RNA vaccines were tremendously successful during the Covid-19 pandemic due to excellent stimulation of immune responses combined with a fast and easy production process.

Traditional vaccines are typically based on subunit, inactivated or attenuated pathogens, which require long development times, making a rapid response to newly emerging pathogens difficult. In contrast, the RNA in RNA vaccines encodes the primary immunogenic protein(s) of the pathogen and can be developed quickly once the genetic sequence of a pathogen is known. This RNA is formulated or ‘packaged’ in lipid nanoparticles, cationic polymers, or hyperbranched polyglycerol (hPG) amines to allow for efficient transfer into host cells where the RNA of interest is translated into an immunogenic protein.

This protein then triggers an immune response against the very same protein expressed by the pathogen. Currently, RNA vaccines are being explored for a wide range of human and animal pathogens. The main advantages of RNA vaccines are that the production of RNA vaccines is technically simple, fast, easy-to-adapt, cost-effective and free of animal material.

There are currently two types of RNA vaccines, the conventional mRNA vaccine and self-amplifying (sa)RNA vaccines. The conventional formula contains a fixed concentration of packaged mRNA while saRNA vaccines may contain less target RNA than the conventional formula, but in addition contain a sequence encoding for an enzyme called ‘replicase,’ which amplifies the RNA sequence of interest, (i.e., itself). The advantage is a ‘smaller’ package with a longer duration of protein expression depending on the replicase half-life.

While vaccines for some pathogens are easy to design because there is only one, or few, immunologically important protein(s) for protection, other pathogens are more complex and require a multiple-target or cascade approach. This is one of the biggest limitations of RNA vaccines, as only a limited number of RNA sequences of interest – up to three – can be packaged into a single product or injection. Another disadvantage of RNA vaccines is storage, as most require storage at either -20°C or even at -80°C.

Equid alphaherpesvirus 1 (EHV-1) and 4 (EHV-4) are common equine viral pathogens. Horses in North America are routinely vaccinated against several viruses, including Western and Eastern Equine Encephalitis virus (WEEV, EEEV) and against West Nile virus, as well as equine influenza virus (EIV), with all of these vaccines performing reasonably well. In contrast, EHV-1 and EHV-4 outbreaks still occur in well-vaccinated populations. Thus, it is not surprising that the first efficacious nucleic acid vaccines have been developed against viruses for which protective immune responses can be induced with one (or few) immunogenic proteins, including coronavirus, flaviviruses and influenza viruses.

Unfortunately, EHV-1 and EHV-4 are not one of these viruses. Currently, challenges of developing effective mRNA or saRNA vaccines against equine herpes viral infections include the inherent antigenic complexity and immunosuppressive functions of EHV-1 and -4. With funding available through the Grayson Jockey Club Equine Research Foundation in Lexington, Kentucky, and in collaboration with Paul Lunn, BVSc, MS, PhD, MRCVS, Dipl. ACVIM, dean of the University of Liverpool’s School of Veterinary Science, in the U.K., and Juergen Richt, DVM, PhD, Director of Kansas State University’s Center of Excellence for Emerging and Zoonotic Animal Diseases (CEEZAD) and NIH COBRE Center on Emerging and Zoonotic Infectious Diseases (CEZID), in Manhattan, we are currently testing the safety and efficacy of EHV-1 mRNA and saRNA vaccines.

Based on previous experience, our vaccine prospects include nucleic acid sequences encoding EHV-1 glycoprotein D and the Immediate Early (IE) gene. The rationale is based on available results from previous studies with EHV-1, and ii) on the Shingrix® vaccine success story of a subunit vaccine that only contains the glycoprotein (g)E (analogous to EHV-1 gD) preventing shingles in adults after childhood varicella-zoster infection (chickenpox).

We expect these two components to be highly immunogenic, and to stimulate both cellular and humoral (antibodies) immunity. This project is ongoing, and we will provide updates on the progress. We anticipate that testing the vaccine in a neurological model of EHV-1 will show at least some protection from EHM which will be an improvement over currently available vaccines. In the future, this strategy can be further refined and tested for duration of immunity or effectiveness as a booster following vaccination with conventional vaccines. Because of the high sequence homology of the selected viral proteins, it is likely that this vaccine would also offer cross protection to EHV-4. Finally, once we can show the applicability of the technology for EHV-1 in horses, it could readily be adapted to other viral pathogens of horses.

Editor’s note: This is an excerpt from Equine Disease Quarterly, Vol. 34, Issue 1, funded by underwriters at Lloyd’s, London, brokers, and their Kentucky agents. It was written by Lutz S. Goehring, DVM, MS, PhD, Wright – Markey Professor of equine infectious diseases at the University of Kentucky’s Gluck Equine Research Center, in Lexington, Gisela Soboll Hussey, DVM, PhD, professor at Michigan State University’s College of Veterinary Medicine, in East Lansing, and Klaus Osterrieder, Dipl. ECVM, dean of City University’s Jockey Club College of Veterinary Medicine and Life Sciences in Hong Kong, professor of virology and chair at Freie Universität, in Berlin, Germany, and adjunct professor of virology at Cornell University, in Ithaca, New York.