With DNA fragments from the teeth of 58 adults and 10 children buried in three imperial-period Italian cemeteries, researchers were able to recover the mitochondrial genome to identify the specific malaria species that infected people.
Their data confirm that it was the malaria parasite Plasmodium falciparum, the same one that is spread by mosquitoes today and kills hundreds of thousands of people every year. Symptoms include fever, chills and flu-like illness.
"Malaria was likely a significant historical pathogen that caused widespread death in ancient Rome," said evolutionary geneticist and study author Hendrik Poinar, director of the Ancient DNA Center at McMaster University in Hamilton, Ontario.
The researchers estimate that malaria killed as many people during the Roman Empire as it does now in Africa. In 2015, there were an estimated 438,000 malaria deaths worldwide, with 91% of them occurring in sub-Saharan Africa, according to the World Health Organization. The organization considers the disease preventable and treatable.
Before this discovery, descriptions of fevers that sounded like malaria were found in historical texts like Hippocrates' "On Epidemics" or Celsus' "De Medicina." The fevers were described as repeated and occurring at particular times of the year, but because many infections cause fevers, it was difficult to classify them as malaria, Poinar said.
The fact that the outbreaks were repeated signified that it might be malaria because no one can build up an immunity to the disease, Poinar said. But researchers didn't have the DNA evidence to point to the species or how it traveled across imperial Italy until now.
The three cemeteries were in three ancient cities in modern-day Italy: Velia, Isola Sacra and Vagnari. The cemetery in Velia, located on a coastal high point, dated from the first to second century AD. Isola Sacra, a low basin of woodlands near the Tiber River, and Vagnari, a low river valley, had cemeteries that dated from the first to third and fourth centuries AD, respectively. Velia and Isola Sacra were both considered port cities where trading occurred. Vagnari, more inland, is considered a cemetery of estate laborers.
More than half of the genome was constructed using DNA from two adult skeletons, one from Velia and the other from Vagnari. None of the parasite was detected in the skeletons from Isola Sacra.
This discovery predates previous studies' detection of the malaria parasite in central Italy by several centuries.
The researchers said their results indicate how malaria affected people across different ecological and cultural environments in imperial-period southern Italy.
"Malaria was indeed present not only along the coast of Italy, and thus blamed on immigrants arriving through the port cities from Africa, but deep inland at rural settings as well," Poinar said. "So it must have been a constant scourge upon the people and ultimately the empire. You can't keep fighting and expanding if your work force and armed forces are weakened by constant repeated infections, can you?"
The hypothesis that malaria caused the fall of Rome is an old and popular one, but it doesn't have direct support in ancient DNA evidence, said biological anthropologist Kristina Killgrove, who was not involved in the study.
"Even if malaria is eventually implicated in demographic changes in Rome at the end of the Empire, it is important to investigate the other health burdens that people suffered from -- including lead poisoning, parasitic infection, venereal diseases and dietary insufficiency -- by studying both ancient skeletons and DNA," Killgrove said. "Most scholars are agreed that there was no single cause of the 'fall' of Rome, but disease certainly played a role in dwindling population numbers."
This type of DNA is hard to find because evidence of the parasites would be found in blood or organs that decompose easily, like the spleen or liver, and wouldn't be available to study 2,000 years later. Malaria also doesn't cause easily identified changes in skeletal remains.
The researchers were able to find incredibly small DNA particles in dental pulp, the cavity within the tooth that houses blood and nerve endings, Poinar said.
They extracted, purified and and enriched the DNA to test for the parasite using modern malaria as a "fishing bait." Small molecules with magnetic beads acted as a way to "pull down" the low concentration of parasite DNA and found its signature. This method has also been used in studies concerning cholera and the plague, Poinar said.
The research team included scientists from McMaster University, the University of Sydney and the Luigi Pigorini National Museum of Prehistory and Ethnography in Rome. Their research was cleared through an ethics board, and they drilled out the pulp to make sure that the teeth remained intact and were returned to the museum.
Knowing more about when and where malaria affected humans may enable the researchers to learn more about how widespread malaria was during the Roman Empire, how it affected people and how it has evolved.
Poinar said he and his team will study more pathogens with ancient roots to understand their origins, which can lead to eradicating and controlling them today. He also believes there is more work to be done based on what they learned from the malaria parasite DNA.
"I hope we can look at the genes within the humans that might provide an example of increased protection or susceptibility for those living under the constant threat of this parasite," Poinar said. "Also, this can't be the only repeated infection, so what else was attacking the Romans while malaria flared in and out?"