Rats weren't to blame for bringing plague to a Spanish island in 1820, argues new research published yesterday in PNAS. The findings of Mallorca's epidemic challenge longstanding narratives that rodents, a reservoir for Yersinia pestis, were the primary drivers of outbreaks in Europe. 

Caused by the bacterium Y pestis, plague can take several forms. The study posits that historical records from the epidemic on the rural Mediterranean island describe a disease pattern that is inconsistent with bubonic rat-borne plague, partly because of the extremely high fatality rate. Instead, they say the epidemic was likely fueled by pneumonic or septicemic plague. 

With pneumonic plague, Y pestis infects a person's lungs and then spreads to others through respiratory droplets. Transmission of septicemic plague occurs when the bacterium infects the bloodstream, often through contact with infectious body fluids or a flea bite.

This analysis highlights how human behavior and mobility can shape epidemic dynamics, wrote authors Pedro Puig, PhD, of the Autonomous University of Barcelona, and Joana Maria Pujadas-Mora, PhD, of the Open University of Catalonia. 

Mallorca's 1820 epidemic was among the last outbreaks of the Second Plague Pandemic, which ravaged communities throughout Europe, North Africa and Western Asia, starting in the 14th century. 

Even though Mallorca instituted strict isolation policies, plague took the lives of more than 2,400 people, nearly a third of the island's population. Researchers found the case-fatality rate to be 78%, which is more in line with that of pneumonic or septicemic plague.

The first cases were reported in May 1820, in the town of Son Servera, which lacked a port. The authors hypothesize that goods carrying plague-infested fleas were transported to the island, possibly by smugglers, via rowboats, meaning rats were unlikely to hitchhike to shore. It's also possible the epidemic started through person-to-person transmission after an infected person arrived on the island and had close contact with others. 

Clinical observations written during the epidemic by physicians don't mention buboes, the painful, swollen lymph nodes that are the hallmark of rodent-driven bubonic plague. Instead, these documents describe symptoms such as fever, exhaustion, and weakness.

Interestingly, they also don't describe classic pneumonic plague symptoms such as chest pain, cough, and bloody sputum.

The illness is described as progressing quickly, sometimes killing a healthy person within a day. The authors note that rapid onset of disease after contact with sick patients also points to septicemic or pneumonic plague.

Once authorities realized a fatal disease was infecting people on the eastern side of Mallorca, they enforced strict isolation measures in Son Servera, along with the towns of Artà, Capdepera, and Sant Llorenç. As a result, the plague did not reach the more than 40 other towns on the island. The decision likely saved hundreds, if not thousands of lives.

Additionally, as the authors detail, segregated camps were established to separate healthy people from those infected with the disease. The military restricted trade and movement off the island, including from parts that were free of plague.

The epidemic on Mallorca was declared over on December 31, 1820, “following the disinfection of the towns and the burning of the belongings of the deceased as well as those used in the camps.”