Newcastle Disease Virus (NDV) is an infectious avian pathogen with a single-stranded RNA genome belonging to the genus Orthoavulavirus and family Paramyxoviridae. NDV belong to a single serotype but genetic and antigenic diversity exists between NDV strains, thus representing a relevant challenge for the control of the disease in endemic countries. Based on the severity of the disease in chickens, NDVs are classified as apathogenic (avirulent), lentogenic (low virulent), mesogenic (moderately virulent) and velogenic (highly virulent). Apathogenic, lentogenic and mesogenic strains have been used for the preparation of vaccines, while the virulent strains are responsible for most of the reported outbreaks. In nature, two classes of NDVs have been identified namely; Class I and II. The class I NDV, isolated predominantly from wild birds, contained a single genotype with three sub-genotypes. The class II viruses have 21 genotypes with multiple sub-genotypes. In Nigeria, five different types of the virulent strains belonging to genotypes IV, VI, XIV, XVII and XVIII have been identified over the years.
Newcastle disease (ND) causes great economic losses, not only due to the cost of control measures and financial losses suffered by farmers but also by restricting trade across the regions. In many developed countries, vaccination programs and the implementation of biosecurity measures have helped to reduce the occurrence of the disease. However, the disease is widespread in poultry, especially in developing countries where the majority of the birds are raised on extensive husbandry system. The disease is enzootic in many developing countries causing severe losses to commercial poultry and backyard poultry and threatening the livelihood of rural communities whose source of protein and income depends on their small holder’s farm.
Over the years, the National Veterinary Research Institute (NVRI) produced several million doses of different strains of the NDV vaccines namely: Hitchner B1 and I2 (apathogenic) strains administered intraocularly, La Sota (lentogenic) strain administered via drinking water, and the Komarov (mesogenic) strain administered subcutaneously.
In the framework of an EU sponsored project, LIDISKI, involving partners from Europe (CIRAD and IZSVe) and Nigeria (NVRI, Ikore and NAERLS), standardized method that combines genetic and antigenic information was used to evaluate the efficacy of the commercial vaccines produced at NVRI against ND strains circulating across the Western African region. Firstly, local velogenic viruses, vaccine strains and corresponding antisera raised in specific-pathogen-free chickens were analysed in vitro using standardized methods to generate maps describing the antigenic match existing between the available vaccines and the currently circulating velogenic strains. These “antigenic maps” demonstrated the existence of a certain degree of antigenic diversity between the different velogenic strains tested, with a Nigerian virus belonging to the genotype XIV identified as a potential escape mutant of vaccine-induced immunity. Seven different NDV vaccination protocols, including boosting with a homologous experimental vaccine, were tested in a vaccination-challenge study in chickens. The homologous inactivated, oil-emulsion NDV vaccine was prepared from the divergent Nigerian genotype XIV NDV isolate using 2 different adjuvants. Considerable differences were identified both in terms of clinical protection offered and the ability to reduce the spread to unvaccinated birds by the different vaccination protocols. Based on the data generated from the experiment, the best vaccination scheme for the Nigerian epidemiological situation, able to induce optimal levels of immunity in the flocks with minimal virus shedding, will be determined and recommended for adoption in the vaccination of poultry flocks across the country.