Equine Herpesvirus Type 1 Mutant Defective in Glycoprotein E Gene as Candidate Vaccine Strain

Abstract

An equine herpesvirus type 1 (EHV-1) mutant, ΔgE, defective in glycoprotein E (gE) was evaluated as a modified live virus (MLV) vaccine. Colostrum-deprived Thoroughbred foals inoculated intranasally (i.n.) or intramuscularly (i.m.) with ΔgE did not exhibit any clinical signs of respiratory disease except for a mild nasal discharge in 1 i.n. inoculated foal on Days 1 and 3 post-infection. In contrast, the intranasal inoculation of foals with the revertant of ΔgE resulted in biphasic pyrexia, mucopurulent nasal discharge and swelling of submandibular lymph nodes. These results indicated that gE plays an important role as regards EHV-1 virulence in horses. The ability of ΔgE to protect against wild type EHV-1 challenge infection was assessed using i.m. vaccinated foals. Foals inoculated twice i.m. with 10⁵ or 10⁶ plaque-forming units (pfu) of ΔgE at an interval of 3 weeks exhibited no clinical evidence of local inflammation, respiratory disease or deleterious systemic responses. Remarkable increases in SN antibody titer to EHV-1 were observed in all vaccinated foals after the 2nd inoculation with ΔgE. Following a wild type EHV-1 challenge infection, vaccinated foals showed milder clinical symptoms than foals vaccinated with a placebo. Specifically, 1 of 3 foals vaccinated with 10⁶ pfu of ΔgE exhibited no clinical symptoms other than a mild nasal discharge for 1 day. Additionally, the virus load of nasal shedding and viremia were reduced by vaccination. These results suggest that ΔgE would be a good candidate as an MLV vaccine.