Duck Viral Hepatitis: Introduction - The Merck Veterinary Manual

Duck Viral Hepatitis: Introduction

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. The viruses that cause hepatitis in ducklings should not be confused with duck hepatitis B virus, a hepadnavirus infection of older ducks.

Etiology:

The originally described, most widespread, and most virulent DHV Type I is an enterovirus in the family Picornaviridae and is readily propagated in chick and duck embryos. It does not produce hemagglutinins. Field experience with DHV Type I indicates that egg transmission does not occur. The disease can be transmitted experimentally by parenteral or oral administration of infected tissues.

Viruses differing from classic DHV Type I have been recognized as causes of hepatitis in ducklings. DHV Type II is considered to be an astrovirus and is difficult to propagate under laboratory conditions; DHV Type III is a member of the Picornaviridae, is antigenically distinct from Type I virus, and can be propagated in duck (but not chick) embryos. A distinct serologic variant of DHV Type I, named DHV Type Ia, has also been described.

Clinical Findings:

The incubation period for Type I virus 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 DHV Type II infection is similar to that of Type I and can occur in ducklings immune to Type I infection. DHV Type III infections occur in ducklings despite immunity to Type I virus. The clinical course of Type III infection is less severe, and mortality is rarely >30%.

Lesions:

The lesions caused by all 3 types of DHV are similar. The liver is enlarged and covered with hemorrhagic foci up to 1 cm in diameter. The spleen may be enlarged and mottled. Kidneys may be swollen, and renal blood vessels congested.

Duck virus hepatitis, 6-7 day-old duckling

Duck virus hepatitis, older duckling

Diagnosis:

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. Type I virus 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. Type II and III viruses are not neutralized by classic Type I antiserum.

Prevention and Treatment:

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 Type I, II, and III viruses, provides parenteral immunity that effectively prevents high losses in young ducklings. The Type I virus 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 DHV Type I vaccine for use in breeder ducks that have been previously primed with live DHV Type I virus 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 Type I virus vaccine also can be used for early vaccination of ducklings susceptible to Type I (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 over 3-4 days.

Antibody against Type I virus, prepared from the eggs of hyperimmunized chickens, administered SC in the neck at the time of initial loss, is an effective flock treatment.