Stability of viability and immunizing potency of lyophilized, modified equine arteritis live-virus vaccine

The Bucyrus strain of equine arteritis virus, previously modified to avirulence and vaccinal virus by 131 serial passages in primary cell cultures of horse kidney followed by 111 passages in primary cell cultures of rabbit kidney, was further passaged in cultures of the E. Derm (NBL-6) cell line, a continuous diploid cell line. Pools of the 16th and 25th passages of the virus in this last equine dermal cell line were lyophilized and stored in lots at 37 C, 23 to 28 C, 4C, and -20 C. The viability of the vaccinal virus deteriorated rapidly during storage at 37 C and at 23 to 28 C, but was relatively stable at 4 C and at -20 C for at least 1 year. The vaccine stored at 4 C for 9 months protected 2 horses, subsequently inoculated with avirulent virus, from arteritis, whereas the 2 nonvaccinated control horses inoculated simultaneously developed severe signs of the disease and died of acute arteritis.