Kristina Lenz & Nils Hybel. Scandinavian Journal of History. Volume 41, Issue 1, February 2016.
The purpose of the present research is to examine various theories concerning the origin of the Black Death, to record its routes of dissemination in the Nordic countries and across the British Isles, and to compare the pattern of that dissemination with trade routes carrying grain throughout northern Europe in the period up to and including 1350.
For the major part of the 20th century the Black Death was afforded little attention by Danish researchers of medieval history. Since the 1930s, and in common with many of their eminent British, French, and German colleagues, they have if not neglected then certainly played down the significance of the Black Death. The first Danish account of the Black Death and ensuing epidemics was not published until 1991. Erik Ulsig’s article ‘Pest og befolkningsnedgang i Danmark i det 14. århundrede’ [The Plague and its Diminishing Effect on the Population in the 14th Century] was seminal for Danish research; at the same time it relied heavily on a historiographic tradition dating back to the second half of the 19th century, when the Black Death was considered to have had a revolutionary impact not only on demography but also on the social and economic development of society. Ulsig demonstrated that outbreaks of the plague in Denmark correlated chronologically with a well-documented pattern known to have been prevalent in northern Europe and he advanced the hypothesis that the plague was bubonic in origin and spread along the trade routes of Europe, carried like metastases by the black rat and its parasitic fleas. The theory that it was primarily the trade in grain that gave rise to an explosive spread of the plague over vast areas was suggested the following year by the Norwegian scholar Ole J. Benedictow, a specialist in the history of epidemiology. In Plague in the Late Medieval Nordic Countries, Benedictow’s principal argument in support of this theory was that the main carrier of the disease, the rat flea, was able to survive on grain. How he reached this conclusion is not entirely clear, partly because at the time of publication the history of the grain trade in northern Europe shortly before the Black Death was obscure, although much substantive evidence has since seen the light of day.
The Epidemiology, Symptomatology, and Mortality of the Black Death
In 1894 microbiologists identified Yersinia pestis as the cause of plague as we know it in modern times. Historic pandemics, like the Black Death, were also attributed to this bacterium. As the Black Death is significantly different from the ‘modern’ bubonic plague of the 20th century, both epidemiologically and symptomatically, this interpretation has since been challenged by doctors and historians. Furthermore, many zoologists have doubted that the rat and its parasitic fleas, responsible for the spread of bubonic plague in the present day, could have survived in northern Europe at the time of the Black Death.
However, since the start of the new millennium, research teams have provided ample examples of incontrovertible evidence in the shape of Yersinia pestis DNA and antigen from victims of the Black Death that leave little doubt that the plague ravaging the Middle Ages was a Yersinia pestis infection. However, before concluding that the Black Death and the bubonic plague of the modern age were spread by the same agent, a number of epidemiological questions need to be addressed.
Yersinia pestis is primarily a bacterium found in rodents and transmitted by fleas. Early 20th-century research into contemporary outbreaks of bubonic plague identified the black rat, Rattus rattus, as the main agent of the disease and also applied this retrospectively to the Black Death. However, there is no evidence to suggest that rats were involved in the spread of the plague in the Middle Ages. No contemporary historical accounts exist of dead rats being found ahead of an outbreak of the plague, as witnessed in Asia during the third pandemic. Until the middle of the 19th century no evidence had been found to suggest that the black rat lived in Europe in the Middle Ages. This may be due to the fact that the tiny bones of rats seem to have attracted little attention from archaeologists; only recently have they begun to search systematically for the distribution of animal remains in archaeological digs, an exercise that has produced plenty of evidence of the existence of the black rat in northern Europe during the Middle Ages.
