Comparison of viral shedding following vaccination with inactivated and live Newcastle disease vaccines formulated with wild-type and recombinant viruses.

Virulent Newcastle disease virus isolates from the 1971 and 2002 U.S. outbreaks are of the same serotype but a different genotype than current vaccine strains. Prior experiments with inactivated vaccines in chickens show significantly less virus shed in birds vaccinated with a homologous vaccine (same genotype as challenge) compared to chickens vaccinated with genotypically heterologous vaccines. Subsequent experiments have compared the protection induced in chickens by live vaccines of B1 and LaSota (genotype II), Ulster (genotype I), and recombinant viruses that express the hemagglutinin neuraminidase gene (HN) or the HN and fusion gene (F) of CA 2002 (genotype V). Vaccinates were challenged with virulent viruses CA 2002 (genotype V) or Texas GB (TXGB, genotype II). After challenge with CA 2002 the birds vaccinated with a live recombinant genotype V virus containing the HN of CA 2002 shed significantly less virus in oropharyngeal swabs compared to B1 and had fewer birds shedding virus compared to B1, LaSota, and Ulster vaccinates. After challenge with CA 2002 birds vaccinated with the recombinant containing both the HN and F of CA 2002 (rA-CAFHN) shed less virus, and fewer birds shed virus compared to LaSota-vaccinated birds. TXGB-challenged LaSota-vaccinated birds shed less virus, and fewer birds shed virus compared to TXGB-challenged rA-CAFHN-vaccinated birds. Genotypic differences between vaccine and challenge did not diminish ability of vaccines to protect against disease, but genotypic similarity did reduce virus shed and may reduce transmission. The development and use of vaccines of the same genotype as the expected field challenge may provide an additional tool for control of this important poultry pathogen.