A study casts doubt on whether the epidemic that struck Mallorca in 1820 was bubonic plague

Is the study of high quality? Are the conclusions supported by robust data?

"I find this a fascinating study of high quality. They have for sure not proved that it is pneumonic plague, but they have strong reasons to assume so based upon the mathematical modelling. It is important to be aware of the fact that in the 19th century the pneumonic plague form what not known – this plague form was first discovered in 1910 by the Chinese professor Wu Lien-teh, a discovery for which he was nominated for the Nobel prize in medicine and physiology (but unfortunately didn’t get it).

I find it interesting that they quite convincingly document that rats or other rodents could not have been involved in the spread of the bacterium between people. Hence, spread between people might likely have been by droplets and/or human ectoparasites such as body flea and lice. This is an important part of the contribution as it clearly shows that rats and other rodents could not have been part of the spread of the bacterium in the human population.

The study is important in showing that archival records can be used for epidemiological modelling, thus making the findings more quantitative".

Are there any significant limitations to bear in mind?

"The limitation is that there in no proof of pneumonic plague – only indications (but rather strong indications)".

What are the implications of the findings?

"Human plague is the most devastating infectious disease though history – as such this study helps us understand our human history better".

If you mention the plague, most people think of the classic features of the Black Death: bubonic plague, spread by flea bites from infected rats, causing painful swellings ('buboes') that turn black and necrotic, giving the disease it's nickname. But this is actually only just one way that the plague bacteria can infect humans. As septicaemic plague, the bacteria enters the bloodstream directly, through injuries breaking the skin, and causes a severe multi-organ infection that quickly results in death if untreated. As pneumonic plague, bacteria are spread through the air by coughing, and aggressively infect the lungs of victims breathing them in, also leading rapidly to death.

Pneumonic plague presents the greatest public health threat: it is the only form known to spread rapidly between humans directly, and in 1910-1911 an outbreak killed around 60,000 people in China. Better contained outbreaks in the past decade have still killed dozens of people. Septicaemic plague and pneumonic plague both have a higher fatality rate (up to 100%) than bubonic plague (30-60%). While the Black Death is widely viewed as bubonic plague, some historians and scientists have recently argued that other forms of plague infection (pneumonic plague in particular) could have contributed to the very high historical death toll of the disease.

Historical disease outbreaks can give us vital insights into the transmission dynamics and pathogenicity of infectious diseases, revealing how these can evolve and change across much longer time scales than we can get from modern records. Unfortunately, we have extremely limited records for plague cases across the second pandemic, when the disease was endemic in Eurasia betwen the 14th and the 19th Centuries.

A new discovery and analysis of an extraordinarily detailed record of cases during the 1820 plague outbreak in Mallorca shows that this outbreak doesnt match what we would expect for rat-borne bubonic plague. Using rigourous methods from modern-day disease outbreak modelling, the authors find that instead, the high fatality rate (78%) and short infectious time among victims implies that pneumonic or septicaemic plague were responsible. Once the first cases arrived from infected sailors, the disease seems to have spread either by coughing (pneumonic plague) or as septicaemic plague spread by bites from human parasites like body lice (instead of the rodent flea bites which cause bubonic plague). This presents a significant challenge to the assumption that epidemics at this time were driven and maintained by continuous infections from rodents as bubonic plague.

These findings are also exciting given the level of statistical modelling that can be applied with this data, showing how this epidemic was shaped by how humans responded to the disease, and that socioeconomic status was likely a major factor in getting infected (accounting, for example, for why so many excess children also died).

This work makes a significant contribution to the study of the plague and, in particular, to the epidemic outbreak that affected the eastern part of Mallorca in 1820, specifically the towns of Son Servera, Capdepera, Artà and Sant Llorenç, although the authors focus solely on the first two.

By combining a rigorous analysis of historical sources with mathematical modelling, the authors challenge the conventional interpretation which attributes a central (or decisive) role to rodents in the dynamics of disease transmission, leading to the bubonic form of the disease. Their findings suggest that person-to-person transmission—whether characterised as pneumonic or via human ectoparasites—may have played a far more significant role than traditionally accepted, particularly in this instance. This explains why many of those affected did not develop buboes (and, at the same time, why doctors of the time were confused about the definition of the disease).

The article thus contributes to the historiographical debate initiated by authors such as Ole J. Benedictow regarding the relative importance of the mechanisms by which the plague spread across Europe. It also offers a convincing explanation for the extremely high mortality rate recorded during the Mallorcan epidemic and provides new insights into the effectiveness of the isolation and containment measures adopted by the health authorities of the time in accordance with the principles of contagionist medicine.

This study proposes that the last known outbreak of plague in Spain (during the second pandemic) was primarily due to septicaemic and pneumonic plague (transmitted from person to person). To support this, the researchers draw on a mathematical analysis of the disease’s dynamics and historical sources.

They base their research on two questions: does the classic rat-flea-human cycle alone explain the introduction, rate of spread and symptoms described? Did transmission occur mainly via ectoparasites from rodents to humans, or was person-to-person transmission (via ectoparasites and respiratory droplets) the dominant route?

The authors propose that the disease was introduced via a smuggling vessel and estimate a case fatality rate (CFR: 78 per cent) and an infectious period (alpha) of 2.05 to 2.86 days.

Palaeomicrobiological identification can sometimes establish the presence of a pathogen, but cannot identify the mechanism of transmission or the form of a disease. Furthermore, in many cases, it is either completely impossible or its interpretation is complicated by issues such as contamination, co-identification of other microorganisms, the failure to preserve the causative pathogen whilst other commensal microorganisms are preserved, and so on.

By contrast, epidemiological parameters such as the infectious period, case fatality rate (CFR) or R0 make it possible to define transmission mechanisms through the dynamics of the epidemic. In this case, the infectious period is very short, the CFR is extremely high and R0 is similar to that of other outbreaks characterised in the literature. These mechanisms are specific to each pathogen or form of the disease. They do not, on their own, allow for a retrospective diagnosis, but they do help to distinguish between possible causes, particularly when there is convergence with historical sources (high case fatality rate, rapid spread, pattern of transmission among contacts of cases, cases without buboes in the clinical description, etc.). Even so, it should be noted that the historical sources used by the authors do not appear to mention the typical respiratory clinical presentation of pneumonic plague that we have come to know better from the most recent epidemics of the 21st century.

This research provides evidence regarding a poorly understood form of the plague in Spain, enables the reconstruction of its transmission dynamics following maritime introduction, and forms part of the body of work seeking to elucidate the role of person-to-person transmission in some of the poorly understood epidemics of the second plague pandemic.