Isolation and Genetic Characterization of Genotype VII Velogenic Pathotype Newcastle Disease Virus from Commercial Chicken Farms in Central Ethiopia, Distinct from the Local Vaccine Strains.

Newcastle disease (ND) is caused by virulent strains of avian paramyxovirus type 1, also known as Newcastle disease virus (NDV). Despite vaccination, the frequency of reported outbreaks in Ethiopia has increased. From January to June 2022, an active outbreak investigation was conducted in six commercial chicken farms across areas of central Ethiopia to identify the circulating NDV strains. Thirty pooled tissue specimens were collected from chickens suspected of being infected with NDV. A questionnaire survey of farm owners and veterinarians was also carried out to collect information on the farms and the outbreak status. NDV was isolated using specific-pathogen-free (SPF)-embryonated chicken eggs and detected using haemagglutination and the reverse transcriptase-polymerase chain reaction (RT-PCR). The genotype and virulence of field NDV isolates were determined using phylogenetic analysis of fusion (F) protein gene sequences and the mean death time (MDT) test in SPF-embryonated chicken eggs. The questionnaire results revealed that ND caused morbidity (23.1%), mortality (16.3%), case fatality (70.8%), and significant economic losses. Eleven of thirty tissue specimens tested positive for NDV using haemagglutination and RT-PCR. The MDT testing and sequence analysis revealed the presence of virulent NDV classified as genotype VII of class II velogenic pathotype and distinct from locally used vaccine strains (genotype II). The amino acid sequences of the current virulent NDV fusion protein cleavage site motif revealed RRQKR↓F, unlike the locally used avirulent vaccine strains (GRQGR↓L). The epidemiological data, MDT results, cleavage site sequence, and phylogenetic analysis all indicated that the present NDV isolates were virulent. The four NDV sequences were deposited in GenBank with accession numbers F gene (PP726912-15) and M gene (PP726916-19). The genetic difference between avirulent vaccine strains and circulating virulent NDV could explain the low level of protection provided by locally used vaccines. Further studies are needed to better understand the circulating NDV genotypes in different production systems.