Caitlyn Stulpin , 2025-05-09 19:11:00
Key takeaways:
- The 1918 influenza pandemic is one of the deadliest in history; however, its genesis remains unknown.
- Researchers hypothesize that horses acted as intermediate hosts and facilitated its rapid spread.
Experts have said that the 1918 influenza pandemic is one of the deadliest pandemics in history, but could it have been facilitated by horses?
This is the hypothesis that Martin Furmanski, MD, and Pablo R. Murcia, DVM, PhD, MSc, posed in a paper suggesting that horses may have acted as intermediate hosts for the virus.
“While research efforts have focused on determining the geographic origin, animal source and timing of the 1918 pandemic influenza A virus (IAV/18), the ecological and epidemiological factors that led to its genesis and rapid spread are not clear,” they wrote.
Horses, they explained, which have historically been recognized as natural hosts of IAVs for centuries, may have acted as “intermediate hosts” and facilitated the spread of prepandemic viruses “that originated IAV/18 via reassortment.”
“I want to emphasize that the article we published presents a hypothesis — a scientific idea based on available evidence, which still needs to be tested through further research,” Murcia, a professor of integrative virology at University of Glasgow’s Centre for Virus Research, told Healio.
We spoke further with Murcia about the hypothesis, as well as the history of horses being drivers of influenza and the importance of this information in today’s context.
Healio: How could horses have facilitated the pandemic?
Murcia: Our hypothesis is that horses may have acted as intermediate hosts — animals that helped transmit a bird-origin influenza virus to humans before 1918.
It has been proposed that the virus that caused the 1918 pandemic was a virus of avian origin (ie, a bird flu virus). So, one scenario is that birds transmitted the virus directly to humans. What we propose is that the avian virus was first transmitted to horses, causing an epizootic of equine flu. This virus started circulating in horses in the United States and then mixed with a human virus, generating the pandemic virus. So, the horses acted as a bridge between birds and humans.
Healio: Can we ever know for sure?
Murcia: Possibly. If we can find preserved tissue samples (such as paraffin-embedded tissues) from horses that had influenza during World War I, and if we are able to extract and sequence the virus from those samples, we could then compare it with the 1918 pandemic virus. If there are genetic similarities, especially shared genes, it would provide stronger evidence supporting the hypothesis.
Healio: Have horses been drivers of any other major outbreaks?
Murcia: Yes. Historically, when horses were widely used for transport and labor (before the automobile), flu outbreaks in horses sometimes occurred just before similar outbreaks in people. More recently, in the early 2000s, a flu virus jumped from horses to dogs, creating a new virus known as canine influenza. So, we know that influenza viruses in horses can cross over into other species.
Healio: What are the odds of archived equine tissues or serum samples being tested and how would that work? Are there other ways of getting samples?
Murcia: It is possible, although the biggest challenge is locating such samples. If preserved tissues exist, we could attempt to extract viral genetic material using the same techniques used to reconstruct the 1918 influenza virus. If freeze-dried (lyophilized) serum samples are available, we could test them for antibodies to influenza, which would indicate prior exposure.
Healio: If horses helped drive the pandemic, what is the importance of that information now?
Murcia: Today, an H5N1 influenza virus — a known pandemic threat — is spreading in dairy cattle in the United States. Understanding how flu viruses spread among domestic animals and how they might jump to humans is crucial for predicting and preventing future pandemics. Learning from the past helps us prepare for the future.
Reference:
For more information:
Pablo Murcia, DVM, PhD, MSc, can be reached at pablo.murcia@glasgow.ac.uk.
