How parasites went on Crusade

When the crusaders of the Order of St John first built a 35-latrine toilet complex in the medieval city of Acre in the Kingdom of Jerusalem, they could scarcely have considered that researchers would be sifting through its contents 900 years later. Yet the 13th-century latrine soil is providing another chapter in understanding the long history of our relationship with intestinal parasites.

Biological anthropologist Dr Piers Mitchell of Cambridge University has been extracting sediment derived from decomposed faecal material and analysing it under the microscope. Long after the many different types of parasites have perished, their tenancy in the intestine of their human host can be deduced by the presence of their eggs, now hundreds or even thousands of years old.


There is a growing body of research worldwide that attests to the fact that parasitic worms have been uninvited guests of the human intestine for millennia. It’s a relationship that is still as strong as ever: today, 740 million people in the tropics have human hookworm according to estimates by the World Health Organization.

One aspect that has captured the attention of researchers is the ability to trace ancient human migrations through the parasites the migrants took with them. As one example, the sequential waves of peopling of the Americas has been timed through the hookworms that infected them. Such research also provides an opportunity to look back to when and how parasites came to cause disease in humans. “We can then understand what impact these infections have had, and will continue to have, upon our evolution,” explained Mitchell.


The crusades were arguably the greatest migration event that took place in medieval Europe. In the 12th and 13th centuries, hundreds of thousands of Europeans travelled to the eastern Mediterranean on military campaigns, pilgrimage and to trade.

“The crusades are often blamed for the spread of disease during the medieval period,” explained Mitchell, whose work was funded by the British Academy. “But only limited research has investigated which diseases might have been spread, in which direction, eastwards or westwards, and what impact this may have had upon the endemic patterns of disease.”

When Mitchell analysed the crusader latrines, he was able to identify the eggs of roundworm, whipworm, beef/pork tapeworm, dysentery and fish tapeworm. He explained why the latter was of particular interest: “Fish tapeworm is found in northern Europe where it infects humans when they eat salted, smoked or dried fish. It’s not found in the Middle East, probably because the environment doesn’t seem to support the lifecycle of the worm.”

“We were able to confirm that the parasite was not there before the crusades. We believe the crusaders brought the parasite with them when they travelled to the eastern Mediterranean with fish tapeworms in their intestines. This is a great example of how migrations in the past can move diseases around the planet. Sometimes they take hold there and become endemic, and sometimes they don’t.”


Mitchell now plans to extend his research even further back in time, focusing on the wider Fertile Crescent – a region that stretches from Jordan to Iran. Here some of the earliest civilisations developed during the past 10,000 years.

“There are theoretical arguments that when our ancestors were hunter-gatherers perhaps they had fewer parasites because they kept moving on. Once they settled and lived in the same places did that make them more predisposed to reinfecting themselves with their parasitic diseases?”

His new research will trace the history of parasitic infections in the Middle East from 9,000 BC till Roman times, and will ask such fundamental questions as: when did intestinal parasites first become common in humans? Did the introduction of farming practices such as irrigation expose people to new species of parasite? And even, what impact did the invention of the humble toilet seat have on public health?


Source: University of Cambridge