Introduction
Newcastle disease (ND) is a highly contagious disease with significant
clinical impact and serious losses in poultry worldwide. The causative
agent, Newcastle disease virus (NDV) or avian paramyxovirus type-1
(APMV-1) is a species within the genus Avian orthoavulavirus 1 , a
member of the family Paramyxoviridae(Rima et al., 2019). The genome is
composed of a negative-sense, single-stranded RNA of approximately 15 kb
coding for at least seven proteins, in the 3β to 5β order nucleocapsid
protein (NP), phosphoprotein (P), V protein (V), matrix protein (M),
fusion protein (F), hemagglutinin-neuraminidase (HN) and large
RNA-dependent polymerase protein (L)
(Steward et al., 1993).The HN protein
recognizes and attaches to sialic acid receptors on the surface of
permissive cells and mediates the fusion activity of the F protein at
the cell membrane for release of the nucleocapsid complex into the
cytoplasm (Lamb & Parks, 2007).
Based on its pathogenicity, NDV is categorized into four groups with
increasing virulence from apathogenic, lentogenic, mesogenic and
velogenic (Alexander, 2000). The molecular
basis for NDV pathogenicity is associated with the amino acid sequence
motif of the protease cleavage site of the fusion protein, and the
abilities of specific cellular proteases to cleave this protein. Based
on phylogenetic analysis, NDV can be distinguished in two distinct
classes, 1 and 2, with one (1.I) and twenty one genotypes, respectively
(2.I-2.XXI) (Dimitrov et al., 2019).
Whereas the majority of class 1 viruses are avirulent and have their
natural reservoir in aquatic wild birds
(Kim et al., 2007), class 2 comprises
virulent viruses that are responsible for the outbreaks in poultry and
evolved over time(P. J. Miller et al.,
2010). Phylogenetic evidence suggests that in the decades prior to the
1970s, genotypes 2.II, 2.III and 2.IV were predominant in North America,
Asia and Europe, respectively. Only genotype 2.II comprises of
lentogenic, mesogenic and velogenic pathotypes
(Czegledi et al., 2002;
Czegledi et al., 2003). and vaccines
derived from lentogenic strains of this genotype
(Seal et al., 1996). Genotype 2.VI
viruses emerged in epizootics in the Middle East and Asia in the 1960s,
while in the early 1970s genotype 2.V (most likely of South American
origin) caused outbreaks in North America and in Europe
(Herczeg et al., 2001;
Wehmann et al., 2003). Later, genotype
2.VI viruses gave rise to strains adapted to pigeons, the so-called
pigeon type paramyxovirus (PPMV-1) (D. J.
Alexander et al., 1985). The new NDV nomenclature proposal
(Dimitrov et al., 2019) placed subgroups
2.VIai and 2.VIaii, encompassing the early strains circulating in
poultry (Chong et al., 2013), into the new
genotypes 2.XX and 2.XXI whereas PPMV-1 is forming the βnewβ genotype
2.VI with several sub-genotypes. Most recently, several sub-lineages of
genotype 2.VII and 2.VIII emerged in the Far East and spread to further
geographic areas (Herczeg et al., 1999).
Genotype 2.VII is currently predominant among velogenic NDV and can be
further subdivided i.e. with sub-lineages 2.VII.1.1 encompassing viruses
that emerged in the 1990s in the Far East, Europe and Asia, and the
Middle East; while 2.VII.1.2 viruses evolved in China, and circulated in
South and East Africa, as well as in Israel and Pakistan
(Dimitrov et al., 2019).
In Egypt, ND is endemic and despite extensive routine vaccination
programs implemented in the commercial poultry farm sectors, new cases
occur continuously, posing a threat to the national poultry industry. In
Egypt, ND was identified for the first time in 1948
(Daubney & Mansy, 1948), and in the last
few years NDV 2.II and 2.VII genotypes were reported from the country
(Radwan et al., 2013). Also,
co-infections with infectious bronchitis and avian influenza viruses
were recorded in Egypt (Moharam et al.,
2019; Naguib et al., 2017;
Samy & Naguib, 2018). This has raised
concerns regarding the efficacy of ND vaccination programs applied in
the country. It has been hypothesized that, due to accumulated mutations
as seen previously with highly pathogenic avian influenza virus
(Grund et al., 2011), new NDV antigenic
variants have arisen, that have an advantage to spread within a
vaccinated population. In consequence, viruses that supersede previous
virus populations should have a gain in fitness, here, mutations within
antigenic relevant sites. To test this hypothesis, four isolates
representing genotype 2-II (n=2), the old genotype 2.XXI (former 2.VIa;
n=1) and the currently circulating genotype 2.VII.1.1 in Egypt (n=1)
were genetically and antigenically analyzed, with special focus on the
viral glycoproteins, that are facilitating receptor mediated virus
uptake and are the main target for protective immune responses.