Abstract
The present study was designed to investigate the epidemiology of the recurrent outbreaks of duck plague, virus isolation and molecular detection of the circulating duck plague viruses followed by phylogenetic analysis targeting UL30 gene from infected ducks of five Haor (wetland) districts of Bangladesh. For conducting epidemiological investigation, a total of 450 randomly selected duck farms containing 1,75,467 ducks were investigated for their morbidity, mortality and case fatality rates during December 2012 to November 2014. Farms were surveyed and the owners were interviewed using a prepared questionnaire about husbandry practice, disease history, bio- security measures, flock and farmer details. Secondary data were collected electronically including web-based information available at OIE and also from the DLS personnel. Primary and secondary data with confirmatory diagnostic results of DP by PCR and PHA test were analyzed for epidemiological analysis of different variables and factors associated with recurrent DP outbreaks. Findings of the epidemiological investigation of DP revealed that overall prevalence of duck plague was 8% at farm level and 3.30% at flock level. On the other hand, morbidity, mortality and case fatality rates due to duck plague at farm level were 52.08%, 29.62%, and 56.86% respectively. Out of 22 variables selected for this study, 16 were found significant (p<0.05) and the remaining 6 were non-significant (p>0.05). Rearing of DP recovered ducks was also found a crucial factor for DP transmission and re-emergence. For isolation, molecular detection of DPV and observation of variation in the genome sequence of selective gene (UL30) between field virus of the study area and the vaccine strain of DPV, a total of 150 (liver-50, intestine-50 and oropharyngeal tissue=50) samples were collected from DP suspected sick/dead ducks of 50 affected farms of Kishoreganj (n=30), Netrokona (n=33), B-Baria (n=27), Habiganj (n=27) and Sunamganj (n=33) districts of Bangladesh. On the contrary, 9 faecal samples and 9 serum samples were also collected and subjected to PCR and PHA test, respectively for the identification of DP recovered duck at farm level. The tissue samples were processed and inoculated into 10 days-old embryonated duck eggs and DEFCC for the isolation of viruses. Presence of virus in the membrane and embryo were confirmed by PCR. Among 150 samples, 90 (60%) isolates were confirmed as DPV by PCR amplification of partial UL30 gene (446bp). Reconfirmation of DPV in the membrane and embryo was done by experimental infection in ducklings and PCR. DP recovered duck was confirmed as carrier of DPV by PCR and PHA test. Two of these isolates BAU/KA/DPV(B1)/2014 and BAU/KA/DPV(B4)/2014 were sequenced and found to be closely related to Chinese isolates of DPV. BAU/KA/DPV(B1)/2014 was found closely related with Chinese isolates Anatid herpes virus (KJ549663) China/2017, Anatid herpes virus BAU DMH/Bangladesh/2015, Anatid herpes virus (JQ673560) China/2014 and Anatid herpes virus (JQ647509) China/2012. On the other hand, BAU/KA/DPV(B4)/2014 was found closely related with Chinese isolates duck enteritis virus UL30 (EF203709) China/2007, duck enteritis virus VAC (EF417996) China/2007, duck enteritis virus UL30 (EF643560) China/2008 and duck enteritis virus UL30 (EF564403) China, 2007. The isolates of DPV, BAU/DPV (B1)/2014 and BAU/DPV (B4)/2014 showed the highest (98%) similarity with each other. Nucleotide sequence of the isolates (BAU/KA/DPV (B1)/2014) showed higher variability in nucleotides (264nt.) when compared with that of the vaccine strain (Accession no. EU082088), which may impact on protein function and additional drug sensitivity. From the findings of molecular study it can be assumed that both Bangladeshi isolates and all the Chinese isolates of DPV may have common ancestry.