Rats, Mosquitos, and the Fall of Rome

In 357 A.D., Emperor Constantius II visited Rome for the first time. He had seen many other great cities, from the new capital at Constantinople to the Syrian metropolis of Antioch. But when he entered the Roman Forum, he was almost overwhelmed by its grandeur. He seems to have spent the next few days, like a modern tourist, seeing the Colosseum, the Pantheon, the great Temple of Jupiter, and other wonders of the city. At the time of Constantius's visit, the population of Rome was probably around 700,000, reduced from the million or so of the early imperial era, but still greater than that of any other city on the planet. According to a count made around this time, Rome had 424 temples, 861 baths, 28 public libraries, and no fewer than 46,602 apartment buildings. Despite a seemingly endless series of invasions and civil wars, the Roman Empire looked much as it had at its height two centuries before. It still stretched from Scotland to Sudan. Its population, though reduced by plague and invasion, was probably in the neighborhood of 60 million. It had an effective currency, thousands of cities, and a professional army of nearly half a million men. In the year 590, two and a half centuries after the visit of Constantius, Pope Gregory I led a procession through the streets of Rome. It had been a hard winter. The Tiber had flooded, sending many torrents racing through the streets. Disease had followed, wracking the survivors with fevers and chills. And now the plague had returned. Dozens had died, then hundreds, including the reigning pope. Gregory had been chosen to replace him. Behind an upheld cross, to the tolling of bells, Gregory processed through an empty city. Perhaps 20,000 people still lived within the Aurelian Walls, a paltry two percent of the population at the Empire's peak. The Forum was overgrown, the Colosseum was a ruin, and gutted apartment buildings lined the streets, their walls collapsing a little more with every gale and frost. The Roman Empire still existed, though now it was centered in Constantinople and rules only a few fragments of Italy. Within a generation, the Islamic conquests would reduce it to an impoverished, rump state. The population of the former Roman world, now divided between the decaying Empire and a shifting caste of Germanic kingdoms, may have fallen to half of what it had been in the reign of Constantius. Dead cities littered the shores of the Mediterranean. These changes were consequences and corollaries of the fall of the Roman Empire. Although the reasons behind the fall are disputed, even the most revisionist scholars give center stage to the emperors and chieftains who dominate our literary sources, as they should. This video, however, will consider the role played by two characters mentioned in no ancient history, the rat and the mosquito. Mosquitoes have always been a nuisance in Italy. Horace complains about the insects who tormented him as he traveled on the Appian Way, and a humorous poem traditionally attributed to Virgil begins with a shepherd swatting a mosquito on his face. But during the Roman period, and indeed until the early 20th century, mosquitoes were much more than a nuisance. They were carriers of malaria, the deadliest endemic disease in Italy. The most dangerous form of malaria, caused by the Plasmodium falciparum parasite, was especially prevalent in and around Rome. Falciparum malaria is carried by mosquitoes of the Anopheles genus. Both the mosquitoes and their deadly parasite appeared to have been established in Italy by the beginning of Roman history. Over time, however, the range of the mosquitoes expanded, exposing more and more people to the threat of disease. To some extent, mosquitoes were spread by direct human agency, insects stowing away on ships, and larvae being carried in water casks. But the environment seems to have also become increasingly favorable, both around Rome and in many coastal cities. The Roman warm period, the exceptionally balmy and moist climatic regime that graced much of the Mediterranean during the first two centuries AD, seems to have been accompanied by regionally higher rainfall. We have accounts, for example, of terrible floods in Rome, even during the normally dry summer months. The pools and puddles left by each flood would have allowed mosquito populations and malaria to flare. A more systemic problem was widespread deforestation. As Rome and other Italian cities expanded, the forests that mantled the hills above the coastal plains were cut, both for fuel and to clear new land for farming. Erosion on exposed and tilled slopes washed huge quantities of sediment into the rivers, choking their channels and producing marshy deltas. Speaking of deforestation, at the end of this video, stick around for a look at how our partner, Planet Wild, established a thriving new ecosystem in a forest devastated by drought. Returning to our topic, the building of Roman roads through marshes and along hillsides interrupted natural drainages, creating additional mosquito habitats. The aqueducts also contributed to the problem, especially in Rome, where they fed hundreds of fountains and countless stagnant pools. As mosquito habitats expanded, malaria became an increasingly dire problem. It has been suggested that the decline of the ancient Greek cities of southern Italy, almost all of which were located on increasingly marshy coasts, was at least partly motivated by debilitating levels of malaria. Silting created a malarial marsh at the mouth of the Tiber, and changes in local drainage patterns caused the Pontine marshes south of Rome to expand. Falciparum malaria was already hyperendemic in Rome by the time of Galen, who described the symptoms of what he called tertian fever with clinical precision. The scourge may have become even worse in late antiquity, when epitaphs from the catacombs reveal huge malaria-driven spikes in mortality during late summer and early fall. The impact of malaria on the health and population of Rome, Italy, and other coastal regions of the empire is difficult to gauge. It obviously increased mortality, probably dramatically in the case of very young and immigrants from non-malarial regions. Pliny the Younger, for example, observed that people in the hills outside Rome tended to live much longer than those in low-lying areas. In the long term, malaria reshaped settlement patterns across Italy as people moved to escape the so-called evil air of the