Submitted to: Avian Diseases
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/21/2001
Publication Date: 12/10/2001
Citation: Interpretive Summary: Clinical Newcastle disease (ND) periodically occurs in free-living pigeons and cormorants as well as racing lofts of pigeons. Newcastle disease virus (NDV) isolates from those outbreaks are more virulent than the NDV isolates that have been prevalent in U. S. chickens and turkeys for over 30 years. If those more virulent isolates become established in U. S. poultry, embargoes of U. S. poultry product export could occur. To simulate the consequences of virus entry and bird to bird spread of these isolates in a chicken flock, selected NDV isolates from pigeons were passaged serially in chickens and the virulence of the isolates before and after passage in chickens was evaluated. Four of the passaged isolates produced mild clinical disease in some of the inoculated birds and severity increased, including some mortality, when younger birds were inoculated. Two of the isolates caused severe disease and mortality even before passage. All of the tested isolates produced a more severe disease than occurs in chickens and turkeys infected with the current field isolates. Viruses similar in origin and virulence to the tested isolates pose an increased risk to commercial poultry flocks and management techniques should be implemented to exclude these viruses from production units.
Technical Abstract: The virulence of six pigeon-origin isolates of Newcastle disease virus (NDV) was evaluated before and after backpassages in White Leghorn chickens. Antigenic characterization with monoclonal antibodies classified four isolates as pigeon paramyxovirus-1 (PPMV-1) and two isolates as avian paramyxovirus-1 (APMV-1). Birds were monitored clinically and euthanized, with collection of tissues for histopathology, in situ hybridization and immunohistochemistry. Pathotyping tests performed before and after passages in chickens demonstrated increased virulence of the passaged PPMV-1 isolates and high virulence of the original APMV-1 isolates. The fusion protein cleavage site sequence of all isolates was typical of the virulent pathotype. Intraconjunctival inoculation of four-week-old White Leghorns with passaged PPMV-1 isolates produced only mild clinical disease limited to depression and some nervous signs in some chickens. However, an nincreased frequency of clinical signs and some mortality occurred in two-week-olds inoculated intraconjunctivally with passaged virus. Histologically, prominent lesions in heart and brain were observed in birds among all four groups inoculated with the PPMV-1 isolates. The behavior of the two pigeon-origin APMV-1 isolates when inoculated into chickens was characteristic of velogenic viscerotropic Newcastle disease viruses that included necro-hemorrhagic lesions in the gastrointestinal tract.