The immune response in horses to vaccination against equine influenza virus. A comparison of recombinant DNA, inactivated-virus, and modified-live virus vaccines

Equine influenza virus is a cause of upper respiratory disease that has economic significance to the horse industry. It is a highly contagious virus that is spread among horses despite vaccination. Due to antigenic changes in surface proteins and the ineffectiveness of vaccines, equine influenza has been maintained as an endemic disease within the equine population.; Various studies have shown that the use of inactivated-virus vaccines results in short-lived immunity that does not prevent infection but reduces the incidence of severe clinical disease. The implications of this partial immunity in equine influenza are two-fold. A sub-clinical infection will often not only show a reduction in performance, but serve as a potential threat to susceptible horses during racemeets and show events.; Because conventional inactivated vaccines against equine influenza are ineffective, novel approaches using recombinant DNA technology have been investigated. This study examines the potency of a recombinant DNA vaccine consisting of an equine herpes-4 virus vector expressing the hemagglutinin and neuraminidase genes of the A/equine/2/Kentucky/94 strain of equine influenza virus. The immune response in horses was determined following inoculation with a recombinant DNA vaccine and compared to that seen after using a modified-live and inactivated-virus vaccine or natural infection. Horses were then challenged with equine influenza virus and the ability of each vaccine to prevent infection or reduce severe clinical disease was determined.; The results of this study indicate that the recombinant DNA vaccine examined here failed to induce an immune response due to over-attenuation. The modified-live and inactivated virus vaccines provided a reduction in the severity of clinical disease compared to that of an unvaccinated control. Protection from severe disease was associated with serum antibodies in the inactivated vaccine recipients and serum, local, and cell-mediated immunity in the modified-live vaccinates. Horses initially infected with live virus were completely protected against infection upon challenge 8 weeks later. This study supports previous investigations and concludes that (1) inactivated-virus vaccines provide incomplete protection through humoral immunity, (2) modified-live vaccines induce serum and local antibody formation, and a cell-mediated immune response, and (3) repeat vaccinations are required to maintain immunity against infection with equine influenza virus.