Newcastle disease virus strain I2--a prospective thermostable vaccine for use in developing countries.

Forty-five avirulent Australian strains of Newcastle disease virus had been examined for antigenicity in chickens and 18 of these were tested for thermostability. Strain I2, chosen for a combination of antigenicity and thermostability, was artificially selected for enhanced thermostability. Master seed material was then prepared in minimal disease eggs, and vaccine by a further two passages in conventional eggs. Strain I2 virus at seed and vaccine level induced adequate levels of antibody in chickens vaccinated by eye drop and usually in their contacts. The serological response to oral vaccination was less certain. Antibody titres indicative of substantial protection against virulent challenge were maintained in a simulated village flock for 38 weeks by vaccination of the foundation flock on two occasions, with subsequent vaccination confined to clutches of chicks as they were produced. Strain I2 virus survived for at least 12 weeks when stored at 22 degrees C in 1% gelatin. Strain I2 is suitable for local production of thermostable vaccine in regional laboratories in developing countries.