Protection of chickens against overt clinical disease and determination of viral shedding following vaccination with commercially available Newcastle disease virus vaccines upon challenge with highly virulent virus from the California 2002 exotic Newcastle disease outbreak.

During 2002-2003, exotic Newcastle disease (END) virus caused a major outbreak among commercial and backyard poultry in southern California and adjacent states. The outbreak raised concerns regarding the protective immunity of commercially available vaccines for prevention and control of this virus in poultry. We sought to determine if existing commercial live and inactivated Newcastle disease virus (NDV) vaccines could provide protection against the 2002-2003 END virus, and whether current commercial NDV-vaccination programs for broiler-breeders (BB) and broilers (Br) would protect against END-challenge. In the first experiment, birds received a single dose of either inactivated or live B1-type vaccine at 2 weeks-of-age and were challenged 2 weeks post-vaccination with a lethal dose of END. In the second experiment, a high (10(6.9)EID50/bird) or low (10(3.9)EID50/bird) dose of live B1 was applied to 8-week-old chickens, followed by lethal END challenge. In the third experiment, NDV field-vaccinated commercial BB (65 weeks-of-age) and Br (36 days-of-age) were challenged against END virus. Results indicated that both the live and inactivated vaccines protected against morbidity and mortality and significantly reduced the incidence and viral titers shed from chickens in comparison with sham controls, but did not prevent infection and virus shedding. In addition, both doses of live vaccine protected birds and significantly decreased the number of birds shedding virus. All unvaccinated control chickens challenged with END died within 6 days post-challenge (pc). Protection from disease correlated with the presence of antibody titers (determined by enzyme-linked immunosorbent assay (ELISA) or hemagglutination inhibition (HI)) at day of challenge. Commercial BB were protected from disease and exhibited low incidence and titer of challenge virus shed. In contrast, commercial Br exhibited 66% mortality and shed significantly more virus than the BB birds. These results underscore the need to develop new NDV vaccines and vaccine strategies for use during outbreak situations to protect birds from both disease and infection to reduce virus shedding.