The Black Death was one of the worst disease outbreaks in human history. This bacterial disease swept across Europe from 1346 to 1353, killing millions. For hundreds of years afterward, this plague returned. Each time, it risked wiping out families and towns. Many people thought rats were to blame. After all, their fleas can harbor the plague microbes. But a new study suggests researchers have given those rats too much blame. Human fleas, not rat fleas, may be most to blame for the Black Death.
Black Death was an especially extreme outbreak of bubonic plague.
Bacteria known as Yersinia pestis cause this disease. When these bacteria are not infecting people, they hang out in rodents, such as rats, prairie dogs and ground squirrels. Many rodents can become infected, explains Katharine Dean. She studies ecology — or how organisms relate to one another — at the University of Oslo in Norway.
The plague’s species “persists mostly because the rodents don’t get sick,” she explains. These animals can then form a reservoir for the plague. They serve as hosts in which these germs can survive.
Later, when fleas bite those rodents, they slurp up the germs. These fleas then spread those bacteria when they bite the next critter on their menu. Often, that next entrée is another rodent. But sometimes, it’s a person. “Plague is not picky,” notes Dean. “It’s amazing that it can live with so many hosts and in different places.”
People can become infected with the plague in three different ways. They can be bitten by a rat flea that’s carrying plague. They can be bitten by a human flea carrying the plague. Or they can catch it from another person. (Plague can spread from person to person through an infected individual’s cough or vomit.) Scientists have been trying to figure out, though, which route was most responsible for the Black Death.
Flea vs. flea
The plague may not be a picky disease, but fleas can be picky eaters. Different species of these parasites are adapted to coexist with different animal hosts. People have their own flea: Pulex irritans. It’s an ectoparasite, meaning that it lives outside its host. People often have to deal with another ectoparasite, as well, a species of louse.
The black rats that lived in Europe during the Middle Ages have their own species of flea. It’s called Xenopsylla cheopis. (Another flea species targets the brown rat, which now dominates in Europe.) All these fleas and the louse can carry plague.
Rat fleas prefer to bite rats. But they won’t turn down a human meal if it's closer. Ever since scientists proved that rat fleas could transmit plague, they assumed those fleas were behind the Black Death. Rat fleas bit people, and people got the plague.
Except that there has been growing evidence that black rats don’t spread plague fast enough to account for how many people died in the Black Death. For one, the fleas found on European black rats don’t like to bite people much.
If scientists needed another explanation, Dean and her colleagues had a candidate: human parasites.
Ancient manuscripts and modern computers
Dean’s team went digging for death records. “We were at the library a lot,” she says. The researchers looked through old books for records of how many people died of plague per day or per week. The records often were quite old and hard to read. “A lot of the records are in Spanish or Italian or Norwegian or Swedish,” Dean notes. “We were so lucky. Our group has so many people that speak so many different languages.”
The team calculated plague death rates from the 1300s to the 1800s for nine cities in Europe and Russia. They graphed the death rates in each city over time. Then the scientists created computer models of the three ways plague can spread — person to person (via human fleas and lice), rat to person (via rat fleas) or person to person (via coughing). Each model predicted what the deaths from each method of spread would look like. Person to person spread might trigger a very quick spike in deaths that fell off quickly. Rat flea-based plague might lead to fewer deaths but those deaths might occur over a long time. Death rates from human flea-based plague would fall somewhere in between.
Dean and her colleagues compared their model results to the patterns of real deaths. The model that assumed the disease was spread by human fleas and lice was the winner. It most closely matched the patterns in death rates seen from human transmissions. The scientists published their findings January 16 in the Proceedings of the National Academy of Sciences.
This study doesn’t exonerate rats. Plague is still around, hiding out in rodents. It probably spread from rats to human fleas and lice. From there, it sometimes prompted human outbreaks. Bubonic plague still emerges. In 1994, for example, rats and their fleas spread plague through India, killing almost 700 people.
Rats still spread a lot of plague, Dean explains. “Just probably not the Black Death. I feel more like a champion for the human ectoparasites,” she says. “They did a good job.”
Not a total surprise
Scientists have suspected that rat fleas might not have played a big role in the Black Death, says Michael Antolin. He is a biologist at Colorado State University in Fort Collins. “It’s nice to see a model that shows [it could happen].”
Studying illnesses of the past is important for the future, Antolin notes. Those long-ago outbreaks can teach a lot about how modern diseases might spread and kill. “What we’re looking for are the conditions that allow epidemics or pandemics to occur,” he says. “What can we learn? Can we predict the next big outbreak?”
Even if rats played a role in the Black Death, they wouldn’t have been the biggest factor, Antolin explains. Instead, environmental conditions that allowed rats, fleas and lice to spend so much time around people would have played a larger role.
Until modern times, he notes, people were gross. They didn’t wash often and there were no modern sewers. Not only that, rats and mice could thrive in the straw that many people used in their buildings for roofing and a floor covering. Hard roofs and clean floors mean fewer places for ratty roommates — and the diseases they might pass on to human fleas and lice.
