The Transmission of the Black Death to Western Europe: A Critical Review of the Existing Evidence

Hans Ditrich. Mediterranean Historical Review. Volume 32, Issue 1, June 2017.

In 1347 the Black Death was introduced from the north-eastern coast of the Black Sea and the Sea of Asov towards southern and western Europe, where it then spread dramatically. A report by the Italian chronicler Gabriel de Mussis of the siege of Caffa (1345-47) is often credited as describing an early deployment of a “biological weapon”, thus triggering the “Black Death” in western Europe. He reports that Mongol troops threw plague victims into the city with catapults, thus contaminating the inhabitants. However, re-evaluation of historical, biological and epidemiological data indicates that the spread of the disease was probably an inevitable consequence of the intense trade relations along the coasts of the Black Sea and the Mediterranean. Therefore, the alleged catapulting of infected corpses would rather have been a marginal contribution to the diffusion of the disease (if it took place at all). The infection was subsequently spread by refugee ships via ports at Constantinople and along the Mediterranean trading routes and harbours towards Genoa, Marseille and Venice, thus initiating the Plague in Europe. The further propagation of the disease inland is still a matter of controversial discussions. However, epidemiological data indicate that the most essential animal vector for further distribution of the plague in central and northern Europe was probably the human louse (Pediculus humanis), instead of the oriental (or tropical) rat flea (Xenopsylla cheopis).

Introduction

In the year 1347 plague was transmitted from the sea of Asov and the north-eastern shores of the Black Sea to several Mediterranean maritime trade centres, thus igniting the “Black Death” in western and central Europe.

The siege of Caffa (now Fedossija, 1345-47) is regarded as an early historical report on an effective deployment of biological weapons in many publications. This assessment is based on a document written by the chronicler de Mussis (Gabriele, ca. 1280-1356, Piacenza, Italy). He describes infection of the city inhabitants from corpses catapulted into the enclosed city—thus an attack with a highly contagious agent in a violent conflict, that is, the deployment of what today would be termed a “bio-weapon”.

Plague bacteria (Yersinia pestis) are classified in the most dangerous category (A) of potential bio-weaponry agents by the Centers for Disease Control and Prevention (CDC), a department of the United States government. An attack involving these germs would therefore be graded as a severe form of assault. However, certain knowledge of the principles (and limits) of infectious diseases, the nature of the plague bacteria and the biology of its vectors would be required to estimate the effects, and such information that was not available at that time. As with all bio-weapons, the production, stockpiling, dissemination and protection of national forces would be critical issues. Nevertheless, these considerations would require an awareness of modern concepts of contagion that the Mongolian commander Khan Dschani (or Jani) Beg could not have possessed at the time.

Irrespective of whether it was a consequence of the conflict at Caffa or of trading with already infected bases at the sea of Asov and the north-eastern Black Sea (or both), within the same year contaminated ships had spread the disease via the Bosporus all along the Mediterranean coasts and harbours. The Genoese outpost Pera (today’s Beyoğlu) near Constantinople was probably infected around the end of June or early July. Further plague outbreaks were reported in Messina in October, in the islands Sardinia, Corsica and Elba in December, and in Genoa and Marseille by the end of 1347. A little later the infection also reached Venice via the Ionian and Adriatic Sea. Upon reaching these prominent Mediterranean trading harbours, landlocked spreading of the contagion became prevalent in western and central Europe.

However, some additional questions remain unanswered. Spreading of the plague beyond areas near the coasts of southern Europe cannot be satisfactorily explained by the classical model of transmission (rat > flea > human). Newer studies in molecular biology, biology and epidemiology seem to justify a reconsideration of this scenario.

De Mussis’s Account

Gabriele de Mussis’s report of the events at Caffa was probably written a few years after the disease spread to Genoa (1347/8); however, the original text is lost and there remains only a later copy (after 1367), which is part of an anthology kept in the Wrocław University Library, Poland (Codex 59). However, it is not certain to what degree the latter text might have been altered, for instance by incorporating religious and classical motifs.

