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Recently, reverse genetics technology has been used to et al., 2012). Although this classification system is not perfect it
enhance the oncolytic properties of NDV (Zamarin and Palese, grouped class I NDV isolates into a single genotype and class II
2012). It was shown that the oncolytic activity of recombinant NDV isolates into 19 genotypes, some with many subgenotypes
NDV was enhanced either by containing a highly fusogenic F (Diel et al., 2012; Snoeck et al., 2013; Dimitrov et al., 2016).
protein (Bian et al., 2005; Elankumaran et al., 2010), express- Class II viruses are most characterized because the majority of
ing immunostimulatory molecules, such as GMCSF, IFN-γ, sequence data currently available are for Class II viruses. Class II
IL-2 and TNFµ (Vigil et al., 2007; Zamarin et al., 2009) or IgG genotypes (I-IV and IX) emerged ‘early’ (1930–1960) and have
monoclonal antibody (Pühler et al., 2008). It was also shown a genome consisting of 15,186 nt; whereas, other genotypes that
that recombinant NDV expressing green fluorescent protein emerged ‘late’ (after 1960) have a genome consisting of 15,192 nt
could be used to detect peritoneal gastric cancer cells and (Czeglédi et al., 2006; Maminiaina et al., 2010). Genotype I
provide important prognostic information (Song et al., 2010). consist of viruses that are of low virulence and some are used
Recently, it was found that the intra-tumoural therapy with NDV as vaccines. Genotype I also include the virulent NDV respon-
could be enhanced by expression of inducible co-stimulator sible for 1998–2000 Australian outbreaks. Genotype II include
(ICOS) ligand molecules (Zamarin et al., 2017). These results viruses of low virulence that are used as vaccines worldwide,
collectively suggest that reverse genetic approaches can improve such as LaSota and B1. It also includes the neurotropic virulent
the safety and oncolytic property of NDV for effective cancer NDV strain Texas GB. Genotype III viruses were mostly isolated
treatment in humans. in Japan before 1960 but have also been isolated in Taiwan in
1969 and in Zimbabwe in 1990 (Yu et al., 2001). Genotype IV
viruses were predominantly isolated in Europe before 1970 and
Antigenic and genetic variations among recently isolated in India (Czeglédi et al., 2006; Kapgate et al.,
NDV isolates 2010a,b). All strains of genotype III, IV and IX contain cleav-
All NDV strains belong to a single serotype, but there is antigenic age site motifs of virulent strains. Genotypes V–VIII were first
and genetic diversity among NDV isolates. Monoclonal antibod- recognized in the mid-1960s (Czeglédi et al., 2006) and contain
ies have been used to demonstrate antigenic variation among only virulent viruses (Miller et al., 2010). Genotype V viruses
NDV strains and isolates (Abenes et al., 1986; Alexander et al., were recognized in South and Central America in 1970 and have
1987; Erdei et al., 1987; Lana et al., 1988; Russell and Alexander, caused outbreaks in Europe (Ballagi-Pordány et al., 1996) and
1983). Using a panel of monoclonal antibodies (mAbs) it was in North America (Wise et al., 2004a). These viruses are still
shown that isolates sharing the same mAb reactivity had the circulating in Mexico (Perozo et al., 2008). Genotype VI viruses
same biological and epizootiological properties (Russell and are mainly isolated from rock pigeons and are enzootic in pigeon
Alexander, 1983). Monoclonal antibodies have also been used populations throughout the world (Mase et al., 2002). Genotypes
to distinguish PPMV-1 from NDV (Alexander et al., 1985a) and VII and VIII were reported in the 1990s from several countries
between vaccine and virulent viruses (Srinivasappa et al., 1986). (Aldous et al., 2003; Abolnik et al., 2004; Bogoyavlenskiy et al.,
Several systems have been proposed to classify NDV isolates 2009; Snoeck et al., 2009). Genotype IX includes the first viru-
based on phylogenetic analysis of partial or complete nucleotide lent outbreak virus from China in 1948 and occasional isolates
sequences of the F gene (Aldous et al., 2003; Czeglédi et al., 2006; from China (Wang et al., 2006). Genotype X contains viruses
Diel et al., 2012). NDV isolates are classified into two genetic of low virulence and were isolated exclusively from Taiwan in
groups, designated class I and class II, based on genome length 1969 and 1981 (Tsai et al., 2004). Genotype XI were isolated
and sequences of F and L genes (Ballagi-Pordány et al., 1996; from apparently healthy chickens in Madagascar (Maminiaina et
Czeglédi et al., 2006; Kim et al., 2007b). The Class I viruses are al., 2010). Genotypes XII were from Asia and South America,
generally avirulent to chickens, have been recovered primarily XIII from Asia, XIV from Nigeria, XVI from Dominican Repub-
from waterfowl and shorebirds, and are occasionally isolated lic and XVII and XVIII from Africa (Snoeck et al., 2013). All
from US live bird market samples (Alexander et al., 1992; Kim genotypes are still in circulation. Genotypes V, VI, VII and VIII
et al., 2007a,b). However, there is one documented case of viru- are the predominant genotypes presently circulating in many
lent class I NDV (Alexander et al., 1992). Class I viruses have a parts of the world. A phylogenetic tree using complete F gene
genome consisting of 15,198 nt (Czeglédi et al., 2006). The Class ORF sequences of representative strains from different NDV
II viruses are commonly isolated from poultry and exhibit a wide genotypes is shown in Fig. 2.9.
range of virulence. Recently, an international consortium of NDV experts have
In 2012, a classification system based on complete F gene developed a consensus NDV classification and nomenclature
sequence was proposed (Diel et al., 2012). In this classification system incorporating phylogenetic topology, genetic distances,
system, a 10% mean nt difference of the fusion protein coding branch support, and epidemiological independence of all avail-
sequence is required to assign phylogenetically distinct groups able complete F gene sequences. The new system maintains the
of viruses into genotypes. Different subgenotypes should have two existing NDV classes and the existing genotypes, but it iden-
an average interpopulational evolutionary distance per site tifies three additional genotypes (thus increasing the genotypes
between 3% and 10%. Genotypes and subgenotypes should be to 22) and reduces the number of subgenotypes (Dimitrov et al.,
assigned only when complete F gene sequences of four or more 2019).
NDV isolates with no epizootiological link are available (Diel Genotyping based on phylogenetic analysis of the F gene has