Newcastle Disease in Vaccinated Commercial Broiler Chicken Flocks Experiencing Subclinical Infectious Bursal Disease in Jordan

Abstract

Infectious bursal disease virus (IBDV) and Newcastle disease virus (NDV) are the most important viruses in poultry, causing catastrophic economical losses to farmers and to chicken breeding companies in Jordan. Fifteen commercial broiler chicken flocks were vaccinated twice against NDV and once against IBDV. Clinical signs of Newcastle disease appeared at 23-25 days of age, mortality and morbidity rates were 30-50% and 65-75% respectively. IBDV maternal derived antibody levels were determined at one day old using commercial Enzyme linked immunossorbant assay (ELISA) kit. Samples of bursas of Fabricius in acute stage were collected and evaluated histologically and the average bursa/body weight ratios were calculated and analyzed statistically. Newcastle disease virus antibody levels at acute and convalescent stages of disease were determined using hemagglutination inhibition (HI) test. Tracheas were also collected in the acute stage for performing NDV reverse transcriptase-polymerase chain reaction (RT-PCR). ELISA results at 1 day of age indicated that all tested flocks had low IBD maternal antibody titers. Histopathological examination of the bursas of Fabricius showed lesions characteristic of subclinical IBDV infection, ranged from moderate to severe lymphoid depletion of follicular lymphocytes, or minimal to absence of follicular lymphocytic presence as a sequel to necrosis. The HI titers for NDV from acute and convalescent sera were 1.9 to 3.3 and 5.3 to 6.9 respectively. Newcastle disease gene was also confirmed in tracheas using RT-PCR test. The results obtained from this study indicate that subclinical IBDV infection at an early age is a significant cause of decreased immune response to NDV vaccine, therefore leading to NDV infection in commercial broiler chicken flocks despite the fact that these flocks were vaccinated twice against NDV.

Keywords

• broiler chicken
• immune response
• newcastle disease
• subclinical ibd