Several texts assume that the notary de Mussis was an eye-witness to the events in Caffa—an interpretation that is based on ambiguous phrasings in the text. The comments by Henschel (1848) already partly challenge this supposition. Moreover, Tononi (1883) indicated that de Mussis was stationed in his hometown Piacenza during the period in question. Consequently, the notary’s vivid descriptions must have been based on narrations from passing travellers (who themselves were perhaps not eye-witnesses either—see below). De Mussis describes the relevant scene—the alleged deployment of the Plague as a bio-weapon—as follows:

The dying Tartars, stunned and stupefied by the immensity of the disaster brought about by the disease, and realizing that they had no hope of escape, lost interest in the siege. But they ordered corpses to be placed in catapults and lobbed into the city in the hope that the intolerable stench would kill everyone inside. What seemed like mountains of dead were thrown into the city and the Christians could not hide or flee or escape from them, although they dumped as many of the bodies as they could in the sea. And soon the rotting corpses tainted the air and poisoned the water supply, and the stench was so overwhelming that hardly one in several thousand was in a position to flee the remains of the Tartar army. Moreover one infected man could carry the poison to others, and infect people and places with the disease by look alone. No one knew, or could discover, a means of defence.

The report stresses two details (among other peculiarities): the reportedly high amount of bodies and the horrible stench they spread. While de Mussis reports the killing of “thousands upon thousands [of Tartars] every day”, it might be assumed that the number of thrown corpses either has been exaggerated, or that several of the bodies were not freshly dead (but rather stored for a while), or both. Storage is also suggested by the mentioned evil odours that—given average temperatures of about 2 °C (January) to 5 °C (March)—would indicate that such corpses had already been dead for at least five days to a week. It might therefore be assumed that the Mongols, if they really decided to get rid of the deceased by catapulting, were facing severe difficulties in providing appropriate burials. This particular problem was reported from most infested cities in the ensuing centuries. Accordingly, the incident might primarily not have been motivated by military strategy, but could rather be seen as a sanitary measure. To consider such an action a deliberate bio-weapon assault would instead imply an understanding of germs and contagion that was not developed before the late nineteenth century, and may thus be regarded as anachronistic. Consequently, the alleged event was hardly an act of “biological warfare” as such.

Critical Considerations

Examining de Mussis’s report on the siege of Caffa and the spreading of the Black Death to central and western Europe, a few central implications need attention. In the absence of fresh historical data, some of these questions can only be answered by aetiological considerations, probability and with decisive use of Occam’s razor. Nevertheless, the accumulating quantity of DNA data has shed new light on various still controversial issues.

As already postulated by Tononi (1884), de Mussis could not have been an eyewitness of the events in Caffa, but rather wrote his commentary after facing the devastating epidemic back home in the vicinity of Piacenza and Genoa. It should, however, also be kept in mind that the notary’s original report is lost, and we have only a later, potentially edited version on which to base our inquiry.

It is possible that de Mussis spoke directly with eyewitnesses of the siege, but this seems unlikely since to do so the narrators would have had to spend around six months on heavily infected ships, in close contact with ill or dying plague victims. It seems more probable that de Mussis gathered his information from retellings by seamen who were not (at least initially) aboard the (infected) ships fleeing from Caffa.

The authenticity of the alleged bio-weapon attack has already been questioned by several authors. Notwithstanding the inaccuracies inherent to retelling, along with the exaggerations and contemporary concepts contained in de Mussis’s report, we can reasonably accept that the Mongols might indeed have catapulted dead bodies into Caffa. That said, the large-scale lobbing of corpses reported by de Mussis would primarily have been a consequence of the growing shortage of burial ground, whether individual or mass graves. Consequently, if the said lobbing of corpses ever happened, it was likely a makeshift solution, given the huge logistical demands of burying a large number of plague victims while also keeping up the siege operations. On the other hand, there is a high probability that the infection was transmitted into the city at about the same time from the supply ships, suggesting that the catapulting story is a classic post hoc fallacy.

