Abstract   

The pathogenicity of recent field isolate (Pm- 10) of Hydropericardium-Hepatitis Syndrome (HHS) showed reduced pathogenicity as compared to the isolates studied on the inception of the syndrome in 1987- 88. Formalinized vaccines for HHS including Aqua Base Liver Organ (ABLO) vaccine and Oil Base Tissue Culture (OBTC) vaccine are effective for the control of the syndrome. ABLO vaccine gives an early immune response as compared to OBTC vaccine, but OBTC vaccine gives better immune response. The vaccination against HHS has positive effect on the immune response of the chicks against Newcastle disease virus.