A research team including CEID member Andrew Park have identified serologic evidence of avian influenza A(H5N1) infections in horses, adding to growing concerns about the ongoing global H5N1 outbreak. Over a two-year period, horses from two distinct ecological regions were tested for antibodies against H5N1. The Arkangai Province, with extensive wetlands and large populations of migratory birds, and the drier Bulgan/Uvurkhangai Provinces, with fewer wild birds, were chosen as study sites. Serologic testing revealed two horses with H5N1 antibodies, indicating subclinical spillover infections. The horses showed no signs of illness, which suggests that H5N1 infections in equines may go undetected, posing a risk for unnoticed viral circulation in animal populations.
The detection of H5N1 in horses reflects a broader pattern of the virus infecting mammals during this unprecedented outbreak, driven by the clade 2.3.4.4b lineage. Initially considered a bird-specific virus, H5N1 has recently spread to various mammals, including foxes, minks, seals, and cattle, raising fears of cross-species transmission. In Europe, a 2022 outbreak at a mink farm demonstrated mammal-to-mammal transmission, a critical step toward the virus potentially adapting to humans. Equine influenza virus (EIV) of the H3N8 subtype is endemic in horses, and researchers warn that horses could serve as intermediate hosts where H5N1 and EIV reassort, creating a novel influenza strain capable of efficient transmission between mammals, including humans.
Given the virus’s expanding host range and its capacity for cross-species infection, the potential for a new influenza strain emerging from interactions between wild birds, livestock, and horses is a growing concern. The ongoing spread of H5N1 into unexpected hosts highlights the risk of viral reassortment, particularly in agricultural settings where contact between species is frequent. The discovery of H5N1 in horses raises the possibility that subclinical infections in equines could go undetected, allowing the virus to circulate unnoticed. This underscores the need for expanded serologic testing in equine populations to identify and monitor spillover infections that could accelerate the emergence of a new pandemic threat.
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