In addition, given that most Mongols of the Golden Horde had already become Muslims under their previous Khan Usbek (r. 1312-42), the idea of taking advantage of the outbreak of disease is questionable, since the Muslim attitude towards disease at that time—according to the Qur’an (e.g., Surah IX and IV) and further instructions by Mohammed—would generally favour letting God’s will take its course (and that an individual should neither leave nor enter an infested region). Moreover, to forfeit one’s chance of future martyrdom by catching the disease might instead constitute a sin on its own. In Islam, the prevailing explanation for diseases was the direct will of God, although communication by contagion was occasionally suspected.

Generally, the most common contemporary explanations of the plague (and many other diseases) were “foul” air (miasmas)—sometimes expelled from the ground by earthquakes—adverse astrological positions of stars (especially comets), poisoning from certain foods (leading to an imbalance of inner juices—the four humours of Hippocrates and Galen), contaminated wells (e.g., by Jews), witchcraft and above all the wrath of God.

Contagion at Caffa

In that fateful 1346 the Mongol army was probably stricken hard by the plague. The epidemic likely originated among Central Asian populations of rodents and lagomorphs such as Tarbagans (Marmota sibirica) and Picas (Ochotona pallasi), and possibly also Bobaks (Marmota bobak) and Great Gerbils (Rhombomys opimus)—all marmot-like mammals that remain repositories for plague bacteria to this day. Climate fluctuations may have influenced the flare of the disease. Furthermore, the pelts of these animals were highly appreciated as fur trimmings, and were therefore in demand as commodities.

In time the disease spread to China, where its effects later might have contributed to the changeover from the Mongolian Yuan dynasty to the Ming (1368), affecting other Asian regions as well. It was also transmitted westwards via the northern branch of the Silk Road, reaching Astrachan in 1345, for example. Subsequently, the southern branch of the Silk Road (via Tabriz and Baghdad) and probably also Red Sea naval routes became further pathways of infection. In the same year the Mongol siege on Caffa was renewed, after a retreat in 1344. The plague spread among the troops, who were plausibly not living in the best hygienic or sanitary conditions. Consequently, the commander in charge (or Khan Jani Beg himself) faced the problem of how to ensure combat capacity and motivation of the troops in the face of an epidemic that probably cost more lives than the fighting itself.

The Mongol trebuchets would normally be aimed at the city bulwarks in order to rupture them, so they had to be repositioned for casting objects higher, over the crown of the walls. Catapulting putrescent material together with stones and containers with incendiary substances was a common practice already in antique warfare, and in the present case the trebuchets were certainly capable of hurling a human corpse over the required distance.

While the act of catapulting corpses of plague victims into the city would certainly have a strong psychological effect on the inhabitants, aside from possible social and religious constraints this ploy for disseminating plague is less efficient than one might presume: without the intervention of a vector (fleas) and/or aerosol dispersal, the infectious potential of a deceased body remains limited, as in order for it to spread the bacteria must enter the bloodstream of the healthy host directly—via an open wound, say. (The same applies to other contaminated objects like textiles: the bacteria have to enter the body in sufficient quantities to generate plague).

A siege is probably the best cordon sanitaire imaginable. It might well have been possible for the black rat to get over barricades, no-man’s-land and city walls. However, it seems rather questionable why such a migration towards the city should have occurred in substantial numbers. Food was probably available on both sides of the front, perhaps even more abundant on the Mongolian side without the walls. It has also been observed that sick rats become lethargic and tend to hide. Thus, the suggestion that the city was infected by rats migrating from the Mongol camp into the city—as proposed by Wheelis—seems untenable.

According to contemporary reports, throughout the course of the siege the harbour and waterways of Caffa were accessible: the Mongol troops were apparently incapable of blocking the seaways, rendering ineffective the common tactic of starving out a besieged city. Support for the city was therefore provided by ships from the surrounding bases. As long as an adequate number of vessels could reach the harbour, this supply might have been sufficient (albeit with restrictions) for prolonged resistance, were it not for the fact that several of the supplying bases—mainly those in the northeast—appear to have become infected by 1346. By this time the Black Death had likely reached the Genoese bases (colonies) at the Sea of Azov (e.g., Asov, Vosporo), and likewise the Venetian base Tana, and possibly the north-eastern coast of the Black Sea as well. It is therefore quite probable that the plague reached Caffa in the same way as other coastal cities did thereafter—namely via transmission by ship rats, infected seamen and contaminated supplies (especially grain and fabrics).

