Research Accomplishment Reports 2009

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Molecular Basis of Attenuation of the Modified Live Virus Vaccine Strain of Equine Arteritis

U. B. R. Balasuriya
Department of Veterinary Sciences


Non-Technical Summary

The United States is the only country with a substantial horse breeding industry that does not have any EAV testing requirements for imported stallions or semen. The situation is complicated by widespread lack of awareness among horse owners and breeders about EAV and differences in breed prevalence of EAV infection. In an effort to clarify the risks associated with EAV infection of horses, we propose to define the genetic determinants of virulence of EAV. The purpose of this study is to identify potential virulence determinants of EAV by sequence analysis of virus strains of defined virulence phenotype.These efforts eventually should facilitate the rapid identification of new strains of the virus of enhanced virulence, more especially their ability to cause abortion in mares. It should also help to better delineate the role of the carrier stallion in generating them, and lead to the eventual development of an improved and MLV vaccine for EAV.

2009 Project Description

Our major research focus has been to definitively characterize the molecular epidemiology and pathogenesis of equine arteritis virus (EAV) infection of horses, and to develop recombinant vaccines to prevent infection of horses with this virus as well as improved tests to diagnose the infection. The principal aim of our ongoing research is to take advantage of the latest molecular virology techniques to develop a safer, highly effective vaccine against EVA based on an infectious cDNA clone of the modified live virus (MLV) vaccine strain of EAV developed in our laboratory. We have used this infectious cDNA clone and reverse genetic technology to generate several recombinant marker viruses that will enable differentiation of infected from vaccinated animals (DIVA).

Furthermore, we are in the process of evaluating the feasibility of developing a novel recombinant vaccine against S. neurona infection based on the infectious cDNA clone of the MLV vaccine strain of EAV. The general strategy is to develop a panel of EAV-SnSAG (Sarcocystis neurona surface antigen) recombinant viruses that express genes encoding the immunodominant SnSAGs. EAV-SnSAG virus strains will be used to immunize horses and the recombinant viruses that yield promising results (e.g. strong immune responses) will be further investigated in future challenge studies to determine their ability to provide immune protection against equine protozoal myeloencephalitis in the future. The resulting EAV-SnSAG viruses will provide bivalent vaccines with the potential for preventing both EPM and equine viral arteritis. These studies are currently in progress.

In addition, the development of several chimeric infectious clones of virulent and avirulent strains of EAV has allowed us to expand our studies to identify the EAV target cell population in equine PBMCs and to carry out a detailed characterization of the interaction between the target cell population and the virus. Using a panel of five recombinant chimeric viruses we demonstrated that interactions between GP2, GP3, GP4, GP5 and M envelope proteins play a major role in determining the CD14+ monocyte tropism while the tropism of CD3+ T lymphocytes is only determined by GP2, GP4, GP5 and M envelope proteins but not the GP3 protein. This study showed, for the first time, that CD3+ T lymphocytes may play an important role in the pathogenesis of equine viral arteritis when horses are infected with the virulent strains of EAV. This study also demonstrated that susceptibility of CD3+ T lymphocyte subpopulations to in vitro infection with virulent Bucyrus (VB) strain of EAV differ among horses and they could be divided into susceptible and resistant groups. Based on this finding, we assumed that a horse's genetic background may play a significant role in determining the clinical outcome of primary infection with EAV as reported in other species.

Efforts are underway in our laboratory to investigate if there is a genetic basis for the differences between horses in the susceptibility of their monocytes and CD3+ T lymphocytes to VB infection by analyzing possible associations with single nucleotide polymorphisms (SNP).

2009 Impact

We believe that this research and expected achievements will lead to the development of genetically modified novel vaccines, and improved means of immunization against EVA. If successful, these strategies will offer an entirely new approach not only to control EVA, but also for the control of other important veterinary viral diseases (e.g. porcine reproductive and respiratory syndrome in pigs). Compared to the existing means of immunization, the new vaccines will provide a) higher efficacy; b) higher safety; c) specific discrimination between vaccinated and naturally infected animals. Finally, these studies will help to better characterize the pathogenesis of EAV infection in horses.

2009 Publications

Smith, K.L., Allen, G.P., Branscum, A.J., Cook, R.F., Vickers, M.L., Timoney, P.J., and Balasuriya, U.B.R. (2009) Increased prevalence of neuropathogenic strains of EHV-1 in equine abortions. Vet. Micro. (2009 Aug 8. [Epub ahead of print]).

Lu, Z., Chambers, T., Boliar, S., Timoney, P.J., Branscum, A.J., Reedy S.E., Tudor, L., Dubovi, E., Vickers, M.L., Sells, S., and Balasuriya, U.B.R. (2009) Development and evaluation of one-step TaqMan real-time reverse transcription-PCR assays targeting NP, M and HA genes of equine influenza virus. J. Clin. Microbiol. 47(12):3907-3913.