The Modern Equine Vet
October 2023
Vol 13 Issue 10 2023
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Infectious Diseases

Can We Reduce Transmission of EIV?

An analysis of the influenza A virus, H3N8, which spilled over from horses to dogs suggests that host population and epidemiology are driving the sustained transmission of equine influenza virus (EIV) clade FC1 among horses.

The data suggest “that well-targeted interventions could in turn result in the extinction of EIV H3N8 FC1 in horses,” the researchers wrote. They advocated for screening horses that travel internationally for influenza before entering a new country, which could reduce transmission worldwide.

“Rational human intervention may be necessary to ‘break’ the host-structure epidemiology that is sustaining this virus,” they wrote.

A recent publication by Brian Wasik, PhD, a research associate in the Parrish Lab at the Baker Institute for Animal Health, at Cornell University, and colleagues investigated the influenza virus that jumped from horses to dogs in the early 2000s to see the epidemiological differences between the 2 hosts. They reviewed genomic data of H3N8 in both species from Cornell sequencing and public repositories starting around 2001.

In this case, this strain of CIV did eventually die out among canines, but has not died out among equines, according to the analysis.

Experts believe H3N8 EIV was introduced to Florida from South America in 1963, and it continues to circulate among horses today. Around the year 2000, the H3N8 EIV strain divided into 2 clades: the Florida Clade 1 (FC1), which primarily affected horses in North America and Florida Clade 2 (FC2), which primarily affects horses in Europe. Although these clades have kept some degree of geographic separation and are not considered competitive with each other, both clades have been seen worldwide, the researchers found.

The incidence of EIV among U.S. horses is less than 10% with seasonal variability, according to voluntary surveillance from the voluntary Equine Respiratory Biosurveillance Program, supported by Merck Animal Health, and lead by Nicola Pusterla, DVM, DACVIM, and the UC Davis Equine Infectious Disease Research Laboratory. About 60% of equine operations administer EIV vaccines, which include antigens from the most commonly circulating strains.

The H3N8 EIV spillover to dogs probably occurred around 1999, and in 2003, the infection was diagnosed among racing greyhounds in a Florida training facility, and it is believed that they spread CIV to several regions of the country. Subsequent transmission occurred among dogs in dense populations, such as animal shelters, kennels and doggie day care centers. It was particularly persistent in the Central and Eastern regions of the country. Scattered outbreaks also were reported in the United Kingdom, Australia and a few other countries. However, by 2016, the H3N8 strain of CIV appeared to become extinct among dogs, according to the researchers.

“Both horses and dogs can be mammalian hosts to influenza viruses. Sometime around 2000 an influenza circulating in horses (H3N8) spilled over into dogs, likely among greyhounds in Florida. Subsequent outbreaks around the United States established the first established Canine Influenza Virus of the modern era. Our last detection of H3N8 canine influenza was in 2016. Our study meant to ask if this apparent ‘extinction’ of canine influenza had to do with the virus itself or to the nature of the 2 host species,” Dr. Wasik explained.

“Our findings, coming from full genome genetic analysis, identified that differences in virus evolution were minimal. We further found that canine influenza was limited geographically, being confined to shelters in major metropolitan centers. In contrast, equine influenza continues to spread near globally and the major clade of the virus appears to be maintained by circulation within the United States. International transfer of horses links equine influenza viruses across continents. Our results suggest that an improved understanding of horse population dynamics and stakeholder cooperation could empower the veterinary world to reduce the burden of equine influenza through screening, quarantine and improved vaccination strategies.” MeV

For more information:
Waski BR, Rothschild E, Voorhees IEH, et al. Understanding the divergent evolution and epidemiology of H3N8 influenza viruses in dogs and horses. Virus Evol. 2023;9(2):vead052.