Outline of Plague Aetiology

The causative agent for Plague—the bacterium Yersinia pestis—was identified by both Alexandre Yersin and Kitasato Shibasaburo almost simultaneously in 1894. Four years later, Paul-Louis Simond demonstrated the classical model of transmission. It involves a population of rats that is infested with oriental (or tropical) rat fleas (Xenopsylla cheopis) as the vector for the bacteria. Other fleas, such as the northern rat flea (Nosopsyllus fasciatus) or the human flea (Pulex irritans), are less efficient vectors, and probably play a lesser role in the initial transmission of the disease.

The black rat (Rattus rattus, a.k.a. house rat, roof rat or ship rat) was abundant in ships, buildings and stores, and had a certain fondness for climbing, unlike the dominant brown rat (Rattus norvegicus) that prefers underground structures like cellars, pipes, sewers and so forth. The ship rat shared man’s resources, but at the time was regarded rather as an annoyance and vermin than as a potential carrier of deadly diseases. Thus, pest control was probably not very rigorously executed.

If an oriental rat flea sucks blood from an already plague-infected rodent, an anterior part of its digestive system (Proventriculus) may become clogged. The flea—attempting to still its hunger and to rehydrate—then bites other rats (or as these become scarce, also humans), thus disseminating the bacteria to hitherto uninfected hosts. However, it should be noted that the flea X. cheopis is restricted to warm or moderate climate and becomes barely infective at low (10 °C) or very high (27-39 °C) temperatures.

Once a human is infected, the bacteria will spread via the blood stream to the regional lymph-nodes and multiply, eventually causing a bubo. Subsequently, the liver, spleen and other organs become infested. If the lung becomes a main site of infection (secondary) pneumonic plague will develop, which is capable of spreading directly from human to human via droplet infection (generating primary pneumonic plague). Massive bacterial proliferation in the blood would result in toxaemia (i.e., septicaemic plague). Other known variants in the progression of the disease (skin, meningitis, pharyngeal, abortive) are rare and probably of less epidemic relevance.

It would usually take from two to six days between intrusion of the bacteria and the development of symptoms like fever, headache, nausea, swollen lymph nodes, and from then eventually up to two days more until death. Rapid infection and high mortality are reported for direct human-to-human transmission via coughing—inhalation of contaminated sputum (primary pneumonic plague). However, only a relatively few (about 5% of infected persons) ever develop pneumonic plague; it always accompanies bubonic plague and its high and rapid mortality (two to five days) hinders the human-to-human distribution of the contagion over longer distances.

Given contact with an open wound and entering of a sufficient number of bacteria into the blood stream, the disease would normally take several days to be lethal. Even assuming an infection via airway—pulmonic plague—it would take at least one to three days from infection to death. Accordingly, assertions like de Mussis’s that “it was as though arrows were raining down from heaven to strike” (stressing the dramatic suddenness of death), remain to be seen in the light of the current scholarship.

Without modern treatment (antibiotics), the mortality is about 40-60% for the bubonic and nearly total (over 90%) for the pulmonic variant. Nevertheless, the properties reported here pertain to the contemporary strains of Y. pestis and may not fully delineate the features of the Black Death strain(s).

It is reported that fleas cannot transmit the disease from human to human as no sufficiently high concentration of bacteria can be reached. Instead a “refuelling” from a plague-ridden rat seems required to advance the infection further. Following this scenario, close contact with severely infested rat populations seems required for epidemic spreading of the plague (disregarding airborne transmission).

This classical transmission model, however, leaves several questions open as far as western and central Europe are concerned:

  • How would a mass infection in a colder climate (without infective oriental fleas) propagate with such a high infection rate?
  • How can we explain such a rapid spread over the continent, superseding the migration potential of the black rat?
  • Why did the mass death of rats go unnoticed, or apparently not happen in some of the communities that were heavily struck by plague?
  • How would the infection propagate once the local rat population was depleted?
  • How would the disease persist, once the rat population was diminished?

