likely diverged from an ancestral African rodent-borne variola-like virus either ≈16,000 or ≈68,000 years before present (YBP), depending on which historical records (East Asian or African) are used to calibrate the molecular clock. One primary clade was represented by the Asian VARV major strains, the more clinically severe form of smallpox, which spread from Asia either 400 or 1,600 YBP. Another primary clade included both alastrim minor, a phenotypically mild smallpox described from the American continents, and isolates from West Africa. This clade diverged from an ancestral VARV either 1,400 or 6,300 YBP, and then further diverged into two subclades at least 800 YBP. All of these analyses indicate that the divergence of alastrim and variola major occurred earlier than previously believed.

Hypothesised spread of variola worldwide

When analysed, variola fell into two large monophyletic clades signifying a historical divide in their genetic relatedness. The earliest representative - or most basal - of the variola major smallpox viruses are the Asian isolates. This suggests that major may have originated in Asia followed by geographic radiation across the Old world and into Africa. Using historical records as a means to calibrate variola evolutionary history, their results indicated that smallpox spread from Asia as much as 1,600 years ago which neatly backed up the historical records of 4th Century China. By the time smallpox reached out of East-Asia, the ancient Greek and Roman civilisations were no more - hinting that the reason they didn't observe smallpox was because at that time, in the Mediterranean region there wasn't any variola virus transmission. Despite this, analysis of the second major clade suggested a split 6,300 years ago placing variola well into ancient history. So, is smallpox a very old or relatively recent human pathogen? And, if so, where did it occur? The molecular data also showed that the clinically 'minor' forms of smallpox - African minor and Alastrim minor are very much related to the major viruses; evolutionarily speaking these viruses are thus very smilier.

A rodent origin of smallpox?

We can investigate the origin of smallpox through the molecular characterisation of other poxviruses. Variolataterapox virus) and camelpox viruses and they all are more related to each other than to other poxviruses, such as monkeypox. When their genomes were compared to that of variola, a time since divergence was estimated at between 16,000 and 68,000 years ago. As taterapox and camelpox are primarily found throughout Africa and Asia this suggests a possible origin of variola and the other poxviruses from ancient endemic poxviruses in Africa, possibly from rodents. Upon human infection this virus may have followed us out of Africa entering Asia and spreading across the globe. Or possibly, the virus emerged in rodent populations only to pop up again in Asia thousands of years later.

Although this period is quite a bit before the development of human agriculture and increased population density as is possibly required for such a highly infectious and lethal virus like smallpox to persist, the ancient variola virus might have behaved very differently from the one we know and fear. Sadly, we cannot say for sure exactly how and where variola emerged because we simply do not know a lot about the natural diversity of poxviruses in rodents or other mammal species and until we do, we will not have an accurate answer.

Future Pox?

Monkeypox from bushmeat? www.lynnjohnsonphoto.com/

What does this history of smallpox say about its impending eradication and the threat of a future virus emergence? Well, sometime in our ancient past variola virus emerged into prehistoric human populations and the data indicate that this may well have occurred within the African continent or nearby in Asia and it is likely that this same ancestral virus emerged into other mammalian species, such as Camels and Rodents. Smallpox then might have followed us through our journey out of Africa and became endemic in large population centres across the Eurasian landmass.

Although we have now effectively eradicated variola from the human population - and soon seek to destroy its last remaining stocks - might another poxvirus emerge just like it did before? We are now in a situation where the human population has very little immunity to variola and other related poxviruses, a situation which last would have existed prior to the initial emergence of smallpox. This provides ample breeding ground for novel poxviruses to emerge and fill the niche emptied by waning population immunity. Alarmingly, the rate of monkeypox infections - another rodent poxvirus - has been increasing (20X)  in the last 3 decades following cessation of smallpox vaccination. May this, or another related poxvirus, be the new smallpox? Could existing smallpox stocks not be used to study poxvirus/human interactions? May it be premature to destroy them in light of possible future pandemics? Only the WHO can decide later ths year.

Gubser C, & Smith GL (2002). The sequence of camelpox virus shows it is most closely related to variola virus, the cause of smallpox. The Journal of general virology, 83 (Pt 4), 855-72 PMID: 11907336

Li, Y., Carroll, D., Gardner, S., Walsh, M., Vitalis, E., & Damon, I. (2007). From the Cover: On the origin of smallpox: Correlating variola phylogenics with historical smallpox records Proceedings of the National Academy of Sciences, 104 (40), 15787-15792 DOI: 10.1073/pnas.0609268104

Raymond S. Weinstein (2011). Should Remaining Stockpiles of Smallpox Virus (Variola) Be Destroyed? Emerg Infect Dis, 17 (Apr) : 10.3201/eid1704.101865

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