duck virus hepatitis

Duck hepatitis A virus (DHAV) was first described in young white pekin ducks on Long Island, New York (Levine and Hofstad, 1945; Levine and Fabricant, 1950). Since then, DHAV has been reported in duck-raising areas worldwide (Woolcock, 2008), and DHAV outbreaks are frequent in China (Guo and Pan, 1984) and Korea (Park, 1985; Sung and Kim, 2000). DHAV-1 is the most common and virulent subtype with global distribution causing more than 80% mortality in ducklings.

DHAV type 2 (DAstV-1) was originally reported in Norfolk, England (Asplin, 1965). A duck astrovirus with very high sequence similarity to DAstV-1 (Todd et al., 2009) has been detected and sequenced in China (Fu et al., 2009), associated with very high mortality in one- to two-week-old commercial ducklings.

DHAV type 3 (DAstV-2) has first occurred in the USA, but is now prevalent in China, South Korea and Vietnam, and has recently been reported in 15 duckling flocks in the Sharkia province of Egypt (Hassan et al., 2020).

For current information on disease incidence, see OIE's World Animal Health Information System (OIE-WAHIS).

There are several approaches for the diagnosis of DH, which involve examining clinical signs and gross pathological changes, or reproducing the disease in susceptible ducklings. However, these techniques are incapable of discriminating DHAV-1 and DHAV-3 because of their similar clinical signs and gross pathological lesions (Liu et al., 2011). Virus isolation combined with polymerase chain reaction (PCR) (Saha et al., 2013), immunofluorescence assays (Zhang et al., 2014; Wu et al., 2015), neutralization test (Hwang, 1969) or enzyme-linked immunosorbent assays (ELISA) (Shen et al., 2015) are reliable for detecting DHAV, but these techniques are all labour-intensive and time-consuming.

DHV type I (DHAV)

The clinical and pathological observations are highly indicative of a DHAV infection. Preparing 20% (w/v) liver homogenate suspensions, from birds that are thought to have had the infection, in buffered saline and inoculating the allantoic sac of eight- to ten-day-old chicken embryos can isolate the virus. Infected embryos will die within 5 to 8 days post-infection and exhibit gross lesions, including dwarfing, enlarged greenish livers with necrotic foci and cutaneous haemorrhage and oedema. Embryo mortality and lesions will occur sooner after inoculation, in 10- to 14-day-old duck embryos from susceptible breeder ducks. The presence of DHAV can be confirmed by one of more of the following tests:

a. Subcutaneous or intramuscular inoculation of the isolate into one- to seven-day-old susceptible ducklings results in death 18 to 48 h post-infection. Gross lesions should be consistent with DHAV infection and the virus should be re-isolated from the livers.

b. Inoculation of serial dilutions of the liver homogenate into the allantoic sac of duck or chicken eggs and observation of clinical changes as described above.

c. Inoculation of liver homogenate suspensions into primary cultures of duck embryo liver cells. DHAV will cause a cytopathic effect (CPE) in the cells. Recently, an attenuated strain of DHAV was reported to replicate in chicken embryo fibroblasts (Zhang et al., 2000). A duck embryo fibroblast cell line has been developed, in which DHAV replicates with cytopathic effect (Fu et al., 2012).

d. An accurate diagnosis of DHAV can be made using direct immunofluorescence on livers from naturally occurring infections or inoculated duck embryos (Vertinskii et al., 1968; Maiboroda, 1972).

e. An ELISA for antibodies to DHAV has been developed using virus protein 1 (VP1) produced in bacteria as antigen (Liu et al., 2010).

f. Reverse transcriptase polymerase chain reaction (RT-PCR) tests have been developed for DHAV (Kim et al., 2007b; 2008; Cheng et al., 2009) and reverse transcription loop-mediated isothermal amplification tests by Song et al. (2012) and Yang et al. (2012), but these tests are labour-intensive and expensive. A rapid and economical one-tube RT-PCR technique for simultaneous detection and genotyping of duck hepatitis A virus subtypes 1 and 3 has been developed recently using a universal primer and type-specific primers targeted to the 5’-NCR sequence (Chen et al., 2019).

DHV type II (DAstV-1)

The virus may be recovered in a 20% (w/v) homogenized liver suspension and can be used to inoculate susceptible ducklings and embryonated chicken eggs. An outbreak of DAstV-1 in China in 2008 killed approximately 50% of one- to two-week-old commercial ducklings (Fu et al., 2009). As sequence data is now available for DAstV-1, RT-PCRs can be used to detect the virus (Fu et al., 2009; Todd et al., 2009).