Infection of Constantinople

Assuming that “among those who escaped from Caffa by boat were a few sailors who had been infected with the poisonous disease” (de Mussis), could these sailors then have been the principal source of the pandemic known as the “Black Death” in Europe?

From the above considerations, there would be no fundamental difference if members of the crews of the fleeing ships had been contaminated by infected corpses catapulted into the city. Indeed, the ship-rat populations and also some provisions were probably already contaminated. Some of the seamen may also have picked up the disease at previous stops of the vessels. Aside from the ships that evacuated Caffa, and were probably crowded with refugees, the typical freight from the northeast of the black sea was textiles (silk), furs, grain and slaves—all of them high-risk goods for plague transmission. It can therefore be assumed that all ships from the Crimean Peninsula or the Sea of Azov might have been contaminated—whether coming from Caffa or not.

Although the (Genoese) galley of that time was equipped with one or two masts, it was mainly powered by 120-150 oarsmen (comparable to the Venetian great galley). The ships were rather crowded, and the rowing benches densely packed. Assuming an average speed of 2-3 knots (3.7-5.6 km/h), a daily travel of approximately 100 kilometres would have been possible under favourable conditions. However, to cover the approximately 700 kilometres separating Caffa from the colony at Pera (Beyoğlu) aside Constantinople, a direct course is unlikely. This bearing would have taken at least 125 hours, assuming travelling also at night and that all other circumstances were favourable, especially the winds. Instead, the ships probably followed the western coast of the Black Sea with frequent landings, thus considerably lengthening the travel (approximately 1200 km).

Unmistakable signs of infestation would have been observed four days after boarding (at the latest). The first cases of death would have occurred after at least around six to eight days of travelling. Consequently, when reaching Pera, numerous people on the incoming ships must have shown advanced signs of the disease, and meanwhile the crews must already have been severely diminished.

Even though it is heavily “inspired” by classical literature, the report by Emperor John VI Kantakouzenos of Constantinople points out that the plague reached the city in early July. John VI had entered the capital as the victor of the Byzantine civil war (Second Palaiologan Civil war, 1341-47) accompanied by about 1000 soldiers, mostly mercenaries, including Serbians, Bulgarians and Turks, in February. We might even speculate that some troopers could already have been infected elsewhere, and that the nascent epidemic had not yet been recognized.

Given the tensions between the Genoese, Byzantine, Venetian and Ottoman political and economic “stakeholders” in the rather small territory that controls access to the Black Sea and Mediterranean trade routes, it is not possible to deduce a clear chain of propagation for the disease. Constantinople might have been contaminated (e.g., by people infected in Pera) around June, but no detailed records are present. Nevertheless, a substantial number of the crew members and passengers on vessels from Caffa landing in Pera must have been in need of care, and probably died soon afterward. Assuming a rather moderate infection rate on board of 50%, at least 50 to 100 severely sick patients per ship would have gone ashore. In the autumn of same year, a report from the port of Alexandria gives a death rate of 86% (287 out of 332) on a trading ship from the Black Sea, and adds that the survivors soon died at the port. Whether these numbers are accurate or illustrative, an unexpected landing of several ships with hundreds of severely ill persons would likely not be regarded as “business as usual” by the local population. Even assuming considerable ignorance, such an event would likely not remain unnoticed by authorities and chroniclers. The time-period reported by John VI seems therefore plausible, with an error margin of a few weeks.

Those travellers who were already infected in Caffa or in transit were probably unfit to continue their voyage. The few uninfected (or not yet sick) and the lucky few that might have survived the disease would likely attempt to continue on their way as soon as possible. Resupplying the ships and hiring replacements for deceased crew members would be a matter of a few days. The ships and apparently healthy travellers could therefore soon resume their journey, as no confinement restrictions existed.

However, only those voyagers that were previously infected and survived the plague already, or recovered from the infection on board during the passage from Caffa to Pera (if there were such) had a reasonable chance of reaching Genoa in the same ships. Thus, only these few persons could be considered as potential eyewitnesses to the alleged “bio-weapons” incident related by de Mussis.