Many scholars have doubted that a subtropical animal like the black rat could survive and multiply north of the Alps in the mid-14th century, a period when evidence suggests that Europe was entering ‘the minor Ice Age’. However, it is significant that these somewhat uncertain signs of climate change have not been supported by accompanying reports of famine, as documented in Europe in the High Middle Ages. Apart from severe frost during the winters of 1305-1306, 1309-1310, 1334-1335, and 1338-1339, studies of the climate in northern Europe in the 14th century have shown that the period leading up to the Black Death was characterized by extraordinarily mild weather devoid of any frost or snow. The great famine in the years 1315-1317, caused by heavy rainfall, is not relevant in the present context. Research into the climate of the latter part of the 14th century, when Europe experienced cyclical attacks of the plague every 10 years, shows a similar pattern. Every new attack was preceded by a period of relatively mild weather. Research also demonstrates that the epidemics were often followed by a period of extremely cold weather, from which it is reasonable to conclude that those rats not brought down by the plague would have perished in the cold.
At this stage one may well ask why climate is seen as such an important factor in this context. Although the rat roamed at large in the warm Mediterranean countryside, it sought the comfort of heated dwellings in its conquest of the north. Through time the rat became more and more closely associated with human dwellings and especially with their granaries, from which they rarely strayed more than 200 metres. To insist that the black rat was unable to exist in the cold climes of the north is to underestimate its ability to adapt, particularly as we know that it can thrive in both the Atlantic climate of the Hebrides and on the sub-Antarctic island of Macquarie.
Like the rat, the existence of its parasitic flea in northern Europe has been doubted by many scholars of medieval history. According to the Indian Plague Research Commission, not all fleas were capable of transmitting the disease to human beings. The rat flea, Xenopsylla cheopis, admittedly had the most favourable conditions to do so, but research demonstrates that it was extremely sensitive to changes in temperature and humidity. Habitually the flea lives in the warm fur of the rat and will only reluctantly abandon its cosy abode if its host succumbs to disease. Fleas are far more likely to migrate to the people visiting a plague-ridden house than venture out into the cold. In this way, even in the depth of winter, fleas would be transported in the garments of visitors to neighbouring houses, where they could generate a new epidemic among the colony of rats in their new dwelling completely independently of climatic conditions.
Perhaps the rat and its parasitic flea were not the only agents in terms of the dissemination of the disease at the time of the Black Death. It is possible that the human flea Pulex irritans played a role, as suggested by the French physician and historian Jean-Noël Biraben. Certainly, research over the past 10 years has demonstrated how several species of human fleas are capable of transmitting Yersinia pestis between mammals within a few days of being infected themselves. This research has challenged the conclusions of previous studies that a blocked flea, i.e. a rat flea, as the only known species with two stomachs, enables the Yersinia pestis bacteria to multiply faster than they would if passed through a single digestive system. Despite persistent efforts, research into various outbreaks of bubonic plague in modern times has not succeeded in identifying the rat flea, while overwhelming numbers of human fleas have been present in dwellings ridden by the plague. Furthermore, American scientists have recently managed to prove the ability of the human flea to act as a vector for the Yersinia pestis bacterium.
As Yersinia pestis is a bacterium prevalent among rodents, we must ask whether other animals could have been responsible for transmitting the plague in the Middle Ages. Just because the bubonic plague was a disease borne by rats in the early 20th century, it does not mean that other ‘domestic animals’ should be ruled out from spreading the disease at the time of the Black Death. In the United States alone, 23 cases have been recorded of cats transmitting the plague bacterium to humans. In Africa, the common shrew has been shown to host the Yersinia pestis bacterium. However, as we have no evidence of these animals playing a part in the transmission of the plague in the Middle Ages, this theory can only be speculative.
