Strong similarities between avian influenza viruses found in South American birds and the equine flu virus have been identified by researchers.
H3N8 equine influenza viruses were first reported in the southern United States in 1963 during an outbreak in horses imported from Argentina.
This emergence resulted in a pandemic that led to international co-circulation of H7N7 and H3N8 equine flu viruses during the 1960s and 1970s, likely causing cross-protection immunity that might have contributed to the eventual extinction of H7N7 lineage.
Today, H3N8 equine flu viruses represent a single genetic lineage capable of inducing serious respiratory disease in susceptible horses.
Genomic analysis suggests that equine H3 viruses are of an avian lineage, likely originating in wild birds.
Equine-like internal genes have been identified in avian influenza viruses isolated from wild birds in the southern region of South America.
However, an equine-like hemagglutinin has not been identified. Hemagglutinin are glycoproteins that cause red blood cells to clump together.
Nicolas Bravo-Vasquez and his fellow researchers, writing in the journal Emerging Infectious Disease, report that they have isolated six distinct H3 viruses from wild birds in Chile that have hemagglutinin, nucleoprotein, nonstructural protein 1, and polymerase acidic genes very similar to the 1963 H3N8 equine influenza virus lineage.
Despite the nucleotide similarity, viruses from Chile were antigenically more closely related to avian viruses and transmitted effectively in chickens, suggesting adaptation to the avian host.
In all cases, the flu viruses were isolated from waterfowl belonging to the Anatidae and Rallidae families, suggesting that circulation of these viruses might be restricted to non-migratory species found only in the southern part of South America.
The findings, they said, show that equine-like H3 hemagglutinin remains in a wild bird reservoir, with direct descendants of the virus that originated the equine influenza virus pandemic continuing to circulate in wild birds in Chile.
The authors noted that although H3 is the most common subtype found in wild birds from North America, typical avian origin H3 subtypes are seldom recovered in South America. To date, only the four isolates in Peru and two in Chile, all resembling contemporary North American avian influenza viruses, have been described.
“Global trade of thoroughbred horses from South America carrying the original 1963 H3N8 equine influenza virus, and not migratory birds, was responsible for the spread of these avian-origin gene segments.
“In comparison, although independent avian-to-equine transmission of lineage H3N8 AIV from Asia to horses was described in eastern Asia in the late 1980s, the epizootic event that followed was self-limited and died out after a few years.
“Subsequent active surveillance of wild birds in Mongolia has shown that avian influenza viruses carrying several gene segments closely related to H3N8 equine influenza virus from Asia are still circulating in wild birds, similar to our findings in Chile.
“This finding,” they said, “suggests that avian-to-equine transmission of H3 influenza A viruses is not an uncommon event.
“The conditions and key genetic signatures that facilitated the species-jump and rapid adaptation from waterfowl to horses in South America and eastern Asia remain unknown.”
However, it is not uncommon to find wild waterfowl next to free-ranging horses in South America, which enables repeated transmission events that might lead to the emergence of a new virus strain with pandemic potential in horses, the authors noted.
“In summary,” they continued, “our data provide evidence that gene segments, including hemagglutinin, that are the closest ancestor of the 1963 H3N8 equine influenza virus, continue to circulate in wild bird reservoirs.
“We recommend increased surveillance to better clarify the role of this subtype in the context of genetic diversity of IAVs [influenza A viruses] in South America, its epidemiology and ecology, and the risk that this new subtype represents to avian and mammalian hosts.”
The study team comprised Bravo-Vasquez, Jiangwei Yao, Pedro Jimenez-Bluhm, Victoria Meliopoulos, Pamela Freiden, Bridgett Sharp, Leonardo Estrada, Amy Davis, Sean Cherry, Brandi Livingston, Angela Danner, Stacey Schultz-Cherry, and Christopher Hamilton-West, variously affiliated with the University of Texas Health Science Center in Houston; the Centers for Disease Control and Prevention in Atlanta, Georgia; the University of Chile in Santiago; and St Jude Children’s Research Hospital in Memphis, Tennessee.
Bravo-Vasquez N, Yao J, Jimenez-Bluhm P, Meliopoulos V, Freiden P, Sharp B, et al. Equine-Like H3 Avian Influenza Viruses in Wild Birds, Chile. Emerg Infect Dis. 2020;26(12):2887-2898. https://doi.org/10.3201/eid2612.202063