Mediterranean Propagation

Homebound Genoese and Venetian vessels would follow their usual courses along the Greek and the Peloponnese coastlines. Soon enough, abandoned ships would also have contributed to infecting these coasts, though we should bear in mind that the naval trade along the other common Mediterranean exchange routes continued uninterrupted. The constant transfer of goods—especially the transport of slaves from infected regions such as the Caucasus—soon spread the disease to other centres of Mediterranean trade, including Rhodes, Crete/Candia, Cyprus and Alexandria, as well as to Trabzon at the south-eastern coast of the Black Sea.

Even though there are no detailed reports on the matter, it was undoubtedly necessary to hire additional crew-members at landings for the ships that came from Caffa, given the travel speed versus infection rate/mortality. Nevertheless, it should not be forgotten that the ship-rat populations must also have shrunk considerably during travel, though it might have been replenished (at least partly) at each stopover. The role of this influx of innately healthy animals to the advance of the disease on board has yet to be determined. One consequence might have been a slight lull in the further infection of humans. Nevertheless, the conditions on board probably accelerated human-to-human transmission, in addition to the rat > flea > human path of infection.

Those travellers from Caffa that proceeded from Pera without already having overcome the plague had a rather low probability of reaching Genoa, Marseille or Venice alive as they had to spend about six months on board plague-infested ships without any form of immunization (a factor that is still disputed). Accordingly, it seems quite possible that several of the original ships may have reached their destinations, albeit the evacuees from Caffa only in exceptional cases.

Irrespective of whether it was initiated by ships with refugees from Caffa or from other vessels heading for Venice or Messina and Genoa, the contagion spread to the European mainland in the course of the ensuing months.

Transmission Beyond Coasts

Several peculiarities of the subsequent spread of the Black Death in Europe emerge from the historical records. Modern microbiological and medical data may shed new light on some of these questions. As already mentioned, the classical model of transmission by Simond (1898) (i.e., rat > flea > human) neither accounts for the high speed of propagation, nor for continuous spreading, despite the perceived absence of large, infected rat populations. These and other inconsistencies convinced several authors to question or refuse the bacterium Y. pestis as the underlying cause of the Black Death. Suspicion first fell on smallpox, an Ebola-like virus, and other pathogens, but DNA samples obtained from the graves of Black Death victims have confirmed that the plague bacterium was indeed Y. pestis, albeit maybe different strains from today’s known type. Furthermore, it should also be considered that not only the pathogen but also the hosts—that is rodents as well as humans—adapt to each other over time.

While the spread of the disease by ships from the Black Sea via infected rats and persons to multiple Mediterranean harbours and finally to Genoa, Marseille and Venice is plausible and fairly well documented, the manner of the plague’s progression is still disputed.

Using mathematical models, Gaudart and his colleagues offer direct transmission of pneumonic plague (type II) as an explanation for the high speed of diffusion (up to 75 km/d). However, if aerosol transmission would have been a main factor, some of the measures that were occasionally adopted later would have been clearly effective. For example “plague covers”—namely, protective suits with conspicuous masks, the “beak” of the latter filled with essences and herbs as worn by many medics—would have provided quite a reasonable protection against droplet infection. Also, confinement measures would have had a significant beneficial impact. On the other hand, the rapid and nearly total lethality of pneumonic plague would itself significantly slow down the spread of infection.

The high propagation speed and the universality of the infection (i.e., being not restricted to certain regions, classes, professions and so on) cannot sufficiently be explained by infected black-rat populations, which tallies with the suggestion of an additional human-to-human vector beyond aerosol transmission (pulmonary plague). Hufthammer and Walløe propose Pulex irritans (the human flea), but also Pediculus humanis (the human louse) as potential vectors. However, the human flea is considered an inefficient vector as it does not become “clogged” by bacteria, and therefore is not driven to sate itself by switching from host to host. Being rather specific to humans, P. irritans also would require a severely septicaemic patient to reach a bacterial load that enables transmission. Other vectors, such as bugs and ticks, have been discussed, but little evidence supports these hypotheses. From the feasible arthropods species, the human louse appears to be the most plausible way for transmitting plague bacteria in the absence of rats and their flea, X. cheopis.

