The Rats, the Flea and the Weather

Although the human flea can transmit Y. pestis, this flea is not the normal vector of this disease in humans. The most common vector is a flea that infests rats and this flea normally prefers rats to any other host. This means that the flea does not usually feed on a human host. So the environmental conditions that can lead to an epidemic of the bubonic plague in humans must also include some reason that forces the rat flea to bite humans. That condition is an increased death rate death in the population of rats. The flea only moves to a non-rat host when the population of rats decreases drastically.

One thing that can kill large numbers of rats is the bubonic plague itself. Rats can tolerate a modest population of the pathogen in their blood stream, but if it invades the lungs or the brain of the rat, this disease becomes lethal to rats (it also kills many domestic animals). Thus an epidemic of the bubonic plague among the rat population of a city can lead to epidemic among the human population of the city and their domestic animals.

Other circumstances can also cause massive die off of rats. For example, in 1994 an outbreak of the plague in Surat India was preceded by massive flooding. The floods drowned many of the city’s rats.

In addition, the climate can influence the severity of the disease. The likelihood of a flea’s bite transmitting the pathogen increases when the temperature is between 20 and 25 degrees C. At this temperature the bacilli multiply in the flea’s stomach in numbers large enough to cause a blockage and threaten the flea with starvation. This causes the flea to try to clear the blockage by regurgitation. If the temperature rises above 27 degrees C, the blockage in the flea’s gut dissolves. Also the larvae of the flea will die if the humidity is too low. Thus a cool, moist climate is the optimum for both the survival of the flea and transmission of the disease. And this was exacerbated by the Fusarium toxins in the food the people of Europe were eating during the Middle Ages.

Today, in the age of vaccines and informed physicians, we lack experience with epidemic diseases and both the general public and modern historians tend to underestimate the full force a new infection can have on a population. Thus we all overlook the role diseases have played in the course of history. We are even less aware of how plant diseases can influence the affairs of humans. Yet both have and will continue to significantly influence human health. And both are influenced by the weather, which, as I am trying to get across, is influenced by the global climate of the time!

“Biology and medicine prove to be crucial driving forces in human history…” Jared Diamond

Next Post: Other Casualties of the Black Death

And There is Even More to it Than That!

If the change in global temperature brings more rain to an area than it is used to, as happened in Western Europe during the Middle Ages, then it also means plant disease can increase and that in turn can increase the devastation caused by human disease. For example, during the Late Middle Ages, a postharvest disease caused by a fungus called Fusarium is believed to have increased the deaths caused by the bubonic plague in areas with increased rain.

Generally this fungus contaminates the seeds either before they are harvested or at the time of harvest. This fungus then grows within the seed during storage. For this fungus to continue to grow it requires a high moisture content in the seeds. Today most grain is stored at a moisture content of 12-14% which is too low for the fungi to grow and the fungi die after a few months in storage and do not infect other seeds. However during the Late Middle Ages, the people had neither the technology nor the knowledge to store grain at this moisture content. During the 13 and 1400’s the moisture content of harvested grain was at the mercy of the weather. Because the weather was moist during this time the moisture content in the seeds must have been high. The fungi in contaminated seed would have continued to grow under these conditions and would have infected other seeds.

As the fungus grows within a seed it not only causes decay of the seed but it also produces substances called mycotoxins. The ingestion of moldy grain containing these toxins can lead to a disease whose symptoms include necrosis of the skin, hemorrhage, liver and kidney damage, and death. Fusarium molds produce a number of mycotoxins. These toxins in both lethal and sublethal doses not only cause the above symptoms but they can also cause an immune deficiency. The toxins attack the lymphoid cells critical for the body’s defense against infection.

According to Mary Kilbourne Matossian in her book Poisons of the Past, the highest moralities of the plague during the late Middle Ages occurred in areas where surplus grain was stored (thus there would also be a surplus of rats) and with the incidence of high humidity, rain, and flooding.

