Credit EID Journal
#18,470
While `conventional wisdom' is that horses are generally only susceptible to equine H3N8 (and previously H7N7), we've previously seen evidence that horses - and other equids - are susceptible to H5N1, and likely other novel flu viruses.
In 2010's Egypt: H5N1 Isolated In Donkeys, we saw evidence of a spillover of H5N1 from poultry to donkeys, while a 2013 paper (Sero-prevalence of avian influenza in animals and human in Egypt) cited the seroprevalence of H5N1 in horses and donkeys ranged from 22.55% (ELISA) to 23.75% (HI).
As with early evidence of the susceptibility of cattle to H5N1 (see A Brief History Of Influenza A In Cattle/Ruminants), this information was treated more as an aberration or a scientific curiosity than a threat.
Which helps to explain why it took months before American veterinarians even thought of testing sick cattle for H5N1.
Last May (see Revisiting The Epizootic of 1872) we returned to a fascinating bit of epidemic history involving horses. In 2010 NIH flu experts David Morens and Jeffrey K. Taubenberger penned an account of a devastating equine epidemic that spread from Canada to Mexico in a matter of months - and brought transportation to a standstill in this country (see A New Look At The Panzootic Of 1872).
Morens and Taubenberger (2010) An avian outbreak associated with panzootic equine influenza in 1872: an early example of highly pathogenic avian influenza? Influenza and Other Respiratory Viruses 4(6), 373–377.
This epizootic didn't just affect horses, as there are anecdotal reports of a concurrent epizootic affecting poultry, and even causing human illness across the nation. Hence the speculation of an HPAI origin.
Previously, we also looked at the potential of equine H3N8 to jump to humans (see Equine H3N8: Looking At A long-shot In The Pandemic Sweepstakes), and in the spring of 2023, we looked at the potential for horses, and other mammalian hosts, to serve as `mixing vessels' for avian influenza (see Viruses: Zoonotic Animal Influenza Virus and Potential Mixing Vessel Hosts).
All of which makes the following EID report something less than surprising.
Research Letter
Evidence of Influenza A(H5N1) Spillover Infections in Horses, Mongolia
Batchuluun Damdinjav, Savitha Raveendran, Laura Mojsiejczuk, Ulaankhuu Ankhanbaatar, Jiayun Yang, Jean-Remy Sadeyen, Munir Iqbal, Daniel R. Perez, Daniela S. Rajao, Andrew Park, Mafalda Viana, and Pablo R. Murcia
Abstract
Recent outbreaks of influenza A(H5N1) have affected many mammal species. We report serologic evidence of H5N1 virus infection in horses in Mongolia. Because H3N8 equine influenza virus is endemic in many countries, horses should be monitored to prevent reassortment between equine and avian influenza viruses with unknown consequences.
Avian influenza viruses (AIVs) of the H5N1 subtype are a cause of concern because they are highly pathogenic in birds and various mammals. H5N1 AIVs have caused outbreaks in both wild and domestic avian species, leading to substantial biodiversity and economic losses from virus-induced deaths and culling interventions.
Surveillance studies have shown an increased incidence of H5N1, particularly of clade 2.3.4.4b, in wild birds (1), which coincides with growing reports of infections in mammal hosts including skunks, raccoons, bears, and foxes (2). In such studies, affected animals were believed to be dead-end hosts, which is consistent with previous perceptions that AIV H5N1 exhibits no or poor transmissibility in mammals.
That perception changed in 2022, when outbreaks of H5N1 clade 2.3.4.4b were reported in fur farms in Europe breeding minks and foxes (3,4) and in populations of pinnipeds (e.g., seals and sea lions) in South America (5). In early 2024, an outbreak of AIV caused by genotype B3.13 H5N1, a descendant of H5N1 2.3.4.4b, was reported in dairy cattle in the United States (6). At the time, infection was also reported in cats, mice, and farm workers, but direct transmission from cattle could not be confirmed. Cow-to-cow transmission was later confirmed by sequencing data and epidemiologic information.
Horses are natural hosts of equine influenza virus (EIV). Two subtypes of EIV have emerged, including 2 H3N8 strains and 1 H7N7 strain. All EIVs are thought to have originated from AIVs. Here, we report serologic evidence of influenza A(H5N1) infection in horses in Mongolia.
In surveillance studies during July 2021–October 2023, we collected serum samples from 10 horses from 24 herds, 3 times per year. We recorded associated metadata including sex, approximate age, clinical status, and main use of the horse, as well as location of the herd. Fourteen herds were in the Ugiinuur area of Arkangai Province, a region that exhibits substantial wetlands and hosts a large population of migratory birds. The other 10 herds were in the Dashinchilen area of Bulgan Province and Burd soum of Uvurkhangai, a dry area near the Gobi Desert with low density of wild birds (Figures 1, 2). All horses were unvaccinated and clinically healthy at the time of sampling. The herders reported no history of respiratory disease in the horses.
(SNIP)
Equids are clearly susceptible to infection by AIV H5N1. Abdel-Moneim et al. (7) described an outbreak of influenza in donkeys in Egypt in 2009; they isolated IAV H5N1 from nasal swabs and demonstrated that ≈26% donkeys sampled had been infected. In addition, H5 antibodies were detected in wild asses (Equus hemonius hemonius) in Mongolia (8); of note, a protein microarray was used in that study and the levels of reactivity were low.
Our findings show that horses are susceptible to infection by H5N1 viruses and that spillover events are likely frequent, highlighting the potential emergence of IAVs by reassortment between H3N8 EIV (the circulating subtype in horses) and H5N1 IAVs.
The ecologic conditions for reassortment are met in North America; 30% of the global horse population is located (9) there, EIV is endemic, avian influenza A(H5N1) clade B3.13 is spreading in cattle, and contact rates between cows and horses are likely to be high in agricultural settings. Consistent with our previous work showing that horses in Mongolia are commonly exposed to H3N8 AIVs in the absence of disease outbreaks (10), our results suggest that H5N1 infections in horses are likely to be subclinical, posing challenges to virus detection.
We recommend serologic surveys in premises that keep horses; such studies would aid early virus detection, provide a comprehensive picture of the changing ecology of IAVs, and inform the design of control measures to prevent influenza emergence.
Dr. Damdinjav is a veterinary researcher who studies transboundary animal diseases in Mongolia. He is the national project coordinator of the Food and Agriculture Organization of the United Nations in Mongolia. He collaborates with various international organizations to promote awareness on animal health issues.
It seems that every time we actually look, we find HPAI H5 in unexpected places.
Unfortunately, there is little appetite for looking for the virus in livestock - particularly if the infection is sub clinical - since any positive finding could result in economic losses for a farm, or even an entire industry.
Of course, we could get lucky, and find that H5N1 isn't capable of sparking a pandemic. But if we aren't willing to look for the most obvious threat, what else are we missing?
