Overview of Duck Viral Hepatitis
Duck viral hepatitis is an acute, highly contagious, viral disease of young ducklings characterized by a short incubation period, sudden onset, high mortality, and characteristic liver lesions. The disease is of economic importance in all duck-raising areas of the world. Three distinct types of duck hepatitis virus (DHV) have been isolated from diseased ducklings. A natural outbreak of DHV Type I has been reported in mallard ducklings; experimental DHV Type I infections have been produced in goslings, turkey poults, young pheasants, quail, and guinea fowl. In Muscovy ducks, DHV Type 1 has been reported to cause pancreatitis and encephalitis. The viruses that cause hepatitis in ducklings should not be confused with duck hepatitis B virus, a hepadnavirus infection of older ducks.
The originally described, most widespread, and most virulent DHV Type I has been renamed duck hepatitis A virus type 1 (DHAV-1) and is now classified in the genus Avihepatovirus in the Picornaviridae family. It is readily propagated in chicken and duck embryos. Two new geno/serotypes, DHAV-2 and DHAV-3, have been identified in China, Taiwan, and Korea. They do not produce hemagglutinins. Field experience with DHAV-1 indicates that egg transmission does not occur. The disease can be transmitted experimentally by parenteral or oral administration of infected tissues.
Viruses that differ from classic DHAV-1 have been recognized as causes of hepatitis in ducklings. DHV Type II, now classified as duck astrovirus type 1 (DAstV-1), is difficult to propagate under laboratory conditions; DHV Type III is also now classified as an astrovirus (DAstV-2) and can be propagated in duck (but not chicken) embryos. Distinct serologic variants of DHAV-1, named DHV Type Ia and N-DHV, have also been described.
The incubation period for DHAV-1 is 18–48 hr. Affected ducklings become lethargic, lose balance, paddle spasmodically, and die within minutes, typically with opisthotonos. Although adults may become infected, clinical signs have not been seen in ducks >7 wk old. Mortality may be as high as 95% in ducklings. Practically all deaths occur within 1 wk after onset of signs.
The clinical course of DAstV-1 infection is similar to that of DHAV-1 and can be seen in ducklings immune to DHAV-1 infection. DAstV-2 infections are seen in ducklings despite immunity to DHAV-1. The clinical course of DAstV-2 infection is less severe, and mortality is rarely >30%.
A presumptive diagnosis can be based on the history and lesions. Sudden onset, rapid spread, and short course, together with characteristic liver lesions, are highly suggestive of duck viral hepatitis. DHAV-1 may be isolated in duck embryos, day-old ducklings, and duck-embryo liver cell cultures, or less easily in chicken embryos. The virus can be identified by neutralization with specific antisera or by inoculation into both susceptible and immune ducklings. DHAV-1 can also be identified by reverse transcriptase PCR. DAstV-1 and DAstV-2 are not neutralized by classic DHAV-1 antiserum.
Prevention is by strict isolation, particularly during the first 5 wk of age. Contact with wild waterfowl should be avoided. Rats have been reported as a reservoir host of the virus; therefore, pest control is indicated.
Immunization of breeder ducks with modified-live virus vaccines, using DHAV-1, DAstV-1, and DAstV-2 , provides parenteral immunity that effectively prevents high losses in young ducklings. The DHAV-1 vaccine is administered SC in the neck to breeder ducks at 16, 20, and 24 wk of age and every 12 wk thereafter throughout the laying period. Three immunizations are advisable for passive protection of ducklings.
An inactivated DHAV-1 vaccine for use in breeder ducks that have been previously primed with live DHAV-1 has been described. A single dose of the inactivated vaccine, given IM before the birds come into lay, provides passive immunity for a complete laying cycle to progeny ducklings.
The chick-embryo origin, modified-live DHAV-1 vaccine also can be used for early vaccination of ducklings susceptible to DHAV-1 (progeny of nonimmune breeders). This vaccine is administered SC or by foot web stab in a single dose to day-old ducklings. Vaccinated ducklings rapidly develop an active immunity within 3–4 days.
Antibody against DHAV-1, prepared from the eggs of hyperimmunized chickens and administered SC in the neck at the time of initial loss, is an effective flock treatment.