“For two years prior to the pandemic of 1348 in Europe the weather was extraordinary rainy and humid and crops were poor. The summer of 1348 was exceptionally wet in England, where the plague began its ravage; on the other hand it was not so wet in Scotland that year and the plague did not spread widely there until the wet summer of 1350.

The coldest and driest regions were untouched by plaque during the pandemic. Iceland, northern Norway and Sweden, Finland and large areas of Russia and the Balkans escaped the blight. So did the mountainous and desert areas of the Near East.”

Mary K. Matossian

Knowing that toxin producing fungi like Fusarium flourish in moist conditions, Matossian speculates that the ingestion of moldy grain by the rats markedly increased the mortality of rats thus increasing the likelihood that the rat flea would move to other animals including humans. (The mortality of horses, cattle, sheep and goats was exceptionally high during the wet period between 1345 and 1350). In addition, the humans were eating bread made with the moldy toxin-infested grain. The ingestion of mycotoxins would have compromised their immune system making them more likely to die when exposed to the plague pathogen. Thus, the increased mortality rates in some places most likely can be explained by a combination of the virulence of Yersinia pesti, the bacteria that causes disease known as the Black Death, and immune deficiencies caused by mycotoxin food-poisoning in both rats and humans

“Man does not live by bread alone - but he must have bread. And he must have bread that is truly the staff of life, not a scepter of death.”

Mary K. Matossian

Next Post: The Rats, the Flea and the Weather

After the Fall of Rome

The first civilizations (Sumeria, Eygpt, Myceanea etc.) were more or less isolated from each other. With the second rise of civilization in Europe and Asia (i.e. the Roman Empire and the Han Dynasty in China) things get a bit more complicated. This was because the world’s population centers were no longer isolated from each other. That meant something new – the movement of human diseases.

Routine travel by sailing ships and by caravan across the length and breadth of the Old World began to reach its peak organization between 100 and 200 CE. During this time thousands of people began to travel the trade routes established between the four centers of civilization, from China through India and the Middle East to the Mediterranean and back. And with them came disease, human disease.

According to William McNeil in his book Plagues and Peoples, between 150 and 600 AD, “new” infectious diseases attacked the people of both China and Rome. These diseases killed millions of people and the social impact of this heavy death toll was devastating. One of those diseases was Bubonic Plague. This disease is believed to have been a part of the demise of the Roman Empire but in the 1300’s it hit Europe with a vengeance.

Let’s take another look at the climate over the last 6000 years (graph above). Look at the temperatures after Rome declines. There is a small dip in the global temperature. That was enough to bring down Rome and plunge the European world into a second Dark Age, or the Early Middle Ages.

The temperature rises again to where it was before Rome declined. This is time period historians call the High Middle Ages. It is also called the Medieval Warm Period. This was a good time in Europe. During this period populations increased in Europe and wheat grew well all the way north into Scandinavia.

Then the global temperature falls again during a time called the Late Middle Ages. This is the beginning of what scientists call The Little Ice Age.

“Around 1300, centuries of European prosperity and growth came to a halt. A series of famines and plagues, such as the Great Famine of 1315–1317 and the Black Death, reduced the population by as much as half according to some estimates. Along with depopulation came social unrest and endemic warfare.”

The Little Ice Age Temperature Graph

The temperature rises slightly again (see graph above) and we have the Renaissance, another good time in Western Europe and then Europe gets hit again with cold temperatures, the coldest period of the time called the Little Ice Age.

In his book, "The Little Ice Age: How Climate Made History, 1300–1850", anthropology professor Brian Fagan of the University of California at Santa Barbara, tells of the plight of European peasants during the 1300 to 1850 chill: famines, hypothermia, bread riots, and the rise of despotic leaders brutalizing an increasingly dispirited peasantry. "In the late 17th century," writes Fagan, "agriculture had dropped off so dramatically that Alpine villagers lived on bread made from ground nutshells mixed with barley and oat flour. Finland lost perhaps a third of its population to starvation and disease.”

Can you see a pattern here? When the weather is good for farming, populations grow and prosper. When weather for is bad for farming, populations decrease because of starvation, disease, and warfare.

Next Post: And There is Even More to it Than That!