An issue that has caused many scholars to question whether the Black Death and the bubonic plague of modern times share the same agent is the mortality rate. Nothing exact is known about the overall mortality rate caused by the plague during the Middle Ages, but several analyses of the existing sources suggest that as many as 75-80% of the population perished in the summer of 1348. One of the first accounts of the demographic consequences of the Black Death seen in a historical context concludes that two-thirds of the clergy in the West Riding and around half of the clergy in the East Riding perished. Subsequent British research confirms a mortality rate of around 50%. In his research into the English inquisitiones post mortem, the American demographer J.C. Russell established the somewhat less dramatic mortality level of 40% for the period between 1348 and 1377, during which England was ravaged by four outbreaks of plague. It is generally accepted that population figures were more or less restored between the outbreaks in 1348-1350, 1360-1361, 1369, and 1375. In his detailed study of the demographic development of the English parish of Halesowen, Zvi Razi estimates mortality to be around 43% in 1348-1349, 14% in 1361-1362, 17% in 1369, and 12% in 1375. These findings parallel a note made in 1382 by Raymundus Chalmelli de Vinario, a personal physician to Pope Clement VII, about the mortality of the disease, which indicates that over time the population of the Middle Ages developed an immunity to the plague.
The third pandemic in Asia did not quite equal these levels, even in India, where most of the outbreaks of plague occurred in the 20th century. In some major cities, like Mumbai, mortality was as low as 3%, although in rural districts like Punjab it reached 36%. It is important, however, to distinguish between case fatality, i.e. the proportion of deaths out of the number of patients actually infected by disease, and the overall mortality of the population at large. In terms of case fatality, today’s figures are 40-60% in outbreaks of bubonic plague, while outbreaks of pneumonic plague see figures soar to almost 100% if the victims are not treated with antibiotics. In terms of the Black Death, we have only scanty information relating to case mortality; however, contemporary accounts suggest that some people infected by the plague did, in fact, survive the disease. Today very few people have been infected by the bubonic plague more than once; however, it is a well-known fact that patients who have been treated with antibiotics do not develop the same degree of life-long immunity as those who have managed to survive without any treatment.
The main controversy, however, in terms of epidemiology has to do with the dissemination of the disease. Based on accounts of the Black Death spreading in just four years to most parts of Europe, critics of the rat-flea model have argued that this pestilence must have been extremely contagious, particularly in comparison with the bubonic plague of the 20th century, which is known to spread over land by no more than 15 kilometres per year from its epicentre.
The incubation period for bubonic plague is three to five days, whereas contemporary accounts of the plague in the Middle Ages attest that up to 21 days could pass from the time the first member of a household was struck by the disease until the next victim was infected. According to the rat-flea model promoted by Simond, it takes exactly 21 days from the point when the Yersinia pestis bacterium has entered a dwelling until infection has wiped out its resident colony of rats, the invading fleas have transmitted the infection and the disease has erupted among the inhabitants of the dwelling.
The dissemination of the disease within a local community is easy to explain. Visitors to plague-infected dwellings were often bitten by the infectious rat fleas abounding there or else carried rat fleas back home in their garments, resulting in an epizooty breaking out in another colony of rats.
It is more difficult to explain how the disease could spread over distances of hundreds of kilometres. Given the fact, already noted, that the rat only rarely moves further than 200 metres from human dwellings or granaries, the spread of the disease could only have been caused by passive transport. The predilection of Rattus rattus for seafaring vessels is well known, but a period at sea lasting more than 14 days would have completely eradicated a colony of rats already infected by Yersinia pestis.
The explanation for this seems to hinge on the rat flea. Since grain is part of the rat’s staple diet, its fleas have probably developed an ability to survive by hibernating in grain even where only tiny residues are to be found, for example in a ship’s hold. This would explain how the rat flea can be passively transported over great distances provided that the climate is not too hot or dry causing desiccation and death. Humidity during transportation enables infected fleas to survive for many weeks and would explain how the plague bacterium could spread over vast distances as long as it was carried by sea.
The Spread of the Black Death in the Nordic Countries and the British Isles
In order to account for the spread of the plague, this study traces the advance of the Black Death in both the Nordic countries and the British Isles. The Nordic sources are sparse but a certain pattern does seem to emerge when they are studied alongside their British counterparts. Only a year after the epidemic had ravaged southern Europe it struck Norway on two fronts: in Oslo, by then the largest town in the southern part of the country, and Bergen on the west coast. From these two points the infection spread inland along main roads and pilgrim routes, both south of the Oslo fjord and all the way up north to the Archdiocese of Nidaros, where the archbishop himself succumbed in 1349.