The body louse (P. humanis humanis) is a highly specific human parasite. It is therefore unlikely that this insect would transmit diseases from an animal to man. Instead, it is conceivable that lice can transmit infected blood from human to human. It should also not be forgotten that lice were ubiquitous and a severe nuisance for all armed forces before the end of the World War II (i.e., before the widespread application of DDT).

Human-to-human transmission via body lice would also be particularly consistent with severe spreading of the disease among the Mongols. Their use of layered silk undershirts as protection (comparable to today’s Kevlar vests) provided an excellent habitat for lice. This garment was supplemented by a thick overcoat, and leather or iron lamellar armour. An analogous type of protective covering from layered fabric had already been used by ancient Greeks (Linothorax), and for medieval armour (Gambeson). Additionally, the fashion for decorating clothing with animal pelt probably also contributed to the initial infection among Mongols and also Italian merchants.

While the role of lice in spreading spotted fever is widely known, this parasite can also carry other infections as well, like tularaemia, trench fever (Bartonella quintana) and borelliosis (Lyme disease). Further results from the Raoult group of researchers indicate that lice can also play a central role in spreading Y. pestis. These results were further supported by field investigations.

Recognition of the body-louse as a principal plague vector for central, western and northern Europe would solve some of the inconsistencies of the traditional rat-flea-human model mentioned before:

  • Notwithstanding some seasonality, body lice would be transported and transferred rather independently of outside temperatures.
  • The speed of spread would be roughly equivalent to human travel speed.
  • No intermediate hosts, and therefore no widespread deaths of rats or presence of plague reservoirs in European rodents, are required.

Conclusions

The “second pandemic”—namely, the plague that devastated Europe in 1347—affected most areas of Eurasia and possibly also several in Africa. It triggered serious consequences on many differing levels (demographic, economic, socio-cultural and so on) in central and western Europe. One reason for this severe impact might be that the contagion was rather abruptly introduced into several of the main centres of commerce (and culture) by trading ships, instead of proceeding more linearly (albeit with “jumps”) via overland routes. Accordingly, the events in the year 1347 that helped to propagate the infection towards Europe’s centres deserve special attention.

Summarizing the above considerations, while de Mussis may have faithfully recorded the reports he received from passing travellers, the latter were likely not eyewitnesses themselves, and/or may have exaggerated some events at the siege of Caffa. The Mongol troops may or may not have contributed to the infection of the citizens (and thus the subsequent spreading to western Europe) by catapulting infected corpses over the city walls. However, this was unlikely to have been a deliberate ploy as a “bio-weapon”, as surmised by many authors, but more a sanitary measure than a military tactic (if it took place at all). The efficacy of an organism as a bio-weapon involves many factors beyond its pathogenicity. The most important parameters are production, stockpiling, distribution/deployment, effectiveness, protection of own forces and, finally, decontamination. Taking these features into account, Y. pestis is far from optimal as an agent for use as a bio-weapon. Consequently, the few recorded historic attempts were only moderately successful.

Notably, contaminated crews and goods from the Sea of Asov/Black Sea area would have spread the contagion as effectively as the ships fleeing from plague-striken Caffa. Thus, the transmission of the infection was an inevitable consequence of persistent trade around the Mediterranean basin, and not least the absence of any effective cordon sanitaire or quarantine measures. An additional supporting factor might have been the relatively rapid transfer of the contagion to several of the key commercial hubs via the main Mediterranean trading routes, leaving both authorities and populations substantially unprepared.

Despite several endemically infected areas of the globe, the plague holds a relatively minor position among modern epidemic threats. Improved general health conditions of the world’s populations may play an important role in its containment today. However, the key factors in control today are hygiene, antibiotics and most importantly, knowledge of the aetiology of the disease, along with a greater understanding of the evolution of pathogens and hosts. The devastating spread of the Black Death in western and northern Europe in the fourteenth century cannot sufficiently be explained by the classical rat > flea > human infection model, and for this reason various inconsistencies in the account of the plague’s effects can be resolved if the human louse is reconsidered as a major vector in its spread.