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

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