A common pest(is)

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McMaster researcher Hendrick Poinar and an international team of researchers have uncovered telling secrets from the grave. The research focuses on the recovery of DNA from fossil remains such as teeth from individuals who died from the bubonic plague in Marseille, France. DNA sequences can be used to trace the past and answer questions about the evolution of infectious disease.

“We are very interested in how diseases emerge and then continually re-emerge. We want to know if reservoirs are local or more distant. Are these epidemics repeatedly stemming from trade routes like the Silk Road from China and the Golden Horn from Kazakhstan or are they a localized epidemic?” Poinar explained.

This involves tracing back to Europe in 1346, during the time of the Black Death, when the bacteria Yersinia pestis wiped out a third of the European population.

“That was an epidemic like we have never seen before and hopefully will never see again,” said Poinar.

After an initial flare, the plague seemed to disappear in Europe. What followed its supposed demise is a series of little outbreaks that Poinar labeled as the “shouldering effect.”

“We are very interested in how diseases emerge and the continually re-emerge. We want to know if resevoirs are local or more distant.

“If you look over the course of decades, you see relatively nasty outbreaks. You have basically 400 years of repeated epidemics in Europe, until it disappears. The question has been if these are repeated epidemics that occur every 30 to 100 years, is the source of these epidemics a migration of pathogen from the East to the West.”

By analyzing the global phylogeny, a method of relating disease sequences and strains to each other, researchers observed that rodents are at the start of many diseases that reach human populations. The aforementioned rodents seem to have these basal strains or ancestral strains appear in the highlands of Mongolia and China and Kazakhstan. These sources are farther east from the European outbreaks, supporting research that the initial flare of Black Death was brought from the East along trade routes.

At least in the case of the plague that overtook Marseille, the pathogen was found to be a descendant of the Yersinia pestis strain. Contrary to the constant dribble of plague down trade routes, the pathogen must have remained in a reservoir closer to home. What these reservoirs are thought to be is the next big question. Rats could be to blame, or soil, however no answer is currently known.

“We have had major outbreaks in Eastern Europe up until the 1800s. In mainland Europe, it hasn’t [popped up] since 1720. We have about 400 years that were clear of outbreaks.” This could be as a result of attenuation in the variance of the bug, or a rise in resistance among humans. One of Poinar’s students is working on just that, looking for signs of selection within the human genome. This involves searching for resistance to the epidemic in the genes of those of European descent whose ancestors survived to pass on their protective genes.

Interestingly, having resistance to the Black Death can also give someone resistance to various other pathogens. A genomic mutation known to give people resistance to the HIV virus by blocking the virus from entering the cell has been found in higher frequency in Europe, despite the virus’ origin in Africa. You would expect the exact opposite, where populations exposed to the virus in greater amounts would undergo greater selection and therefore greater resistance.

“When you try to date the [onset of resistance], most of those dates show up around the time of the Black Death. So there is the issue that those that underwent a selection against plague and survived because of this genomic deletion, they were protected from the bacteria of the plague and now protected from the virus of HIV,” explained Poinar.

Rising concerns about antibiotic resistance has shifted a focus towards what causes the reappearance of bacterial infections. By attempting to dig up the roots of infectious diseases, researchers like Poinar are looking to uncover more about pathogens and their fluctuating attacks.

Photo Credit: Jason Lau

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