Research Accomplishment Reports 2007

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

U. B. R. Balasuriya
Department of Veterinary Sciences

 

Project Description

In 2006, a multi-state occurrence of equine viral arteritis (EVA) was confirmed in Quarter horses in the State of New Mexico and nine other states. Until this occurrence, there was very little evidence of equine arteritis virus (EAV) infection in the Quarter horse population in the United States. Although the current modified live virus (MLV) vaccine against equine viral arteritis (EVA) is safe and efficacious, there is resistance to using it in horses in many countries. One of the major concerns is the safety of the current MLV vaccine in pregnant mares, in particular the ability of the attenuated virus to cross the placenta and infect the unborn foal. The vaccine is not recommended for use in pregnant mares, especially during the last two months of gestation, or in foals less than 6 weeks of age, unless they are at high risk of natural exposure to infection. Furthermore, horses that are vaccinated with the current MLV cannot be distinguished from naturally infected animals.

Following the 2006 EVA outbreak in the United States there is a strong industry demand for a marker vaccine to distinguish vaccinated animals from naturally infected animals, as well as to develop a MLV vaccine that is totally safe for use in pregnant mares. 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 MLV vaccine strain of EAV. To achieve this, we plan to target the major envelope protein (GP5) of EAV to generate an attenuated marker virus, and develop a companion serological assay that will enable differentiation of infected from vaccinated animals (DIVA). In addition, we propose to develop a panel of "disabled infectious single cycle" (DISC) EAV mutant viruses that are incapable of progressing beyond a single round of replication within the infected cell/animal.

DISC viruses combine the safety advantages of inactivated vaccines with the immunogenic activity of live viral vaccines. DISC viruses can infect their target cells in much the same fashion as native virus but they will not produce progeny virus particles, resulting in an abortive infection. Recombinant mutant DISC viruses will produce all the other viral proteins (as well as any inserted marker gene) with the exception of the deleted minor envelope protein and are capable of inducing a strong humoral and cellular immune response to these proteins. Prerequisite for these studies was to develop an infectious cDNA clone of the MLV vaccine strain of EAV.

Impact

In 2007 we developed an infectious cDNA clone of the current MLV vaccine (ARVAC(registered tm)) strain of EAV (prMLVB) in our laboratory. The cloned vaccine virus (rMLV) has 100% nucleotide identity with the master sequence of the parental MLV vaccine strain. We are now in a position to use this infectious cDNA clone of the MLV vaccine strain to develop a panel of DISC mutant viruses as well as marker viruses as potential recombinant vaccines against EVA. We believe that this research and expected achievements will result in 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. Patent pending (University of Kentucky Case 1502): An infectious cDNA clone of the modified live virus strain of the equine arteritis virus.