On 22 December 1874, HMS Dido arrived in Fiji from Sydney, Australia, with about 200 people and an invisible cargo on board. A Fijian king and his son, who were on board, were infected with measles. When they landed, they started an epidemic that killed 20,000 people in Fiji, up to a quarter of the population, who had no immunity to the disease.
But in the days when people traveled by sail or steam, such events were the exception rather than the rule. A new report, published last week in The Proceedings of the National Academy of Sciences, uses mathematical models to show how viruses had to overcome very long obstacles to spread by sea. More often, the study found, infectious diseases died out on board before the ships even docked.
In the contemporary world, new diseases and older infectious threats are expected to spread almost instantaneously around the world, as has happened with Covid-19. But where is the inflection point? Elizabeth Blackmore, a Yale doctoral student, and James O. Lloyd-Smith, an ecologist at the University of California, Los Angeles, set out to find the point at which viral transmission began to change.
John McNeill, a historian at Georgetown University who was not involved in the study, said Blackmore’s use of sophisticated mathematical modeling “has accomplished something that no historian or anyone else has done before: quantify the probabilities of transmission.”
Kyle Harper, a historian at the University of Oklahoma who was also not involved in the study, said the work “breaks new ground.”
Ms Blackmore said she and Dr Lloyd-Smith came up with the idea to study shipping while she was working on her master’s degree. They learned that the first cases of smallpox in California were not reported until 1806 and 1838. Smallpox was first reported much later elsewhere in the Pacific.
She also read “Pox Americana: The Great Smallpox Epidemic of 1775-82” by historian Elizabeth Fenn. Blackmore said she was “surprised to learn that Boston had 20- to 30-year intervals between smallpox epidemics throughout the 18th century.”
“These two cases led us to ask why and how it took so long,” Blackmore said. “And that led us to look at the ships.”
She pointed out that the only way for a disease to spread after a ship had boarded was for there to be a chain of infections on board that lasted at least as long as the ship’s voyage. In the days of sailing ships and even many steamships, this was simply not possible. The usual situation was that by the time a ship reached its destination, everyone on board who might have had a disease had been infected and had either recovered or died.
The researchers studied the transmission of three infectious diseases — influenza, measles and smallpox — with a mathematical model that Simon Levin, a mathematical ecologist at Princeton, called “beautiful.”
Influenza is the most difficult disease to spread because the infection period is very short: on average, people are contagious for only three days.
Measles, which has an average contagious period of about nine days, and smallpox, which has an average contagious period of about 20 days, are more likely to be transmitted because people are contagious for longer periods of time.
The researchers then studied the risks of disease transmission on a list of 18 ships, including the Santa Maria, which carried Christopher Columbus to the Americas, and the Mayflower.
They calculated that if a person had contracted influenza while traveling on the Santa Maria in 1492, there would have been less than a 0.1 percent chance that the disease would have been transmitted to the New World. If a person had contracted measles, the risk would have been 24 percent. For smallpox, it was 33 percent. The Santa Maria’s voyage, with 41 people on board, lasted 35 days, so the small number of people on board and the length of the voyage contributed to the low risk of spreading the disease.
The Mayflower’s voyage in 1620 took longer: 66 days. So even though there were 127 people on board, the risk of transmission in the New World was even lower. For influenza, it was less than 0.1 percent. For measles, it was 13 percent and for smallpox, 17 percent.
Yet ships can be hotbeds of disease, Blackmore said. Contemporary reports of living conditions on ships in centuries past were horrific: stifling and unhygienic.
A newspaper report of what happened on a 77-day voyage in 1801 involving the ship Nancy is typical. The ship travelled from Sligo Harbour in Ireland to New York with 417 passengers, most of whom quickly fell ill. Mrs Blackmore and Dr Lloyd-Smith included an account of the conditions in their study:
Partly due to the lack of strength and assistance among the sick and partly due to the lack of a sense of decency, the space between the decks, occupied by nearly 300 people, became the receptacle of all excremental matter, so much so that it came out in streams from the scuppers.
By the time the ship reached its destination, 90 people were dead and 180 were suffering from an unspecified illness, according to their accounts.
A ship that sailed from Panama to San Francisco in 1851 crammed as many passengers as possible before it sailed, according to a contemporary report.
It was only when it was found that there was barely enough room for the people on board that she dropped anchor.
Passengers on the trip were suffering from “fever and dysentery,” the report said.
The risk that ship passengers could spread a pathogen suddenly increased in the mid-19th century, when steamships accelerated and transformed travel.
While this seems intuitively logical, Dr. Levin said, the researchers’ mathematical analysis “shows why and makes it quantitative.”
Mrs Blackmore and Dr Lloyd-Smith saw the effects of steamships when they examined steamships travelling to San Francisco from 1850 to 1852, during the gold rush.
Steamships were much faster and carried many more people than earlier sailing ships: an average of 196 people on voyages from Panama and a maximum of 1,050. Sailing ships carried an average of 53 people and a maximum of 287.
There were many more steamship voyages on the Panama-to-San Francisco route than sailing voyages. Using their mathematical model, the researchers calculated the probability that passengers on a San Francisco-bound steamship who had influenza, measles, or smallpox would still be contagious when they arrived at the dock.
Only ships from closer destinations like Panama arriving in San Francisco had voyages short enough to ensure that some passengers would still be contagious.
The risk of influenza transmission remains below 0.1 percent. In contrast, the risk of measles transmission is 70 percent and that of smallpox 74 percent.
Researchers have found that the risk of transmission increased during World War I, when extremely fast ships could transport between 1,000 and 1,500 soldiers to the front lines. Historians say that troop movements were one reason why the 1918 flu spread so quickly around the world.
A Royal Navy commander in chief during the First World War described the extreme overcrowding on board ships. When men lay in their hammocks, their heads were “less than three feet apart”. Large numbers of men were confined in an area “less than one-fiftieth of a square mile”. The report concluded that “every person on board receives a dose of the infectious agent sufficient to cause influenza”.
Blackmore is continuing this line of research as part of her doctoral work at Yale, seeking to test her models against “real outbreaks on real ships” described in the historical record.
“Part of what motivates me is that if we want to convince ourselves that measures like social distancing will work, it’s important to have examples of infectious diseases spreading rather than saying, ‘They’re spreading like wildfire,’” Blackmore said.
“We need to talk about how slowly a disease can spread,” she added.