From Norway the plague must have been transmitted to Denmark, where, in the autumn of 1349, it erupted in the port of Halmstad in Halland on the east coast of the Kattegat. Closer scrutiny of the sources reveals that in the same year the infection also reached Ribe, then an important port on the west coast. The following year the Black Death had the entire country in its grip according to contemporary chronicles, annals, and anniversary of deaths registers.
With the exception of Gotland, where the plague reached Visby, none of the contemporary sources refers to the Black Death in Sweden. Iceland avoided the infection the first time round but was hit by the plague in the following century. No sources shed light on whether Finland was hit by the second pandemic.
History paints quite a different picture in terms of British sources. By all accounts, the Black Death spread from France in the summer of 1348 to the port of Weymouth on the southern coast of England, from whence it travelled very rapidly to other ports in both directions along the coast. It progressed up through the Bristol Channel to Bristol before advancing along the Severn to Gloucester. From here it spread inland towards the east along the main routes to London, but also north and north-west, eventually invading Wales. Simultaneously, as proved by research, the infection spread along the Thames from east to west to reach London towards the end of 1348.
In the same year the Black Death established itself in ports along the coast of the North Sea. The eastern parts of England were thus attacked on two fronts. From Lynn at the mouth of the Great Ouse the plague penetrated Cambridgeshire and the Diocese of Ely. From Grimsby and Hull, both situated at the entrance to the Humber, the epidemic advanced inland along the rivers of Trent and Ouse until it eventually reached the cathedral city of York in May 1349. From York the progress of the plague towards the north is less well documented; however, according to English chronicles it is believed to have reached the southern part of Scotland towards the end of 1349, while the rest of that country was invaded the following year.
According to an account written by the Irish monk John Clyn, the Black Death arrived in Dalkey and Drogheda, two small towns situated in the English colony known as the Pale on the east coast of Ireland, in August 1348. The explanation for such an early outbreak in this location must be that the infection was carried in provisions brought by sea direct from one of the ports along the south coast of England already hit by the epidemic. Figure 1 traces the routes of dissemination of the Black Death in the Nordic countries and in the British Isles.
Graph: Figure 1 The spread of the Black Death in northern Europe, 1346-1351.
The Trade Routes of Northern Europe
Research into the trade routes of northern Europe has shown that even the very early Middle Ages witnessed well-developed trade relations enjoying an extensive traffic network of vessels, partly along the coastline but also along the major rivers and canals, at that time constituting an extremely important part of the transport system of Europe. It is well documented that trade in northern Europe grew exponentially in the 12th century: the rapid growth of towns and markets, especially from the latter part of the century onwards, and advancements in maritime technology are just some of the signs of this commercial expansion. The most significant development in this context is that trade in heavy goods like timber, cloth, artefacts in limestone and lead, and not least grain over long distances rapidly gained impetus from the middle of the 12th century. Trade of this kind influenced patterns of consumption, technology, and specialization in agriculture; however, sadly, it also created routes of dissemination for epidemic diseases centuries before the Black Death devastated the population of Europe. In order to substantiate a hypothesis that the Black Death was a bubonic plague spread by rats and their parasitic fleas and aided by the transport of grain, we need to examine the trade in grain in northern Europe in the period preceding the Black Death. It should be emphasized at this stage that grain, like many other commodities in the Middle Ages, was transported by sea as mixed cargo. In other words, even vessels not regularly carrying grain would doubtless have shipped it on previous occasions, so that residues remained in the hold. In addition, all ships would obviously have carried victuals for the crew.
