Adrian Hugo Aginagalde
Spokesperson of the Spanish Society of Preventive Medicine, Public Health and Health Management (SEMPSPGS) and Associate Professor of Public Health at the University of Deusto
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.