Improved Newcastle disease vaccine strategies to reduce shedding of virulent virus from infected birds

Abstract

Newcastle disease threatens the poultry industry throughout the world, adversely affecting poultry producers, backyard farmers, and the economies of entire nations. Newcastle disease virus (NDV) is also known as avian paramyxovirus serotype -1 virus (APMV-1). APMV-1 all belong to a single serotype, and by definition any Newcastle disease (ND) vaccine strain should provide protection against morbidity and mortality from any NDV challenge. However, available vaccines do not protect against infection. Vaccinated birds exposed to a virulent NDV likely will be infected and shed virus without showing symptoms of ND. Vaccination with live and killed ND vaccines is universally practiced in the United States (U.S.) and in many other countries, with some exceptions. The vaccine seed strains are viruses of low virulence that were originally isolated in the 1940s, and genetically are distant from most recent virulent viruses. Currently, when a flock has birds infected with virulent NDV the birds exposed or possibly exposed are depopulated to contain the spread of infection. The most recent outbreaks in the U.S. are believed to have originated from viruses spread from birds that entered the U.S. from Mexico. Viruses that cause future outbreaks likely will be in the same genetic lineage as these recent outbreak viruses. The experiments described were performed to test the hypothesis that increasing the genetic relatedness of the ND vaccine virus to the likely virulent challenge virus will produce more specific neutralizing antibodies and decrease the amount of challenge virus shed from vaccinated poultry. Various inactivated and live vaccines including four recombinant viruses, some of which contained genes of the California 2002 outbreak virus were tested against virulent challenges with California/212676/2002 and chicken/U.S./(TX)GB/1948. Inactivated homologous vaccines reduced the amount of virus shed orally better than heterologous vaccines. The inactivated vaccines made with recombinant viruses with varying relatedness to the challenge virus again showed that vaccines more genetically related to the challenge virus worked best to reduce oral viral shedding. All of the live vaccines provided better protection with less virus shed even compared to the homologous inactivated vaccine. However, birds vaccinated with live vaccines more genetically related to the challenge virus often shed less virus in oropharyngeal swabs and had significantly fewer birds shedding virus compared to live vaccines less similar to the challenge virus.