During the second half of the 13th century, the trade in grain overseas from English ports along the coast of the North Sea was dominated primarily by the export of wheat to Norway. However, in the first decade of the 14th century the export of grain was in decline. In 1303 Edward I had granted privileges to foreign merchants in the Carta Mercatoria; these included free trade and the avoidance of certain excise duties. These privileges were revoked in 1309 and the export of grain virtually ground to a halt. In contrast, the import of grain increased after 1322 when the Carta Mercatoria was once more reinstated. Imported grain comprised primarily oats and especially rye from the hinterland of the German towns on the Baltic Sea. The export of grain from north-east Germany and Poland to western Europe, by now heavily urbanized, can be traced far back in history. From very early on, towns in the Low Countries were supplied with grain from northern France, and by the 13th century France was the most important supplier of grain to Flanders. However, German grain also made its way to Flanders during the first half of the 12th century. These early supplies were derived from the areas along the Rhine and the Elbe, but during the 13th century import from Pomerania and Prussia increased and by about 1300 grain was imported to Flanders from all of the southern and south-eastern parts of the Baltic region. The grain was transported either along the traditional Hanseatic route, Lübeck-Hamburg, or ummeland, i.e. through Danish waters from the Baltic Sea to the Kattegat, passing the Skaw en route to the North Sea or Norway. As early as the second half of the 13th century, the supply of grain to the western part of Norway (particularly Bergen) by Hanseatic merchants was significant. However, research has shown that in the period leading up to the Black Death, English towns along the Channel between Exeter and Sandwich also nurtured close trade relations with both France and Spain. Furthermore, these English ports had well-developed trade relations with the English dominions in the west of France. The ports and towns of Flanders had grain delivered not only from Spain but also on occasion from Italy. Maritime traffic from the Mediterranean through the Straits of Gibraltar to Flanders and England gained impetus in the late 13th century, and Italian merchants played a progressive role in this development. This information concurs with contemporary accounts that the plague could have reached the southern coast of England on board ships direct from France or even Spain or Italy.
London, already a principal trade centre in the Middle Ages, was in regular contact with the rest of Europe, particularly through its relations with Flanders and Bruges. At the same time, trade routes from Europe incorporated the south-eastern part of England, particularly the ports along the Channel. For this reason the plague could easily have reached London very soon after ships had berthed in Weymouth; alternatively, it could already have been introduced by vessels arriving in London direct from the continent.
Poundage accounts that have survived since the granting of the Carta Mercatoria in 1303 testify to the fact that Lynn, Hull, Ipswich, and Boston, on the English coast of the North Sea, were vital hubs for the trade in grain. Through the shipping trade these ports were in close contact; at the same time they maintained trade relations with the major towns and cities in their hinterland thanks to a well-developed system of rivers. Consequently, once the plague reached one port, there was a real danger that the infection would spread inland and contaminate large swathes of the country.
Some grain exports, however, seem to move counter to the routes of dissemination of the plague. It is true that England exported wheat to Norway, but the lists of levies show that at the same time oats were imported from Flanders, not to mention rye from the Baltic countries. It is important to note that rat fleas do not necessarily need a full cargo of grain to travel the world. Any vessel engaged in trade could be the transmitter of the disease, provided that there was just a fraction of grain left in the hold. Whether on disembarkation the crew brought goods ashore or simply shook out sacks that had contained grain, fleas carried in the cargo would quickly rouse from their dormant state and target the nearest colony of rats only to set off a new epizooty.
The Icelandic Sagas describe the plague reaching Bergen from England by boat in 1349; this seems more than probable. A surviving register of vessels going in and out of Hull demonstrates the extent to which England exported wheat just before the Black Death; at the same time, other documents make it clear that as late as 1349 the King of England allowed the export of grain to Norway.