Gross lesions will be similar to field cases. Chicken eggs can also be inoculated, either by the amniotic cavity or the yolk sac. This results in very little mortality and stunted green necrotic livers are the only observable pathology. There are no cell culture systems for DAstV-1.

DHV type III (DAstV-2)

The virus can be recovered from homogenized liver suspensions and isolated by duckling inoculation, or inoculation onto the chorioallantoic membrane of ten-day-old embryonated duck eggs. There will be some embryo mortality 7 to 10 days post-inoculation and the membrane will appear dry and crusty. DAstV-2 is less virulent than DHAV.

Serologic tests

Serologic tests have not been useful because of the acute nature of the clinical disease. However, various virus neutralization (VN) assays have been described that are useful for virus identification, titration of serologic response to vaccination and epidemiologic surveys. The VN tests may achieve greater significance if DAstV-1 and DAstV-2 become more widespread. The VN tests described include a DHAV neutralization test in chicken embryos (Hwang, 1969), an agar gel diffusion precipitin (AGDP) test for identification of type I (Murty and Hanson, 1961) and a plaque-reduction test for VN antibodies (Woolcock et al., 1982). A duck embryo fibroblast cell line has been developed, in which DHAV replicates with cytopathic effect (Fu et al., 2012).

Differential Diagnosis

Although the sudden onset, rapid spread and acute course of the disease are characteristic of DHAV, the virus must be isolated or demonstrated by RT-PCR to confirm DHAV infection. Other potential causes of acute mortality in ducklings include salmonella and aflatoxin. Neither of these causes the liver lesions characteristic of DHAV infection, but will produce rapid onset mortality and ataxia, convulsions and opisthotonos in the case of aflatoxicosis.

Husbandry Methods and Good Practice

DHAV can be prevented by strict isolation during the first 4 to 5 weeks of life. In areas where the disease is prevalent, achieving the necessary degree of isolation may be very difficult and vaccination may be required.

Immunization and Vaccines

DHV type I (DHAV)

Resistance against DHAV in ducklings can be achieved through three methods:

1. Injection of immune serum or yolk from eggs produced by hyperimmune breeder ducks, or yolk from eggs produced by specific-pathogen-free chickens hyperimmunized with DHAV.

2. Immunization of breeder stocks with a live attenuated virus vaccine. The vaccine is produced in embryonated chicken eggs to ensure high levels of passively transferred antibodies in ducklings. Alternatively, breeder ducks can also be vaccinated with an inactivated vaccine if they have already been primed with, or exposed to, live DHAV.

3. Direct immunization of ducklings with live avirulent strains of DHAV by foot web-stab, intramuscular, intranasal or subcutaneous injection. DHAV-1 vaccines may not be fully effective against type 2 and 3 DHAV. Kim et al. (2009) have produced an attenuated DHAV-3 vaccine using a strain that had been circulating in South Korea and China, but there is no vaccine which is licenced for use in China. Another live vaccine candidate against DHAV-3 has been developed in China (Wu et al., 2020) using an attenuated field isolate strain (SD70). The minimum effective dose of SD70 after subcutaneous inoculation was 10^2.5ELD₅₀. A single dose of the SD70 provided good protection to susceptible ducklings against DHAV-3 strain without causing any clinical signs of disease or mortality in one-day-old ducklings and there was no virulence reversion. The attenuated SD70 strain exhibited good safety, stability and protection, and is a promising vaccine candidate for the prevention of DHAV-3 infection.

DHV type II (DAstV-1)

A live virus, DAstV-1 vaccine, protected ducklings under experimental conditions but has never been used commercially.

DHV type III (DAstV-2)

Experimentally, an attenuated live-virus vaccine given to breeder ducks confers immunity to hatchling ducklings. Also, convalescent sera obtained from DAstV-2-infected ducks effectively controlled outbreaks in the field.

National and International Control Policy

As defined by the OIE the incubation period for DVH is 7 days. The Veterinary Administrations of importing countries should require the presentation of an international veterinary certificate attesting that:

• The ducks showed no clinical signs of DVH on the day of shipment.
• The ducks come from establishments that are free from DVH.
• That the ducks are either vaccinated or not vaccinated against DVH.

There are further requirements for the importation of day-old ducks and duck embryonated eggs.