The VG/GA strain of Newcastle disease virus: mucosal immunity, protection against lethal challenge and molecular analysis.

The Villegas-Glisson/University of Georgia (VG/GA) strain of Newcastle disease virus (NDV) isolated from the intestine of healthy turkeys has been proposed to replicate in the respiratory and intestinal tract of chickens. In the present study, the virus distribution, the mucosal and systemic immune response, the efficacy against lethal challenge and the full genome sequence of the VG/GA strain were compared with the La Sota strain of NDV. The VG/GA strain was detected at different time points in the respiratory and intestinal tract of chickens with a preferential tropism for the latter. Both the VG/GA and La Sota strains induced NDV-specific immunoglobulin A (IgA) at the upper respiratory tract. IgA levels were higher in the trachea for the La Sota strain, while they were higher in the bile and intestine for the VG/GA strain. Positive correlation between virus distribution of the viruses and IgA production was observed. Despite the presence of the maternal antibodies in broilers, early vaccination with the VG/GA strain afforded 95% to 100% protection against lethal challenge, equivalent to the protection conferred by the La Sota strain. Full genome sequence analysis classified the VG/GA strain within class II, genotype II viruses, which also include most of the respirotropic vaccine strains. Differences with the La Sota strain at the nucleotide and amino acid levels that may explain the differential phenotype of the VG/GA were observed; however, verification of the significance of those changes is required. Taken together, these results validate field observations on the efficacy of VG/GA vaccination and demonstrated the unique characteristics of the strain.