Humoral Immune Response of the Duck to Duck Hepatitis Virus: Virus-Neutralizing vs. Virus-Precipitating Antibodies

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Abstract
Ducks were induced to develop high-level duck hepatitis virus (DHV)-neutralizing antibodies (IgG and IgM) by inoculation with a chicken-embryo-adapted DHV via s.c., i.m. or intratracheal routes. Administration of the DHV orally in a gelatin capsule failed to stimulate an immune response in the ducks. Contact controls of these ducks also remained negative for anti-DHV antibodies. The DHV administered orally, in a gelatin capsule, failed to infect the ducks. No duck anti-DHV immune sera, with virus-neutralizing activity in the range of 1.8-5.57 log median-embryo-infective-dose neutralization index, developed precipitin lines against a variety of DHV preparations tested in low- and high-ionic-strength agar. Thus the agar-gel immunodiffusion test is unsuitable for serologic testing of duck sera for anti-DHV antibody activity. Immunodiffusion tests of Sephadex G-200 fractions of 1 day old duckling sera with monospecific rabbit anti-duck IgM (DIgM) serum failed to detect DIgM. The IgM seemingly was transferred from the dam to newly hatched ducklings. Seven and 14 day old ducks had DIgM in their sera; this IgM had no DHV-neutralizing activity. It was apparently newly developed by the ducklings, which had no active DHV immune response, not having exposed to DHV.