Abstract
Outbreaks of different diseases are the major constraints in poultry farming in
Bangladesh. Newcastle disease (ND), popularly known as Ranikhet disease, is one of
the major diseases that can cause severe damage to the poultry farming. Very little is
known about the genetic characteristics of Newcastle disease virus (NDV) circulating
in Bangladesh. The present study focused on the detection and genotypic
characterization of NDV of chickens from two wholesale live bird markets (LBMs) as
well as from field outbreaks, and sequential pathology in chickens on experimental
infection. A total of 513 dead chickens were collected from Kaptan Bazar and Karwan
Bazar LBM of Dhaka city during the period from August 2011 to July 2012. The
carcasses were first screened for type A influenza virus using a rapid antigen test kit
and a piece of tracheal tissue was collected from representative samples for molecular
characterization of the avian influenza virus (AIV). Influenza rapid test negative
samples (n=420) were subjected to routine necropsy and pooled tissue samples of the
trachea, lungs, spleen, proventriculus and caecal tonsils were collected from ND
suspected carcasses. In addition 18 ND suspected samples were received from field
outbreaks. Moreover 10 haemagglutination-positive but AIV-negative samples of
migratory birds were also received from Bangladesh Livestock Research Institute. All
samples were tested for the detection and molecular characterization of AIV and NDV
by RT-PCR and real time RT-PCR (rRT-PCR). A selected NDV isolate from a chicken
was used for sequential pathogenicity study in chickens on experimental infection. A
pigeon isolate of NDV from a previous study was also used for comparison. Out of 513
dead birds from LBM, 93 (18.21%) were positive for AIV in rapid antigen test and 85
(16.56%) were suspected at necropsy to have died of ND. Samples were collected from
80 out of 85 ND suspected carcasses. On PCR analysis, 59 out of 80 ND suspected
samples were positive for NDV, of which 18 were found to be concurrently infected
with NDV and AIV; and among the remaining 21 NDV negative samples 6 were
positive for AIV. All the 18 ND suspected samples from field outbreaks and 10
migratory bird samples were positive for NDV. Nucleotide sequences of a 535 bp
fragment of F gene of 19 selected NDV isolates were established. Phylogenetic analysis
revealed that all Bangladeshi isolates belonged to the currently spreading genotype
XIII, however, they formed two separate clusters away from previously described subgenotype XIIIa and XIIIb with a nucleotide divergence of 5.8% to 6.7% and having two
and six unique amino acid residues, respectively. These two groups of Bangladeshi
isolates might have evolved into two new sub-genotypes and are proposed to be
designated as sub-genotype XIIIc and XIIId. In the experimental pathogenicity study
the chicken isolate was found to be highly pathogenic for chickens. The visceral lesions
were typical of velogenic viscerotropic NDV. Progressive necrosis and depletion of
lymphocytes in various lymphoid organs were seen. Kidney tubular necrosis with
karyorrhexis appeared to be a novel finding. However, the pigeon isolate produced
minimum gross lesions; the microscopic lesions were also less severe. Both the chicken
and pigeon isolates produced lesions in the brain. However, the brain lesions were
more pronounced with the pigeon isolate. The information generated in the present
study on genotypic and pathogenic properties of currently circulating NDV strains of
Bangladesh would help developing a better vaccine candidate for controlling ND and
better understanding of the pathogenesis of the disease.