Short CommunicationEpidemiology and molecular characterisation of duck hepatitis A virus from different duck breeds in Egypt
Graphical abstract
Introduction
Duck virus hepatitis (DVH) is a highly contagious disease of ducks which causes severe morbidity and mortality particularly in ducklings less than 4-week-old (Woolcock, 2003). The virus can be transmitted by parenteral and oral routes, while vertical transmission has not been yet reported. Vaccination of breeder ducks protects the offspring through maternal antibodies (Woolcock, 2003, OIE, 2010) and vaccination of ducklings or transfer of hyperimmune sera can be also protective (Woolcock, 2003, Kim et al., 2009). The disease is caused by three different viruses. Duck hepatitis A virus (DHAV) type I, genus Avihepatovirus, a member of the RNA family Picornaviridae is the most common cause for DHV worldwide. Meanwhile, DHAV types II and III, were recently classified as duck astrovirus 1 and 2 (DAstV-I and DAstV-II) and were reported in the United Kingdom and the United States of America, respectively. Both viruses belong to the family Astroviridae and are antigenically distinct from DHAV type I. The latter has three distinguishable serotypes designated serotype 1, 2 and 3. Serotype 1 is the most widespread serotype worldwide, whereas serotype 2 was reported in Taiwan and serotype 3 in South Korea and China (Kim et al., 2007, Li et al., 2013).
DHAV is a nonenveloped virus with single-strand positive-sense ∼7.8 kb RNA genome that encodes a single viral polyprotein (VP) flanked with untranslated region (UTR) at the 5′ and 3′ ends (Pan et al., 2012). The 5′UTR possesses probably distinct internal ribosome entry site (IRES) element that is important for translation initiation and RNA synthesis of the virus. The VP is a polyprotein of ∼2200 amino acids in length that is further divided into several structural proteins including VP0 (VP2/VP4), VP1 and VP3 and nine non-structural proteins (2A1, 2A2, 2A3, 2B, 2C, 3A, 3B, 3C and 3D). The VP1 protein is the highly variable one (Wang et al., 2008, Gao et al., 2012, Wei et al., 2012, Xu et al., 2012) and probably plays a vital role in receptor binding, virulence, immunogenicity and protection against DHV (Liu et al., 2008). The 3D protein is RNA-dependent RNA-polymerase which is responsible for the synthesis of the viral RNA (Kok and McMinn, 2009). The evolution of the virus is driven by accumulation of point mutations and/or recombination between different serotypes (Wei et al., 2012). A limited number of sequences of DHAV are available which are only from China, South Korea, Taiwan, Vietnam, UK and USA (Tseng et al., 2007).
Although the virus was described in Egypt in the late 1970s (Shalaby et al., 1978), little is known about the current situation of the disease (OIE, 2009). Vaccination of breeder ducks is applied in the commercial sector in Egypt (Abd-Elhakim et al., 2009) using attenuated vaccines produced from E52 Rispens strain (Anon., 2015, Ellakany et al., 2002). In this study, we investigated the prevalence of DHAV in ducklings with a history of high mortality using reverse transcription polymerase chain reaction (RT-PCR) and virus isolation in embryonated duck eggs (EDE) from several provinces in Egypt. Furthermore, the sequence of 5′UTR, VP1 and 3D genes of the vaccine strain and 15 field viruses were generated and analysed. To the best of our knowledge this is the first published sequence for DVAH in the Middle East and Africa.
Section snippets
Samples
Samples were collected from different breeds of ducks in 46 commercial farms in Egypt; Pekin (n = 29), Muscovy (n = 9), Mallard (n = 5) and Green Winged ducks (n = 3). Samples were taken in 2012 (n = 4), 2013 (n = 12) and 2014 (n = 30). Farms’ capacities ranged from 400 to 10,000 birds in 11 different provinces (Fig. 1). The surveillance targeted commercial farmed ducks with a history of nervous signs and high mortality rate during the first 2 weeks of life (Table 1 and Supplementary Table S1). Samples were
Virus detection and isolation
All RT-PCRs have relatively comparable sensitivity. Respectively 17, 16 and 15 positive samples were detected by RT-PCR targeting the 5′UTR, VP1 and 3D genes. Out of the 17 samples tested positive by RT-PCR, only 5 samples were positive in virus isolation (Table 1) in terms of stunting, subcutaneous haemorrhages of the embryos (5/5 cases), haemorrhagic enlarged liver (4/5), enlarged mottled spleen (4/5) and rarely enlarged kidneys (1/5). In 17 positive farms, mortality ranged from 15% (case no.
Discussion
Since decades, DHAV causes severe losses in duck industry. Yet, no information about the prevalence of the virus outside Asia, particularly in the Middle East and Africa, is available. Diagnosis of DHAV may be confirmed by rapid onset of clinical signs and mortality in inoculated susceptible ducklings, inoculation of tissues homogenate into the allantoic sacs of EDE and embryonated chicken eggs or infection of cell culture (OIE, 2010). In contrast to the laborious and time-consuming culture
Acknowledgments
The authors are grateful to the colleagues and co-workers at the national laboratory for veterinary quality control on poultry production, animal health research institute, ministry of agriculture, Egypt for their valuable support during this study.
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