Читать книгу The Power of Plagues - Irwin W. Sherman - Страница 43

Yersin discovered the plague microbe, but he did not find the means whereby the disease could be transmitted. He asked himself: was it airborne, or did it have to do with rat feces or dried rat urine? The answer to the question came from Paul-Louis Simond (1858-1947), a French army physician who was sent by Pasteur to Vietnam and India to follow up on Yersin’s observations. Simond noted that not only were there large numbers of dead and dying rats in the streets and buildings, but that 20 laborers in a wool factory who had been cleaning the floor of dead rats had died of plague, while none of the other factory workers who had no contact with rats became ill. He began to suspect that there must be an intermediary between a dead rat and a human, and that the intermediary might be the rat flea Xenopsylla cheopis (Fig. 4.7). Simond found that healthy rats groomed themselves and had few fleas, but that sick rats unable to groom their fur had many; and when the rats died, the fleas moved off onto other healthy rats or onto humans. To prove this he did an experiment: a sick rat was placed at the bottom of a jar, and above this he suspended a healthy rat in a wire mesh cage; although the healthy rat had no direct contact with the plague-infected one, it did become infected by exposure to its fleas (which he determined could jump as high as 4 in. without any difficulty). As a control Simond placed a sick rat without fleas together with healthy rats in a jar. None of the healthy ones became sick, but when he introduced fleas into the jar, the healthy rats developed plague and died. On June 2, 1898, he wrote Pasteur that the problem of plague transmission had been solved.

Figure 4.7 Flea as seen with the scanning electron microscope. Courtesy CDC/Janice Haney Carr

The most common scenario for the origins of the Black Death, i.e., the second pandemic, is that its source was microbes left over from the first pandemic (the Justinian plague) that had moved eastward and remained endemic for 7 centuries in voles, marmots, gerbils, and the highly susceptible black rats (Rattus rattus) of the arid plateau of central Asia (roughly corresponding today to Turkestan). Plague-infected rats moved westward along the caravan routes between Asia and the Mediterranean known as the Silk Road, and in this way plague traveled from central Asia, around the Caspian Sea, to the Crimea. There the rats boarded ships and moved from port to port and country to country, spreading plague to the human populations living in filthy, rat-infested cities. This has been described poetically in Robert Browning’s poem “The Pied Piper,” which is based on a legend that on June 26, 1284, the German city of Hamelin became infested with rats. A Pied Piper comes to Hamelin and agrees to rid the city of rats for payment of a large sum of money. He is able to enchant the rats by playing his flute, leading them to the river, where they drown. But when the city fathers refuse payment, the Pied Piper leads the children to a cave, where they disappear. Versions of the tale were gathered by the Grimm brothers (1812), and this plague-inspired tale has over time come to have a moral interpretation: evil will befall those who do not carry out their promises.

Simond’s hypothesis was that the transmission of plague was from the black rat (R. rattus) via the rat flea (X. cheopis) to humans. This has been the accepted scenario for decades. Recently, however, this has been called into question. First, there is a complete lack of evidence of the involvement of rats and rat fleas in the historical epidemics in Europe; and second, the speed of transmission of the epidemics was very different: while the medieval epidemics spread extremely rapidly, the modern epidemics spread rather slowly. Although rats may have been important in warmer countries such as China and India, in northern Europe there are no historical references to rats. For example, there is no mention of rats during the plague epidemics in London during the 1600s, and Samuel Pepys, who in his diary described trifling events in great detail, makes no mention of sick or dead rats. It is the view of Hufthammer and Walløe that there were in fact very few rats in northern Europe at the time, and collections of ancient rat bones suggest that they were patchily distributed. It has been proposed that the Black Death cannot be attributed to a singular introduction of Y. pestis but to repeated climate-driven reintroductions from a reservoir of gerbil populations in Asia into Europe via caravans. It is hypothesized that since rats were uncommon in medieval Europe it is unlikely that they were responsible for the dissemination of human plague during the second pandemic. Instead, it is suggested that the mode of transmission of Y. pestis during the time of the Black Death was not, as shown by Simond, via the black rat and the rat flea X. cheopis (during the third pandemic), but rather directly from human to human by the human flea Pulex irritans and the human body louse Pediculus humanus humanus. Hufthammer and Walløe add that “they were present in all European countries and in sufficient numbers to be real candidates during ancient human epidemics” and that they were present in people’s clothing and bedding in the Middle Ages and early modern times. Further, it has been shown experimentally that P. irritans is capable of transmitting plague from a dying human plague victim to guinea pigs and rats and that Y. pestis can remain in soil in burrows dug by small mammals.

According to this hypothesis, plague was carried over long distances by people in their clothing and in the wool and other goods they transported. It was caravans passing through Asian plague foci that were responsible for transporting plague between Asia and Europe; camels are known to become infected relatively easy from infected fleas in plague foci and can transmit the disease to humans. After a plague infection established itself in a caravan (in its animals, in the traders or in in fleas in its cargo) the disease could have spread to other caravans during the time when goods and animals were redistributed and transported across Eurasian trade routes.

This hypothesis, the authors claim, unlike the rat model, can account for the rapid spread of plague epidemics and also explains why all members of one household in a town might have become plague victims while neighboring households escaped. The maritime import of plague is most evident during the second pandemic in the isolation of ships before unloading their cargo, and the black rat may have played a role in maintaining plague outbreaks on ships as well as importing plague into harbors, but its role as a potential plague reservoir in Europe can be questioned.

DNA sequences obtained from skeletons in Germany spanning the 14th to 17th centuries suggest that plague outbreaks in Europe did not arrive from the East but rather that a virulent strain of Y. pestis remained (or was reimported) for several centuries in Europe, while over time it worked its way back to Asia, causing outbreaks and killing millions in 19th-century China, including the one that reappeared in 1994 in epidemic form in countries including Malawi, Mozambique, and India, and that afflicts Madagascar today.