What stops plague isn’t medicine or killing rats, Antolin says. “Sanitation is what fixes plague.”
bacteria (singular: bacterium) Single-celled organisms. These dwell nearly everywhere on Earth, from the bottom of the sea to inside other living organisms (such as plants and animals).
biology The study of living things. The scientists who study them are known as biologists.
bubonic plague A disease caused by the bacterium Yersinia pestis. It’s transmitted by the bite of a flea that had previously bitten some rodent (or other mammal) infected with the germ. This form of plague causes fever, vomiting and diarrhea. It also inflames the lymph nodes, causing them to swell. Those swollen tissues, called buboes, give this form of the disease its name. Known as the Black Death, bubonic plague killed millions of people in Europe during a series of outbreaks during the Middle Ages.
colleague Someone who works with another; a co-worker or team member.
computer model A program that runs on a computer that creates a model, or simulation, of a real-world feature, phenomenon or event.
death rates The share of people in a particular, defined group that die per year. Those rates can change if the group is affected by disease or other deadly conditions (such as accidents, natural disasters, extreme heat or war and other sources of violence).
ecology A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
ectoparasite A parasite such as a flea or louse, which lives outside of its host.
epidemic A widespread outbreak of an infectious disease that sickens many people (or other organisms) in a community at the same time. The term also may be applied to non-infectious diseases or conditions that have spread in a similar way.
gland A cell, a group of cells or an organ that produces and discharges a substance (or “secretion”) for use elsewhere in the body or in a body cavity, or for elimination from the body.
host (in biology and medicine) The organism (or environment) in which some other thing resides. Humans may be a temporary host for food-poisoning germs or other infective agents.
immune (adj.) Having to do with the immunity. (v.) Able to ward off a particular infection. Alternatively, this term can be used to mean an organism shows no impacts from exposure to a particular poison or process. More generally, the term may signal that something cannot be hurt by a particular drug, disease or chemical.
immune system The collection of cells and their responses that help the body fight off infections and deal with foreign substances that may provoke allergies.
infect To spread a disease from one organism to another. This usually involves introducing some sort of disease-causing germ to an individual.
lymph A colorless fluid produced by lymph glands. This secretion, which contains white blood cells, bathes the tissues and eventually drains into the bloodstream.
lymph glands (or lymph nodes) Small nodules located in the armpits, groin and stomach, these organs are part of the lymph system. They secrete lymph and also serve as a storage place for some cells in the immune system.
model A simulation of a real-world event (usually using a computer) that has been developed to predict one or more likely outcomes. Or an individual that is meant to display how something would work in or look on others.
organ (in biology) Various parts of an organism that perform one or more particular functions. For instance, an ovary is an organ that makes eggs, the brain is an organ that makes sense of nerve signals and a plant’s roots are organs that take in nutrients and moisture.
organism Any living thing, from elephants and plants to bacteria and other types of single-celled life.
outbreak The sudden emergence of disease in a population of people or animals. The term may also be applied to the sudden emergence of devastating natural phenomena, such as earthquakes or tornadoes.
pandemic An epidemic that affects a large proportion of the population across a country or the world.
parasite An organism that gets benefits from another species, called a host, but doesn’t provide that host any benefits. Classic examples of parasites include ticks, fleas and tapeworms.
plague A term for any horrific infection that spreads easily and kills many people, usually quickly. Best known are the infections caused by the bacterium Yersinia pestis. Indeed, they are commonly referred to simply as the plague. In one form, people pick up the germ from the bite of infected fleas. This inflames the lymph nodes, causing them to swell. Those swollen tissues, called buboes, give this form of the disease its name: bubonic plague. When the disease is instead transmitted by inhaling the bacteria, people develop what’s known as pneumonic plague. This form of the disease can be spread when sick people cough. Pneumonic plague is the most deadly form, often killing its victims within 24 hours.
Proceedings of the National Academy of Sciences A prestigious journal publishing original scientific research, begun in 1914. The journal's content spans the biological, physical, and social sciences. Each of the more than 3,000 papers it publishes each year, now, are not only peer reviewed but also approved by a member of the U.S. National Academy of Sciences.
reservoir A large store of something. Lakes are reservoirs that hold water. People who study infections refer to the environment in which germs can survive safely (such as the bodies of birds or pigs) as living reservoirs.
rodent A mammal of the order Rodentia, a group that includes mice, rats, squirrels, guinea pigs, hamsters and porcupines.
sanitation The protection of human health by preventing human contact with our own bodily wastes, through hand washing, use of things like use of toilets or latrines, separation of disposal of wastes from drinking-water sources and water, and cleaning water to rid of disease causing agents disinfecting foods and materials that may be ingested or otherwise enter the body.
sewer A system of water pipes, usually running underground, to move sewage (primarily urine and feces) and stormwater for collection — and often treatment — elsewhere.
species A group of similar organisms capable of producing offspring that can survive and reproduce.
transmit (n. transmission) To send or pass along.
Yersinia pestis The bacterium that causes plague, both the bubonic and pneumonic forms.
Journal: K.R. Dean et al. Human ectoparasites and the spread of plague in Europe during the Second Pandemic. Proceedings of the National Academy of Sciences. Published online January 16, 2018. doi: 10.1073/pnas.1715640115.