Horse industries around globe remain at high risk from equine flu – review

: Transmission of Equine Influenza Virus (EIV).
Transmission of Equine Influenza Virus (EIV). Image: Raj K. Singh, Kuldeep Dhama, Kumaragurubaran Karthik, Rekha Khandia, Ashok Munjal, Sandip K. Khurana, Sandip Chakraborty, Yashpal S. Malik, Nitin Virmani, Rajendra Singh, Bhupendra N. Tripathi, Muhammad Munir, and Johannes H. van der Kolk, CC BY 4.0, via Wikimedia Commons

Horse-related industries around the world remain at high risk of equine influenza outbreaks, the authors of a just-published review concluded.

Much of what is known about the equine flu virus has followed from techniques and findings in the human field, Fleur Whitlock, Pablo Murcia and Richard Newton wrote in the journal Viruses.

Influenza A viruses have a main natural reservoir in wild birds. They are highly contagious, continually evolve, and have a wide host range that includes horses, pigs, and humans.

The trio, with the University of Glasgow Centre for Virus Research, said equine influenza remains endemic in many countries, and seasonal human influenza epidemics occur annually.

Influenza epidemics and pandemics affect both species, usually after the introduction of a novel influenza A virus. “Ever changing, their threat is constant, and furthering our understanding of host-pathogen interaction is integral to inform control and prevention measures.”

Their review examined a range of aspects surrounding equine flu, ranging from elements of the virus itself to disease progression, immune response, clinical aspects, and factors relating to its spread. They also explored existing evidence around surveillance and preventive measures such as vaccines.

Examination of the development and progression of infections, including viral-host interactions, remains fundamental in combating influenza A viruses in humans and horses, they said.

Although much is known about human risk factors for infection and outcomes, similar information is still lacking in the equine field.

Equine veterinary surgeons should be encouraged to understand their role in improving scientific understanding of the equine flu virus by sampling suspect cases, reporting unexpected events following infection, and encouraging post-mortem examinations, the review team said.

“The epidemiology of infection in both species is similar, but with more emphasis on the ability of equine influenza virus to travel large distances by aerosolization and human viruses mainly relying on close contact for transmission.”

Virus, host, and environmental factors all have a strong influence on the specifics of disease occurrences, they said, including the clinical severity, the extent of spread, and success of control and prevention measures.

“Surveillance in both species has similar challenges to overcome and includes improving engagement and addressing the disparity in data quantity and quality between countries.”

Equine flu strains do not seem to have the same diversity and rate of change as human influenza viruses. As a result of this, together with a smaller commercial pharmaceutical market, equine vaccine strain updates are not employed in as timely a manner as they should be, the authors said.

“Alongside a relatively poor coverage by vaccination and variable implementation of additional biosecurity measures, equine industries remain at high risk of equine influenza occurrences.

“The main question to pose to the equine world would be to consider what the current preparedness for strains with pandemic potential is and how would prompt vaccine development and production be achieved.”

Looking at the threat from cross-species transmission, the researchers said it usually resulted in limited onward transmission in the new host.

“Although equine influenza infections among humans with horse exposure or through experimental infection have been confirmed serologically, human-to-human transmission has not been reported,” they noted.

With significant differences between certain receptors in equine flu virus and human influenza virus, there remains a host range restriction for influenza A viruses, they said. “Additionally, innate immune responses in the host prevent viral infection, although influenza A viruses have evolved strategies to overcome such barriers.”

However, interspecies transmission of the H3N8 equine flu virus occurred in dogs in the late 1990s, with sustained circulation in the canine population in North America, and sporadic spill-overs in Britain and Australia.

“Equids, therefore, have been an intermediate host for cross-species transmission of influenza A virus,” they said, noting that the H3N8 equine flu virus has been isolated from a camel, pigs, and infected cats under experimental conditions.

“Although cross-species transmission of influenza A viruses remains a concern, the relative human disease threat from equine influenza virus seems to be low, but given the ever-changing nature of influenza A viruses, open communication and a collaborative, joined-up approach between all parties are essential.”

They continued: “Given the zoonotic potential of influenza A viruses, an interdisciplinary approach in all species is essential.”

Whitlock, F.; Murcia, P.R.; Newton, J.R. A Review on Equine Influenza from a Human Influenza Perspective. Viruses 2022, 14, 1312.

The review, published under a Creative Commons License, can be read here

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