A new intranasal, modified-live virus vaccine for equine H3N8 influenza

Background: Equine-2 (H3N8) influenza is the most frequent cause of infectious upper respiratory disease in horses in the USA. Conventional inactivated-virus vaccines have been largely ineffective for prevention of this disease, even following repeated doses. We developed a modified-live virus vaccine derived by cold-adaptation from a wild-type strain of equine-2 influenza virus, and demonstrated its safety and efficacy in equines. Methods: The wild-type influenza A/equine/2/Kentucky/91 virus was serially passed in embryonated eggs at temperatures reduced stepwise from 34 to 26 T. Temperature sensitivity and stability was tested in MDCK cells. Safety and efficacy tests, including experimental challenges with wild-type viruses, were conducted in seropositive and seronegative equines. Results: Randomly selected clones from the 49th passage were temperature sensitivity (ts) with 34/39.5degrees infectivity ratios > 10(4). The ts phenotype was retained through five serial passages in MDCK cells. The clone selected as vaccine candidate (P821) was further tested in equines and had the desired degree of attenuation: it successfully infected seropositive animals based on detectable virus shedding, yet induced no significant disease symptoms in seronegative animals. The candidate vaccine was tested for stability of ts phenotype through five serial horse-to-horse passages; for safety and efficacy in exercised-stressed animals; for efficacy against challenge with heterologous strains of equine-2 influenza virus; and for safety in large-scale field studies. Each of these tests was passed successfully. Efficacy following single doses was established on the basis of protection from clinical disease, as the vaccine induced little serum antibody in single-dosed animals. Conclusion: A modified-live virus vaccine for equine-2 influenza has been developed which is safe and effective in horses following a single dose administered intranasally. (C) 2001 Elsevier Science B.V. All rights reserved.