Plague Bacteria Discovered in 20 Million Year Old Flea

Today we have the discovery of not one but two ancient diseases wrapped up in one tiny amber package with a brand new species of flea thrown in for good measure.

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Fossilised flea in amber from the Dominican Republic. Credit: George Poiner Jr at Oregon State University

The find comes from the amber mines of the Dominican Republic and takes the form of a beautifully preserved fossil flea complete with  a gut-full of pathogens. The flea itself was unusual enough for the team to conclude that it was a totally new species from a totally new genus. Its discoverer named it Atopopsyllus cionus and concluded that it was probably a member of a larger flea family called the Spilopsyllinae. Today this group includes many important animal-living fleas including one which spreads the virus responsible for myxomatosis in rabbits. Finding a new flea is itself very exciting but of course not half as exciting as discovering a new, disease carrying flea. Using a high powered microscope the researchers were able to take a very close look at the tiny organisms intestines and made a fascinating discovery. Stuffed inside the rectum, and also preserved in a tiny blood droplet forever balanced on the fossil’s proboscis, were tiny bacteria, some shaped as rods and some which were near spherical. Of all the pathogenic bacteria that use fleas for their transmission there is one that has this mixture of shapes seen in the amber. That bacteria is Yersinia pestis, better known by the name of disease it causes, bubonic plague.

Today there are many different diseases spread by fleas but arguably the most notorious is bubonic plague. This disease still kills people today despite being treatable with antibiotics, but it is best known of as a Medieval scourge that depopulated much of Europe. Until now though we’d had very little direct evidence of its true antiquity. Genomic studies of the disease had suggested that the modern life cycle we see today evolved around 20,000 years ago but that date now seems far too recent. Instead the discovery of these bacteria associated with a flea that appears to have been feeding on the blood of a larger organism just before it died seems to confirm that this life cycle actually evolved at least 20 million years ago. When the flea was alive that area was a lush tropical rain forest so it would have been spoiled for choice of host. Since many of its modern relatives feed on rodents it seems likely that this flea did as well and probably spread it bacteria to them. Today Y. pestis is a very serious disease for the rodents it infects and often kills them in vast numbers during outbreaks. In fact large numbers of dead rats often acts as a precursor to a human outbreak of plague.

So can we be certain these bacteria are really Y. pestis? The answer, unfortunately, is no. We can’t be absolutely certain because no DNA is preserved but there are some very good reasons to think that they are. Firstly the size and shapes of the bacteria match modern Y. pestis which are cocobacillus bacteria and come in a mix of tiny rods and spheres. More intriguing still is where the bacteria were found. As well as loitering in the midgut there were also numerous bacteria in the blood droplet at the end of the probiscus. When modern fleas take a blood meal from a Y. pestis infected host the bacteria often form a sticky mass that the flea can’t swallow. It can become partially lodged at the base of the oesophagus impeding the animal’s ability to feed. This triggers the flea to try again, feeding on a new host at which point the sticky mass is regurgitated and passes the bacteria into the blood of the unfortunate rodent/Medieval plague victim. This sticky mass in the process of being regurgitated could be what is preserved in the Dominican amber.

Now I did promise you two diseases. We’ve had potential bubonic plague so what else was lurking in the guts of that tiny flea. The answer is trypanosomes. You might not have heard of these before, but they are single-celled protozoa that cause a variety of human diseases. They are parasites, carried by fleas which infect hosts through their saliva. Unlike Y. pestis trypanosomes have a very long fossil history, dating back around 300 million years and today are best known for causing leishmania and sleeping sickness. Finding a population of trypanosomes preserved inside a fossil flea is not quite as sensational as finding bubonic plague but they are equally important disease agents and the more we know about their evolution the better equipped we are to deal with their modern spread.

This is all fascinating stuff. Diseases and parasites have a huge impact on modern ecosystems and doubtless did on ancient ones too. Discovering any evidence of prehistoric disease is important but the discovery of one of the most notorious diseases in human history makes it just that bit more exciting.

 

Reference: Poiner, G.Jr. 2015. A New Genus of Fleas with Associated Microorganisms in Dominican Amber. Journal of Medical Entomology. DOI: http://dx.doi.org/10.1093/jme/tjv134

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