PATHOGENICITY, IMMUNOGENICITY, PROTECTION EFFICACY, AND SPIKE PROTEIN GENE SEQUENCE OF A HIGH-PASSAGE TURKEY CORONAVIRUS SERIALLY PASSAGED IN EMBRYONATED TURKEY EGGS

Experimental infection of a high-passage turkey coronavirus passaged serially in embryonated turkey eggs for 344 times (P344 TCoV 540) showed no enteritis-related clinical signs, decreased body weight gains, gross, and microscopic lesions. TCoV spike (S) protein specific antibodies appeared from 14 days post infection (dpi) and increased gradually. Virus neutralization (VN) titers of the serum from P344 TCoV 540-inoculated turkeys were 1:13 at 14 dpi, 1:16 at 28 dpi, and 1:36 at 56 dpi against P344 TCoV 540. P344 TCoV 540-inoculated turkeys were protected against the challenge by homologous P344 TCoV 540 completely or low passage P3 TCoV 540 partially as revealed by lack of histopathological alterations, absence of TCoV by immunofluorescent antibody assay in the intestines, and reduction in TCoV viral RNA loads in the intestines and feces. The serum from P344 TCoV 540-vaccinated turkeys had higher VN titers against P344 TCoV 540 than those against P3 TCoV 540. P344 TCoV 540 had 52 amino acid substitutions as compared to those of P3 TCoV in the S protein. The results indicated that a high passage TCoV can induce protective humoral and cellular immune response and have potentials to become an attenuated vaccine.