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| Credit NIAID |
#14,038
While equine influenza (H3N8 for the past 50+ years), isn't considered to be a zoonotic disease, it has jumped species (to dogs in 2004), and has been shown experimentally capable of infecting both pigs (see J.Virol.: Experimental Infectivity Of H3N8 In Swine) and cats (see Equine influenza A(H3N8) virus infection in cats).
In 2016, in Epizootics, Host Ranges, and Conventional Wisdom, we looked at the (limited) scientific and historical evidence that suggests that equine influenza may have infected humans in the past, and could possibly do so again someday.This is a scenario we looked at again in 2018 in Equine H3N8: Looking At A long-shot In The Pandemic Sweepstakes. Canine and equine flu are of particular interest because the H3N8 and H3N2 subtypes they carry are similar to pandemic strains of the past (see chart below).
There is a lot of debate over these pre-1900 influenza pandemics, with conflicting views over whether the 1889-93 `Russian flu’ was due to the H2N2, H3N2, or H3N8 virus, but with most attributing the 1900 outbreak to H3N8 (see Transmissibility and geographic spread of the 1889 influenza pandemic).
Already in 2019 we've looked at two high profile equine influenza epidemics (see UK BHA: Update On Equine Influenza Outbreak and OIE: Senegal Equine Influenza - 275 Outbreaks, 2700 Horses Lost), which makes the following EID Journal Historical Review particularly timely.I've only posted a few excerpts from a much longer review, so follow the link to read it in its entirety.
Volume 25, Number 6—June 2019
Historical Review
Equine Influenza Virus—A Neglected, Reemergent Disease Threat Metric Details
Alexandra Sack, Ann Cullinane, Ulziimaa Daramragchaa, Maitsetseg Chuluunbaatar, Battsetseg Gonchigoo, and Gregory C. Gray
.C. Gray)
Abstract
Equine influenza virus (EIV) is a common, highly contagious equid respiratory disease. Historically, EIV outbreaks have caused high levels of equine illness and economic damage. Outbreaks have occurred worldwide in the past decade. The risk for EIV infection is not limited to equids; dogs, cats, and humans are susceptible.
In addition, equids are at risk from infection with avian influenza viruses, which can increase mortality rates. EIV is spread by direct and indirect contact, and recent epizootics also suggest wind-aided aerosol transmission. Increased international transport and commerce in horses, along with difficulties in controlling EIV with vaccination, could lead to emergent EIV strains and potential global spread. We review the history and epidemiology of EIV infections, describe neglected aspects of EIV surveillance, and discuss the potential for novel EIV strains to cause substantial disease burden and subsequent economic distress.
(SNIP)
Humans are also a potential host for EIV. Experimental infection of antibody-negative human volunteers in the 1960s saw >60% of them seroconvert and have positive virus cultures from throat swabs collected 2–6 days after nasal inoculation. Most of the human volunteers also shed virus from day 2 through day 5 but rarely shed past day 6 (34,35).
In the same study, horses became infected by strains of EIV passed through humans (34). During 1958–1963, human serum samples were tested in the Netherlands for EIV antibodies. Less than 0.5% of people < 60 years of age had elevated antibody titers, but 11.5% of people > 60 years of age had elevated EIV antibodies, with > 40% EIV antibody elevation among people >70 years of age. .
The authors surmised that a virus resembling the 1963 EIV strain infected humans during 1896–1900 (36). The study was performed before the human H3 influenza virus was recorded and determined to have crossed from ducks to humans in 1965, although the equine H3 strain is older (37). The evidence suggests past equine-to-human interspecies transmission.
(SNIP)
Future Challenges
Equine influenza is a highly contagious virus with the potential to cause global harm. The 2007 EIV outbreak in Australia demonstrated the economic impact the virus can have when introduced into a previously unexposed equine population (18). Furthermore, potential novel and virulent avian influenza virus strains could cross into horses and rapidly spread despite previous equid vaccinations (31,33). Risk from avian strains is compounded by EIV’s potential for infecting humans (13).
Although the role of humans in EIV evolution is unknown, historical and serologic evidence suggest EIV has zoonotic potential and is known to infect other nonhuman species (26,28,30). Historical review suggests the 1889 human influenza pandemic might have been of equine origin, with equids playing the role that swine play in modern outbreaks (12). With all this in mind, we posit that EIV should be recognized as a potential epidemic, if not pandemic, threat.
At the time of this research, Dr. Sack was a postdoctoral associate with the School of Medicine and the Global Health Institute at Duke University. She is now a postdoctoral fellow at Tufts Clinical and Translational Science Institute, Boston, Massachusetts, USA. Her primary research interests include zoonoses, specifically involving, human, livestock, and wildlife interactions.
For a particularly fascinating account of the 1972 equine epizootic, and its possible spillover into poultry, and humans, you may wish to revisit 2010's Morens and Taubenberger: A New Look At The Panzootic Of 1872.
Highly recommended.
