It is common knowledge that microbes hitched rides on long sea voyages like that of Christopher Columbus. But it was never inevitable that diseases like smallpox and measles would survive these early ocean crossings, which took a month or more. Now, researchers have used mathematical modelling to predict the historical risk of pathogens lingering in a ship’s population long enough to disembark with passengers.
“These ships aren’t just carrying a pathogen like it’s an extra piece of cargo – it needs to be spreading through the population,” says at Yale University. “People have thought about this using common sense, but until this study, no one had asked the more general question of what limited the historical spread of pathogens across the world.”
Blackmore and her co-author, at the University of California, Los Angeles, tailored a long-standing mathematical model of disease spread to hypothetical ship environments. It incorporated factors like journey time, ship size, transmission intensity, susceptibility of those on board and the biology of the bug in question. They ran predictions for how long outbreaks of measles, influenza and smallpox would have been likely to last in various scenarios, and then compared their model’s output to data about historically significant voyages between 1492 and 1918.
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Their findings indicated that smallpox especially, but also measles, could probably have persisted on certain early voyages, which lasted an average of five to 10 weeks. The risk of one of these diseases spreading and maintaining itself on board depended on how many infected individuals came aboard, and in particular on how many susceptible were present as well as how densely packed the passengers were.
In 1801, for example, a ship set sail from Ireland to New York crammed with 417 passengers, so many that additional housing had to be built on the deck. In these crowded conditions, typhus and dysentery broke out, and by the time the ship docked 77 days later, 90 people had died and 180 had been sickened.
Unlike smallpox and measles, influenza is a faster-burning bug, and the authors found that it was unlikely to make a full oceanic crossing – at least early on. When steam travel was introduced in the 19th century, however, it sped up travel and probably accelerated transmission of all three pathogens to the places where these ships docked.
The “fascinating” new study uses modern computational tools to understand a “key historical puzzle”, says at Brown University in Rhode Island. “The combination of the historical, biological and epidemiological evidence utilised by the researchers is unique and will hopefully encourage new efforts at the intersection of history and mathematical disease modelling.”
PNAS
