Newcastle disease virus (NDV) is one of the most important infectious agents in the poultry industry . NDV is synonym of avian paramyxovirus 1 (APMV-1), a member of the genus Avulavirus in the family Paramyxoviridae. The enveloped virus has a negative-sense, single-stranded RNA genome of 15186, 15192, or 15198 nucleotides (nts) in length, encoding six proteins: nucleocapsid protein (NP), phosphoprotein (P), matrix protein (M), fusion protein (F), hemagglutinin–neuraminidase (HN), and large protein (L) [3–5]. Phylogenetically, NDV is divided into two distinct classes (I and II). Almost all virulent NDVs belong to class II which can be subdivided into sublineages and genotypes: early sublineage of genotypes I–IV with genome length of 15186 nts, late sublineage of genotypes V–VIII with genome length of 15192 nts and genotype IX belonging to early sublineage but having a genome length of 15192 nts [6–9].
The pathogenicity of NDV isolates can be assessed by determining the mean death time (MDT) in chicken embryos, the intracerebral pathogenicity index (ICPI) in 1-day-old chicks, or the intravenous pathogenicity index (IVPI) in 6-week-old chickens . MDT is used to divide virus strains into velogenic, mesogenic and lentogenic pathotypes. ICPI and IVPI are used to differentiate high pathogenic from low pathogenic NDVs. The velogenic strains are further divided into a velogenic viscerotropic pathotype, which is an acute lethal infections with necro-hemorrhagic lesions most obvious in the gastrointestinal tract, and a velogenic neurotropic pathotype, which cause predominantly respiratory and neurologic signs with high mortality [5, 10, 11]. The amino acid sequence at the protease cleavage site of the F protein has been postulated to be a primary molecular determinant of NDV virulence. As a rule, the virulent isolates have the cleavage site motif 112R/K-R-Q/K-K/R-R-F117, while the avirulent NDV isolates present the motif as 112 G/E-K/R-Q-G/E-R-L117[4, 12–14].
A wide variety of wild, domestic and cage birds can be infected with NDV under natural and experimental conditions, while the chicken remains the most susceptible and important natural host . The waterfowl are usually considered as the natural host of avirulent NDVs belonging to the class I and genotype I of class II. Moreover, the waterfowl such as geese and ducks have been thought to be resistant to strains of NDV even most virulent for chickens [11, 16, 17]. However, outbreaks of a novel disease entity of high morbidity and mortality with pathologic lesions of the necrosis in spleen and the extensive necrotic foci in the intestinal mucosa in goose flocks in Southern and Eastern China have been reported frequently since late 1990s [6, 18–21]. The disease was described as “goose paramyxovirus (GPMV) infection” in earlier reports of Chinese literature although later work has confirmed that the concurrently emerging genotype VIId NDV in chickens is the etiologic agent of the disease [6, 22, 23]. Although NDVs isolated from both chickens and geese in the past 15 years are predominantly VIId viruses, published data on comparison between goose- and chicken-originated ND viruses are scarce and controversial.
Yu et al. first showed that the goose isolate Ch/97-1 belongs to subgenotype VIId and shares very high sequence homology with VIId NDV TW/98-4 from chicken. This suggested that Ch/97-1 might have derived or evolved from a chicken-originated VIId NDV isolate . In 2003, Liu et al. demonstrated that there is epidemiological link between subgenotype VIId virus and the novel ND entity in geese based on the pathotypical and genotypical characterization of NDV strains isolated from geese and chickens in China during the period 1985–2001 . The close phylogenicity of the goose-originated NDVs and concurrent epidemic chicken-originated NDVs has also been mentioned in other reports [18, 25, 26]. In contrast, Kong et al. demostrated the genetic difference between the chicken- and goose-originated NDVs, and used this information to establish a multiplex RT-PCR method for the differentiation between the chicken- and goose-originated genotype VIId NDVs . In addition, Li et al. showed antigenic variation between genotype VIId NDV of goose-origin and an early lineage strain of genotype IX NDV from chicken. Thus, the difference and relationship between NDVs of goose- and chicken-origin remain unclear.
To fully elucidate the genetic and pathological correlation between the goose- and chicken-originated VIId NDVs, ten entire genomic sequences and 329 complete coding sequences of individual genes from goose- and chicken-originated VIId NDVs were analyzed genetically. Two goose- and two chicken-originated VIId NDVs were randomly selected for pathological comparison in both geese and chickens with genotype IV strain Herts/33 as a reference.