lowlands. Some Roman cities were completely abandoned as the regions around them became dangerous. Malaria also emptied parts of Rome itself and reduced the Roman Campania, once a region of lush gardens and sprawling villas, to the lonely wasteland it remained through the 19th century. Profound though the impact of the mosquito was on the health of the Roman Empire, another pest proved even more dangerous. Rotus rotus, the black or ship rat, is native to Southeast Asia. It gradually spread west following trade routes and seems to have reached the Mediterranean during the Roman Republican era. The spread of rats is difficult to date archaeologically, both because rat bones are fragile and because excavators have historically paid little attention to the remains of rodents. Literature is of little help since both Greek and Latin use the same word for rat and mouse. Artistic evidence is also lacking. There are, however, several indications that rats spread rapidly through the Roman world. One may be the growing popularity of cats, the classical world's sole line of defense against rodents. Another, as we'll see, is the growing visibility of a terrifying disease. But the best evidence is simple rat biology. Rats breed constantly and prolifically. A female produces up to five litters a year, and newborns reach maturity within a few months. The only factor that can limit population growth is the availability of food, and in the Roman world rats found one of their favorite foods in unprecedented quantities. Rats love grain. So, of course, did the Romans, who consumed most of their calories in the form of bread. Every city and each military camp had granaries, and almost every substantial port was visited by grain barges. These became a rodent superhighway, carrying rats from Egypt and other grain-exporting regions throughout the Mediterranean. By late antiquity, to judge from bones found in Roman contexts, rats had reached northern Britain and the Rhine frontier. Besides being a nuisance to farmers and anyone else trying to store grain, rats can spread bubonic plague. Yersinia pestis, the plague bacterium, inhabits the guts of several species of flea. During a flare-up of the plague, infected fleas bite rats, who sicken and die. As the rat population dwindles, the fleas seek out other mammals, including humans. The modern form of Yersinia pestis seems to have evolved about 3,000 years ago among the gerbils and marmots of Central Asia. At some point, it made the leap to rats and began its slow migration toward the Mediterranean. By the first century A.D., isolated plague outbreaks were beginning to erupt in the eastern Roman provinces. Secondhand reports came to the attention of a few medical authors who cataloged the symptoms. Swollen lymph nodes, gangrene, vomiting, and fever, as a curiosity. Plague never reached Rome and appears nowhere in the voluminous works of Galen. But as the years passed and rats multiplied, the danger of a pandemic grew. In the year 541, the storm broke. Plague appeared in Pelusium, near Alexandria, and spread rapidly through Egypt and along the coasts. By February of 542, the pestilence had reached Constantinople. Within weeks, if we can believe our sources, thousands were dying every day, and half the population perished in a matter of months. The plague pressed on by sea and land, reaching Rome and Gaul in 543, and Britain by the year after. Levels of mortality seem to have approached those of the Black Death, which killed nearly half the population of Europe. This pandemic is usually called the Plague of Justinian, after the Roman Emperor at the time of its appearance. Justinian, famously, set out to revive the Roman Empire. Before the plague, he had reconquered North Africa and Italy, codified Roman law, and built Hagia Sophia. After the plague, he seemed able to do little more than cling grimly to his conquests. Did the plague wreck Justinian's plans? Or, to return to the first part of this video, did malaria weaken Rome? In each case, the obvious answer would seem to be yes. But a better answer would be possibly to some degree. Malaria certainly didn't help the Romans, and there can be little doubt that it drove the depopulation of many coastal areas in Italy and elsewhere. But malaria had been prevalent for centuries before the empire began to decline. The city of Rome and the Roman Empire flourished despite it, and declined for reasons basically independent of it. Malaria robbed the Romans of human resources they could ill afford to lose. But to claim that it crippled the empire is to press the evidence too far. Plague made Justinian's project of reconquest, always a tenuous enterprise, more difficult. But even in this seemingly unambiguous case, we should be cautious. The evidence for overwhelming mortality is far from secure. And even if the plague killed half the empire's population, as some scholars believe, it should be remembered that the Romans had shown remarkable resilience in the face of the Antonine Plague and the Plague of Cyprian, two previous pandemics. Justinian's problems did not ultimately stem from the plague. The plague just made them harder to cope with. History is hindsight, and hindsight has a way of seeming clear. The job of an ancient historian is to build narratives, to seek causes, to draw conclusions from evidence that is scattered and often contradictory. The testimonia take you so far. The rest, for better or worse, is a leap of faith. History, in short, is never simple, and there are few historical questions more complex than the fall of Rome. If you're interested in the relationship between humans and their environment, I highly recommend Planet Wild's YouTube channel. Whether they're tracking Europe's last bison or cleaning coral reefs, Planet Wild releases a video every month documenting their missions to rewild the planet and repair damaged environments. These videos are not only entertaining, they also combine education with real environmental action. And it's all driven by a community that anyone can join to help our planet bounce back. Their monthly missions are based on community votes and vetted for high impact. I've already joined myself because I believe they're making a real difference. Planet Wild's latest videos, linked here, explore how they've transformed a dying forest into a strong, climate-resilient ecosystem, and how they're striving to save this tiny owl from extinction. You can check them out now. Thanks for watching.