Why Were Medieval Monks So Susceptible to Intestinal Worms?

The Augustinian friars of Medieval Britain pledged themselves to a life of poverty, but their friaries offered a pretty high standard of communal living. The monks dwelled in buildings with sophisticated stone and glass work, studied in libraries, and dined on the products of bountiful gardens. When nature called, they enjoyed dedicated latrines and hand-washing facilities, complete with running water systems that were rare even among the era’s wealthiest households. But new research on human remains from a friary buried below the University of Cambridge shows that the monks suffered greatly from a gastrointestinal affliction—worms.

Scientists unearthed centuries-old parasite eggs, buried with monk skeletons in the friary’s private graveyard, and to contextualize their find, compared their abundance with the parasites found among commoners of the same era in a nearby Cambridge cemetery. The friars were afflicted with intestinal worms at almost double the rate of Cambridge’s general population—despite the fact that many of the city’s inhabitants lived with sanitary facilities no better than a hole in the ground.

“We expected, if there was going to be a difference, it would be the monks who had fewer intestinal parasites of the kinds spread by poor sanitation,” says Piers Mitchell, an osteoarchaeologist at the University of Cambridge. “Because they had hand-washing and nice toilets, while it would be the poor peasant who may not have a toilet at all, or even have freshwater to wash their hands, who would have more parasites.”

A study co-authored by Mitchell, published today in the International Journal of Paleopathology, is the first to compare parasite abundance among individuals who lived different lifestyles in the same Medieval community. Such work may help untangle which factors have historically promoted parasite afflictions and which factors helped humans to keep them at bay. The study suggests that while sanitation will always be key, other factors may also play a role.

Founded in the 1280s, Cambridge’s Augustinian friary was a leading house for the reading and study of manuscripts and hosted clergy from across Britain, and Europe, for about 250 years. In 1538 it was closed, like many English monasteries, when Henry VIII broke with the Roman Catholic Church. Left behind were the graves of deceased friars, shared with the intestinal pests that were with them in life.

Mitchell and colleagues sampled the grave soil that, during decomposition, fell into the body cavity where the intestines once resided. This soil mingled with remnants of the abdominal cavity and its contents, including the eggs of worms that lived in the intestines while the corpse was alive. Using digital light microscopy, the team sifted through the soil to spot and sort the ancient parasite eggs still found with each skeleton after many centuries.

“The eggs of most intestinal worms are pretty tough, otherwise you’d digest them and they’d never get to reproduce and infect other people,” says Mitchell, who was part of another team that recently found parasites in the 4,500 year-old poop left by the builders of Stonehenge. “The tough wall that stops you digesting them also makes it hard for soil fungi and bacteria to break these things down. So many of them can survive in soil for hundreds or thousands of years in the right kind of conditions.”

While examining the remains of 19 monks buried on the friary grounds, most from the 13th and 14th centuries, the team found that at least 11 (58 percent) were infected by worms. When they similarly tested the remains of 25 adults from the cemetery of All Saints by the Castle parish church, just eight of those townspeople (32 percent) had parasites.

The team wasn’t surprised that some 30 percent of Medieval Cambridge’s citizens suffered from parasites; those numbers line up with results found in other studies of the period around Europe. But the high rate of infection among the monks, nearly double this average, raised eyebrows.

Why such a high prevalence of roundworm and whipworm infections? The scientists speculate that the monks may have picked up the parasites when putting feces to work as a fertilizer, either by emptying their own latrines to manure crops, or by bringing in outside fertilizer contaminated by parasites in human or pig excrement. These hands-on practices were definitely used in Medieval and Roman times, just as they are today in parts of the world where other fertilizer options are scarce.

“It’s a practice that works, but the problem is you have to make sure you break the cycle of infection,” Mitchell notes. The parasites, which in the case of roundworms can grow to nearly a foot in length, pass their eggs in human feces. When humans then ingest food or water contaminated by this feces, they become infected and host a new generation of parasites. To prevent infections, farmers can compost human waste so that it reaches temperatures high enough to kill off pathogens, creating safe fertilizer, but that can be a tricky business.

Many of those afflicted with the worms would have suffered gastrointestinal distress but never known that parasites were the reason. In other cases, the obvious presence of visible worms in feces would have made the problem nauseatingly clear. But Medieval medical experts, while aware of worms, did not know they could be spread from person to person and especially by poor sanitation.

The learned inhabitants of Medieval Cambridge wrote of how parasitic worms were an unpleasant part of life, and in the process displayed a misunderstanding of the problem. Seventeenth-century medical practitioner John Stockton wrote a manuscript suggesting that different intestinal worms were created by imbalances in the body’s four humors; blood, phlegm, black bile and yellow bile. “Long round worms form from an excess of salt phlegm, short round worms from sour phlegm, while short and broad worms came from natural or sweet phlegm,” he wrote. Stockton’s treatment included bitter medicinal plants like wormwood, which can kill off some parasites, though likely at the cost of persistent diarrhea. A 15th-century monk named Symon Welles swore by an even less palatable cure, a curative drink made from powdered moles. For some, the Welles method involving the small mammals may have made living with worms seem like not such a bad option after all.

Scientists have delved into many ancient latrines to uncover evidence of ancient diets and intestinal parasites. Studying such parasites yields a lot of important information, but it does involve some challenges. For example, one expert pointed out that the small sample sizes used in studies like this one can make it difficult to draw broader conclusions. Another expert cautioned that what seem like parasite eggs can sometimes be confused with plant or fungal remains.

But studying such pests can inform the best ways to control and eradicate them today. That often means improved sanitation, which generally leads to lower levels of harmful parasite problems.

Medieval Cambridge was home to several friaries and nunneries, as well as the other typical inhabitants of a Medieval city including merchants, craftspeople, laborers, farmers and even early students of the university that centuries later is spearheading efforts to delve into their past.

Studies like this one that compare parasitic infection rates among discrete subsets of a single community are uncommon, because when bodies have mingled in a cemetery for centuries it’s hard for researchers to know exactly who’s who. But the monastery’s private graveyard provided a unique opportunity. Burials were mostly restricted to the brothers who lived there, with the exception of some wealthy outsiders who paid for the privilege of interment in sacred soil. These could be easily spotted and omitted from the study because they weren’t wearing the telltale robes and surviving rusty belt buckles, in which the friary’s monks were buried.

Such unique historical situations, like the castle toilets that preserved parasites from crusaders’ feces, offer chances to learn in more detail about how different humans coped with the intestinal pests that were all too common in the past—and to help eradicate their descendants who remain all too uncomfortably with us today.