For epidemics to cross oceans, viruses on ships had to overcome adversity

On December 22, 1874, the HMS Dido, carrying about 200 people, arrived in Fiji from Sydney, Australia. invisible charge. The King of Fiji and his son, who were on the ship, were infected with measles. When they disembarked, an epidemic broke out that killed 20,000 people in Fiji—up to a quarter of the population—who had no immunity to the disease.

But in those days, when people traveled by sail or steamboat, such events were the exception rather than the rule. Modern reportpublished last week in The Proceedings of the National Academy of Sciences, uses mathematical models to show how viruses had to overcome very long odds to be carried across the sea. More often than not, the study found, infectious diseases burned out on board, before the ships even docked.

In today’s world, modern diseases and older infectious threats are expected to spread almost instantly across the globe, as Covid-19 has done. But where was the tipping point? Elizabeth Blackmore, student at Yale University and James O. Lloyd-Smith, an ecologist at the University of California, Los Angeles, set out to find the point at which virus transmission begins to change.

John McNeill, a historian at Georgetown University who was not involved in the study, said Ms. Blackmore’s operate of advanced mathematical modeling “achieved something here that no historian or anyone else had been able to do before — quantify the probability of transmission.”

Kyle Harper, a historian at the University of Oklahoma who was also not involved in the study, said the work “breaks modern ground.”

Ms Blackmore said she and Dr Lloyd-Smith got the idea to look at shipping while she was working on her master’s degree. They learned that the first reports of smallpox outbreaks in California were not 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. Ms. Blackmore said she was “astonished to learn that Boston experienced 20- to 30-year gaps between smallpox epidemics in the 18th century.”

“Both of those cases made us wonder how and why it had taken so long,” Ms. Blackmore said. “And that led us to the ships.”

She noted that the only way the disease could have been transmitted once a ship docked was if there had been a chain of infection on board that lasted at least as long as the ship’s voyage. Often in the days of sailing ships, and even many steamships, this simply could not have happened. The usual situation was that by the time a ship reached its destination, everyone on board who was susceptible to the disease had already been infected and had either recovered or died.

Scientists have considered the transmission of three infectious diseases — influenza, measles and smallpox — using a mathematical model that Simon Levin, a mathematical ecologist at Princeton, called “attractive.”

Influenza is the most complex to spread because the infectious period is very low – on average, people are infectious for only three days.

Measles, which has an average infectious period of approximately nine daysand smallpox, the average infectious period of which is about 20 days, are at greater risk of transmission because people are infectious for a longer period of time.

The researchers then looked at the likelihood of disease transmission on 18 ships, including the Santa Maria, which brought Christopher Columbus to the Americas, and the Mayflower.

They calculated that if one person had the flu on the Santa Maria in 1492, the chance of the disease being transmitted to the Modern World was less than 0.1%. If one person had measles, the chance was 24%. For smallpox, the chance was 33%. The Santa Maria, with its 41 people on board, took 35 days to complete its voyage, so the narrow number of people on board and the length of the voyage contributed to the diminutive 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 probability of transmission in the Modern World was even lower. For influenza, it was less than 0.1 percent. For measles, it was 13 percent; for smallpox, 17 percent.

But ships can be breeding grounds for disease, Ms. Blackmore said. Contemporary reports of conditions on ships in centuries past were appalling—stufficent overcrowding and lacking in sanitary facilities.

Typical was the newspaper report of what happened during the 77-day voyage in 1801 involving the Nancy. The ship sailed from Sligo, Ireland, to Modern York with 417 passengers, most of whom soon fell ill. Mrs. Blackmore and Dr. Lloyd-Smith included a description of the conditions in their study:

Partly because of the lack of strength and assistance among the ailing, and partly because of a lack of a sense of decency, the space between decks, occupied by about 300 persons, became a reservoir for all the excrement that flowed in streams from the drains.

According to their accounts, by the time the ship reached its destination, 90 people had died and 180 were ill with an unspecified disease.

According to a report from that time, a ship sailing from Panama to San Francisco in 1851 took on board as many passengers as possible before departing.

Only when it was ascertained that there were only a few standing places on deck did the anchor pop out.

According to the report, the cruise participants suffered from “fever and dysentery.”

The risk of spreading pathogens by ship passengers suddenly increased in the mid-19th century as steamships made travel faster and changed the way we travel.

While this seems intuitively logical, Dr. Levin said the researchers’ mathematical analysis “shows why this happens and makes it quantitative.”

Mrs Blackmore and Dr Lloyd-Smith saw the effects of steamships when they looked at steamboats sailing to San Francisco between 1850 and 1852, during the Gold Rush.

The steamships were much faster and carried many more people than the earlier sailing ships—an average of 196 on the Panama route and a maximum of 1,050. The sailing ships carried an average of 53 people and a maximum of 287.

And there were far more steamboat trips on the Panama-to-San Francisco route than sailboat trips. Using their mathematical model, the researchers calculated how likely it was that passengers on a steamboat to San Francisco who had the flu, measles, or smallpox would still be contagious upon arrival at port.

Only ships arriving in San Francisco from closer destinations, such as Panama, had voyages low enough to be certain that some passengers would still be infected.

The probability of transmitting influenza was less than 0.1 percent. But there was a 70 percent chance of transmitting measles and a 74 percent chance of transmitting smallpox.

The risk of transmission increased during World War I, researchers found, when extremely quick ships could carry 1,000 to 1,500 soldiers to the front lines. Historians say troop movements were one reason the 1918 flu spread so quickly around the world.

The Royal Navy’s Surgeon General during World War I described the extreme crowding on board ships. When men lay in hammocks, their heads were “less than three feet apart.” Immense numbers of men were confined to an area “less than one-fiftieth of a square mile.” The report stated that “every man on board received a dose of the infectious agent sufficient to produce influenza.”

Ms. Blackmore is pursuing this line of research as part of her doctoral thesis at Yale, hoping to test her models on “real epidemics on real ships” described in historical archives.

“Part of what motivates me is that if we’re going to convince ourselves that things like social distancing are going to work, it’s crucial to have examples of how infectious diseases spread, rather than saying, ‘It’s spreading like wildfire,’” Ms Blackmore said.

“We need to talk about how slowly the disease can spread,” she added.