The experience gained with mRNA vaccines against Covid-19 could soon see this technology in use against pathogens such as the equine influenza virus, according to the authors of a just-published review.
mRNA, or messenger RNA, is the genetic material that tells the body how to make a protein. An mRNA vaccine delivers pieces of mRNA that tell the body how to make a viral protein that will be recognised as foreign by the immune system. This enables the immune system to be primed in case it is challenged by the Covid-19 virus. In the case of mRNA vaccines against Covid-19, they target the spike protein on the surface.
Equine influenza virus is a constantly evolving pathogen. There is currently no evidence of circulation of the original H7N7 strain of the virus worldwide; however, the EIV H3N8 strain, first isolated in the early 1960s, remains a major threat to most of the world’s horse populations.
The ability of the virus to constantly accumulate mutations in its antibody-binding sites enables it to evade host protective immunity, Fatai Oladunni and his fellow researchers noted in their paper published in the journal Viruses. This, they said, makes it a successful viral pathogen.
Protection against viruses is achieved by stimulation of strong cellular and humoral immunity in vaccinated horses. However, despite vaccine updates over the years, the virus still causes problems because the protective effects of vaccines decay and allow subclinical infections that result in transmission into susceptible populations.
In their review, the researchers describe how the evolution of the virus drives repeated outbreaks, even in horse populations with supposedly high vaccination coverage.
They traversed the approaches employed to develop effective vaccines for commercial use, the types of vaccines employed, and the existing system for recommendations on updating vaccines based on available clinical and virological data to improve protective immunity in vaccinated horse populations.
Understanding how the virus’s biology can be better harnessed to improve vaccines is central to controlling the pathogen, they said.
The authors noted that equine influenza virus strains in circulation undergo antigenic drift, albeit at a much slower rate than human, avian, or swine influenza A viruses.
“Therefore, vaccine manufacturers, to keep up with antigenic drift, should periodically update the virus strains in their vaccines. However, vaccine manufacturers are not required to do so, and it is possible for a product to remain on the market long after it has become antigenically obsolete.”
They said leading manufacturers understand the scientific need for updating, with guidance having been available from the international Equine Influenza Expert Surveillance Panel from the mid-1990s.
The review team says the economic burden of equine influenza runs into billions of dollars, and the disease exacts a considerable cost in developing nations where equids are widely used as working animals.
“As a result, reinforced efforts aimed at preventing equine influenza outbreaks in horse populations are valuable contributions to the continued survival of large and small equine operations.”
The constant evolution of the virus, the transboundary nature of the disease, and evidence of poor vaccine performance necessitated the ongoing international surveillance effort, unique for animal influenza viruses, advising on the updating of strains incorporated in commercial vaccines.
They said that, despite the availability of different types of vaccines against the equine influenza virus, there are certain challenges to effective immunization in horses. These include the constant evolution of the virus, vaccine breakdown, vaccination-induced short-lived immunity, immunity gap, the inability of vaccines to induce sterilizing immunity, and, in young horses, interference with maternally derived immunity.
“Many equine influenza vaccine technologies are used or being investigated for their safety and ability to induce long-lasting protective immunity,” they said.
“So far, it appears that only modified live-attenuated vaccines can induce a long-lasting protective immunity by triggering multiple arms of the immune system.
“Considerable practical experience with mRNA vaccines has been gained from their use against SARS-CoV-2,” they noted, “and this technology may soon find veterinary applications against pathogens such as equine influenza virus.”
Many groups are evaluating equine flu vaccines to understand how to induce long-lasting sterilizing immunity, reduce the effect of strain differences, and close the immunity gap between vaccine doses in young horses, they said.
“As with human influenza, the prospect of a universal equine influenza virus vaccine targeting conserved domains is tantalizingly close.”
Since neither equine influenza vaccine coverage nor surveillance is universal, influenza in horses is not going away in the foreseeable future, they said.
“However, the results from these studies should lead to more improved equine influenza vaccines that will reduce the severity and spread of disease outbreaks into minor episodes and also help to combat newly emerging equine influenza virus strains in the future.”
The review team comprised Oladunni, with the Texas Biomedical Research Institute in San Antonio and the Department of Veterinary Microbiology at the University of Ilorin in Nigeria; Saheed Oluwasina Oseni, with the Department of Biological Sciences at Florida Atlantic University; Luis Martinez-Sobrido, with the Texas Biomedical Research Institute; and Thomas Chambers, with the Gluck Equine Research Center at the University of Kentucky.
Oladunni, F.S.; Oseni, S.O.; Martinez-Sobrido, L.; Chambers, T.M. Equine Influenza Virus and Vaccines. Viruses 2021, 13, 1657. https://doi.org/10.3390/v13081657