Serological response of chickens either vaccinated or artificially infected with Haemophilus paragallinarum.

The serological response of chickens either vaccinated or artificially infected with Haemophilus paragallinarum (Hpg) serovar A or C was investigated using both a specific hemagglutinin (HA) antigen and a common HA antigen. With Hpg serovar A, both vaccinated and artificially infected chickens produced hemagglutination-inhibition (HI) antibodies to Hpg serovar-specific and Hpg common HA antigens. Most chickens vaccinated with Hpg serovar C had detectable HI antibodies to both types of HA antigen by 3 weeks postvaccination, after which titers gradually declined. In contrast, most chickens artificially infected with serovar C produced HI antibodies to only the common HA antigen; very few of these chickens produced HI antibodies to the serovar-specific HA antigen.