Infectious Diseases

Did WWI Warhorses Drive the 1918 Influenza Pandemic?

By Adam Marcus

Could horses have played a role in one of humanity’s most devastating pandemics?

A new study makes the case, proposing the massive global movement of equids during World War I may have helped spread the virus that led to the 1918 influenza pandemic that killed some 50 million people worldwide.
The evidence so far is circumstantial, but the researchers lay out a convincing argument for how epidemiologic and demographic factors leading up to the war provided the ideal conditions for the outbreak. And horses are at the center of the story.

“Even today, with technology that allows us to recover genetic information from almost any biological material, we still don’t know where the virus that caused 1 of the deadliest pandemics in human history actually came from,” said Pablo Murcia, DVM, MSc, PhD, a professor of integrative virology at the University of Glasgow Centre for Virus Research, in Scotland, and a co-author of the study. “It’s possible we’ve been looking in the wrong place all along.”

Equine influenza viruses circulating in horses for the last 70 years are thought to have emerged from avian flu that adapted to infect mammalian hosts. These spillover events have occurred throughout history, likely including an 1872 outbreak in the United States and another in China in 1989.

As Dr. Murcia and his colleague, the physician and medical historian Martin Furmanski, MD, noted, horses in North America were hit with a particularly severe epizootic of equine influenza, or “shipping fever,” in 1915, just after the global conflict erupted in Europe.

Although the virus may have jumped directly from birds to people, Drs. Murcia and Furmanski hypothesize a different route: from birds to horses, and then from horses to humans. “We propose that a putative H7N1 avian virus caused a large epidemic in horses. This H7N1 virus then reassorted with an also-putative human H1 virus. These 2 viruses reassorted, and this reassortment event originated the 1918 pandemic H1N1,” Dr. Murcia said.

“Rates of equine communicable diseases (overwhelmingly shipping fever) and deaths in individual remount depots reached up to 19% and 32%, respectively,” Drs. Murcia and Furmanski write. “During the equine mobilization of 1914 to 1918, the railway network transported large numbers of infected horses to densely populated cities and crowded training camps.”

One place that received many equids was Camp Funston, in northeast Kansas, where in March 1918 the first U.S. cases of the so-called Spanish flu emerged.

Another piece of the puzzle: the outbreak of equine influenza that began in 1915 was marked by a high rate of secondary bacterial pneumonia, which also was a distinguishing feature of the 1918 pandemic in humans.

To Dr. Furmanski, the most interesting aspect of the equine mobilization effort in North America during the war was the confinement of the animals to holding facilities prior to transport. Once equine influenza appeared, it had a constant stream of new, immunologically virginal hosts.

“This was a ‘stationery epizootic,’ in which fresh, immunologically naïve animals were continuously introduced to ongoing epizootic transmission,” he said. “And since human equine handlers are intimately exposed to equine respiratory mucus in routine exams, fitting bridles and other surfaces, it was a great way to get equine flu variants fostered in the ‘blowtorch’ stationery epizootic to transfer to humans.”

Of course, Drs. Murcia and Furmanski’s hypothesis is just that. Other explanations for the emergence of the pandemic exist—including the most direct one: that a virus jumped directly from birds to people.

“We do think that given the size of the horse population at the time, their mobilization due to WWI, and their contact rate with humans, the path to emergence that we propose, while less parsimonious, is entirely feasible from a biological perspective.”

The strongest evidence in support of horse-human transmission would lie in preserved specimens of equine tissue from the early 1900s, which could contain the viral proof of the link.

“We think that archived sera or paraffin embedded tissues from diseased animals might provide compelling evidence supporting our hypothesis,” Dr. Murcia said. “I have been looking for sources of material but so far had no luck. I am hopeful that tissues will be stored somewhere.” MeV

An Interview with
DR. MARTIN FURMANSKI: METICULOUS RECORDS

Martin Furmanski, MD, a physician and historian of medicine, became interested in the potential role of horses in the spread of the Spanish Flu after learning of a WWI-era effort by the German military to use biologic weapons, particularly anthrax and glanders, to sabotage Allied cavalry.

“Although cavalry charges were vanishingly rare—and disastrous due to the advent of machine guns—horses were essential to the war effort of all armies in WWI. They were essential for transport of material from railhead to fighting line, and for frequent repositioning field artillery to avoid antibattery counter fire,” Dr. Furmanski said.

The numbers tell the tale: At its peak, the British army used roughly 1 million horses that required shipments totaling 5.4 million tons of forage to feed—more than the 5.2 million tons of ammunition the British guns required, or even the 3.2 million tons of food British soldiers consumed.

In the years before the conflict, militaries in Britain and Europe created an “equine draft,” Dr. Furmanski said, “registering domestic equids that would be suitable for military service if mobilization was declared. This did not supply enough equids, and many horses were still needed for the still largely non-mechanized agriculture of the UK and European powers.”

But because not enough home-bred animals existed, the military had to import equids, which it did at scale: drawing roughly 1.4 million horses, donkeys and mules from North America alone during the war years, and a smaller but significant number from South America.

Thanks to the British Army Veterinary Corps (AVC; now the Royal Army Veterinary Corps), which emerged after the Boer War in South Africa from 1898-1900, during which some 300,000 equids were killed, Dr. Furmanski had access to what he called “meticulous records.”

“The British AVC records were a godsend,” he said. “I looked over all the reports of infectious diseases in the AVC records for glanders, and found only 1 major outbreak, that may or may not have been due to German sabotage, though warnings of the sabotage were sent about that time to the AVC posts at the front.”

The outbreak of Burkholderia mallei, which causes glanders, was quashed by a blitz of bacterial testing of every horse on the battle front —a success story for the AVC and public veterinary health practice!” Dr. Furmanski said.

But in studying the AVC’s infection reports, Dr. Furmanski began to notice the periodic mention of equine influenza – but only in groups of equids fresh from countries beyond the corps’ jurisdiction. “This intrigued me, and eventually, I was able to find the records of the British Remount Commission in another archive. The BRC was tasked with buying, caring for and transporting equids from North America. This was the goldmine of the equine flu story that makes up part of the paper Dr. Murcia and I published.” MeV