Safety, efficacy and immunogenicity of a modified-live equine influenza virus vaccine in ponies after induction of exercise-induced immunosuppression

Objective-To determine safety, efficacy, and immunogenicity of an intranasa l cold-adapted modified-live equine influenza virus vaccine administered to ponies following induction of exercise-induced immunosuppression. Design-Prospective study. Animals-Fifteen 9- to 15-month old ponies that had not had influenza. Procedure-Five ponies were vaccinated after 5 days of strenuous exercise on a high-speed treadmill, 5 were vaccinated without undergoing exercise, and 5 were not vaccinated or exercised and served as controls. Three months la ter, ail ponies were challenged by nebulization of homologous equine influe nza virus. Clinical and hematologic responses and viral shedding were monit ored, and serum and nasal secretions were collected for determination of in fluenza-virus-specific antibody isotype responses. Results-Exercise caused immunosuppression, as indicated by depression of ly mphocyte proliferation in response to pokeweed mitogen. Vaccination did not result in adverse clinical effects, and none of the vaccinated ponies deve loped clinical signs of infection following challenge exposure. In contrast , challenge exposure caused marked clinical signs of respiratory tract dise ase in 4 control ponies. Vaccinated and control ponies shed virus after cha llenge exposure. Antibody responses to vaccination were restricted to serum IgGa and IgGb responses in both vaccination groups. After challenge exposu re, ponies in all groups generated serum IgGa and IgGb and nasal IgA respon ses. Patterns of serum hemagglutination inhibition titers were similar to p atterns of IgGa and IgGb responses. Conclusions and Clinical Relevance-Results suggested that administration of this MLV vaccine to ponies with exercise-induced immunosuppression was saf e and that administration of a single dose to ponies provided clinical prot ection 3 months later.