Source material pertaining to the Black Death in Denmark and Sweden is too sparse for any conclusion to be made about the spread of the disease in these lands, but there are strong indications that the plague entered Denmark via the two ports of Halmstad and Ribe as early as 1349. It is probable that the infection was introduced by ships either from Oslo or, more likely, by the so-called ‘ummeland voyagers’ travelling north of the Skaw into Danish waters en route to the Baltic countries to pick up grain for export. The disease could also have been brought into the country by Danish merchants or by the many foreign merchants who facilitated the extensive commercial trade between Denmark and the rest of Europe in the Middle Ages. Although evidence of the import of grain into Denmark is scanty, it is likely that more grain was imported into the country than exported. On the other hand, there is plenty of evidence to suggest that Danish agricultural exports were dominated by animal products and livestock. The trade routes of grain in northern Europe are indicated in Figure 2.
The Trade Routes of Grain and the Routes of Dissemination
If we compare Figure 2 with Figure 1, showing the spread of the bubonic plague, it is tempting to conclude that shipments of grain in northern Europe before 1350 played a significant part in the spread of the disease. However, as noted earlier, the infection was not spread directly along these trade routes. Rats and fleas and thus the contagion itself could also be spread by transport going in the opposite direction, carrying residues of grain and grain-based provisions that would feed the carriers of the disease on their journey. The traffic of vessels between western France and the southern parts of England and between the Low Countries and eastern parts of England were potential routes of contagion, as was the export of grain from eastern parts of England to Bergen. Along these routes the spread of the infection matched directly the trade routes of grain from France and the Low Countries to England and from England to Norway. The dissemination of the disease from England to Denmark via Ribe took place both along and counter to the trade route of grain. In the latter part of the 13th century Ribe was granted permission to import grain on a number of occasions. Merchants from that city are shown in English customs records from the first part of the 14th century as importing grain from ports on the east coast of England. The population of Norway was not infected from England alone. The considerable export of grain from Germany to Norway was equally important for the transmission of the disease, and with cargo going back along the coast of Halland and through Danish waters to the Baltic Sea the plague reached Denmark from the north along the same route that saw the populations of the Prussian towns and their hinterland fall victim to it. We know for certain that ships would dock at Skanør and the Skaw and other ports as they passed through Danish waters.
The spread of the infection from the coast further into Poland was probably caused by commercial traffic along the inland waterways, as occurred in England and mainland Europe. Trade in grain along the rivers of Europe was widespread: for example, we have evidence that grain was transported from central Poland and Ukraine along the Vistula to Torun, where it was bought up by merchants from western Europe and transported by boat to Norway, the British Isles, and the Low Countries.
This account of the spread of the Black Death in the British Isles and the Nordic countries, comparing the evidence with the trade routes of grain, demonstrates how it was possible for a Yersinia pestis infection to spread very rapidly over huge distances by means of the rat and its parasitic flea. Unlike the third pandemic in Asia, in which the disease was carried over land, geographical and trade-related conditions in Europe made it possible for the rat flea and thus the plague bacterium to be carried by ship overseas and inland on board vessels sailing along the rivers and extensive canal systems already well developed in the 14th century. As early as the 11th century, Adam of Bremen noted that marine transport was relatively speedy. In his survey of European shipping routes and the duration of voyages, he writes that it took two days to reach Flanders from Ribe and one and a half days to reach England. We are not suggesting that the trade in grain by sea was the only route of dissemination, but it appears to have been the most important factor in the regions that we have researched, not least because it gave rise to an almost explosive spread of the Black Death.
Despite the fact that an increasing quantity of clear evidence based on DNA and protein-based studies indicates that the Black Death was caused by the Yersinia pestis bacterium, there are still scholars who doubt this. By means of the present study, in which we have compared the trade routes of Europe, and the transport of grain along these routes, with the dissemination of the plague, we believe that we have demonstrated that it was possible for the bubonic plague to spread in giant leaps across the whole of Europe in just four years. Based on this research we believe that there can be no doubt remaining that the Black Death was an infection caused by Yersinia pestis and disseminated by the black rat and its parasitic flea.