Mathematics reveal the secret of the plague that killed 6,000 Majorcans in 1820
A study published in PNAS points out that the epidemic that devastated Son Servera, Capdepera and other municipalities was not only bubonic plague and that more aggressive forms played an important role.
PalmaThe plague that ravaged part of Mallorca in 1820 could have been much more lethal than previously considered. A study published in the scientific journal PNAS suggests that the outbreak was not exclusively bubonic plague, but that the pneumonic and septicemic forms also played an important role. Researchers have reached this conclusion based on statistical analysis of data recorded over two centuries ago.
The epidemic caused around 6,000 deaths between May and November 1820 in four municipalities in Mallorca. The research has focused on Son Servera, where the outbreak began, and Capdepera, the two localities for which the most complete records have been preserved.
The study is the work of Pedro Puig, Professor of Statistics and Operations Research at the Autonomous University of Barcelona, and Joana María Pujadas-Mora, ICREA Acadèmia researcher and Senior Lecturer at the Open University of Catalonia. Pujadas-Mora had already researched the epidemic from a historiographical perspective, but the discovery of some registers with daily data on the outbreak's evolution allowed for a further step and the application of mathematical models. The documents, preserved at the Royal Academy of Medicine of the Balearic Islands, collect the data that the Superior Health Board received daily from the affected municipalities from June 1820 onwards. The records indicate the number of new cases, deaths, and recovered individuals, differentiated by sex and also between adults and children.
A lethality of 78%
The analysis of this data has allowed us to estimate some of the main parameters of the epidemic. Both Son Servera and Capdepera registered a global fatality rate of 78%, with higher mortality among children than among adults. Furthermore, the period during which an infected person could die or recover was about 2.5 days.
These records are particularly revealing because the speed of the disease's progression and the high mortality do not entirely fit with what would be expected from an exclusively bubonic plague epidemic. According to the researchers, the very short periods of contagion, the historical descriptions of clinical deterioration, and the fatality rate suggest that the pneumonic and septicemic forms contributed substantially to the transmission.
Bubonic plague is usually transmitted through the cycle between rats, fleas, and humans and can have a mortality rate of 60% or 70% without treatment. Pneumonic and septicemic forms, on the other hand, can be transmitted by respiratory droplets or by contact with fluids and have much higher mortality rates, close to 100% in the absence of treatment. However, the authors warn that this is a reconstruction based on mathematical models and historical sources. "We haven't touched bone," points out Pujadas-Mora, who calls for caution when interpreting the results.
An epidemic that arrived by sea
The research also provides clues about how the plague was introduced to Mallorca. Researchers consider it unlikely that the outbreak strictly followed the classic transmission mechanism through rats and humans. Historical documentation indicates that the epidemic arrived rapidly after the entry into Mallorca of a ship from Tangier.
The merchandise was transferred ashore using small rowing boats and, moreover, inhabitants of the area would have gone directly to the ship to buy products there. Therefore, the team considers it plausible that transmission occurred through fleas or lice associated with humans.
The documents even allow the reconstruction of the first cases. The first identified victim was Francisca Brunet, who was quickly followed by her husband, her son, and several neighbors. Faced with the spread of the outbreak, the authorities established a triple cordon sanitaire: by land, by sea, and between the four affected municipalities. According to Pujadas-Mora, these measures "worked very well" and prevented the epidemic from spreading to the rest of the island.
The outbreak that boosted public health
Beyond the victims, researchers consider that the plague of 1820 acted as a catalyst for public health in the Balearic Islands, which at the time had one of the highest life expectancies in the State. The epidemic showed the population the need to adopt sanitary measures and to trust in medical and scientific knowledge. Two hundred years later, the preserved data now allow us to reconstruct that crisis from a perspective that combines history, epidemiology, and mathematics.
The plague, however, also left its mark on popular memory. For years, a legend circulated that the epidemic had begun after a teenage shepherd put on a cloak he had picked up from a tomb on the beach, where a person from a ship had just been buried. The story is part of the memory of Son Servera to this day. In 2008, the City Council erected a small statue dedicated to the shepherd as a tribute to the victims of the 1820 epidemic.