Newcastle Disease
Etiology
Newcastle disease (ND) is an acute viral infection of great economic significance to the poultry industry worldwide. The disease is caused by the Newcastle disease virus, also called avian paramyxovirus-1 (APMV-1), that is an RNA, negative sense, single stranded, non-segmented, enveloped virus which belongs to the family Paramyxoviridae and the genus Avulavirus.
Although there is only one serotype, there are a number of different strains of Newcastle disease virus. Within a given host such as the chicken, variations in Newcastle virus strains are associated with a wide range of clinical signs. Much of this variation is the result of differences in 2 surface glycoproteins, hemagglutinin-neurominidase (HN) and fusion (F), which play an important role in the binding and fusion of the virus to host cells, initiating infection. Furthermore, the key to viral immunity involves a response to these glycoproteins.
Historically, strains of NDV have been grouped into five pathotypes or forms, based on the clinical signs seen in infected chickens. These have been defined as:
- Viscerotropic velogenic: a highly pathogenic form in which hemorrhagic intestinal lesions are frequently seen
- Neurotropic velogenic: a form that presents with high mortality, usually following respiratory and nervous signs
- Mesogenic: a form that presents with respiratory signs, occasional nervous signs, but low mortality
- Lentogenic or respiratory: a form that presents with mild or subclinical respiratory infection
- Asymptomatic enteric: a form that usually consists of a subclinical enteric infection."
Currently, ND virus strains are classified according to the World Organization for Animal Health (OIE) system. This classification system serves as the basis for determining whether an outbreak of ND should be reported to the OIE and other local veterinary authorities to facilitate global disease control measures.
The OIE definition for a reportable outbreak of ND is as follows:
Newcastle disease is defined as an infection of birds caused by a virus of avian paramyxovirus serotype 1 (APMV-1) that meets one of the following criteria for virulence:
a) The virus has an intracerebral pathogenicity index (ICPI) in day-old chicks (Gallus gallus) of 0.7 or greater.
or
b) Multiple basic amino acids have been demonstrated in the virus (either directly or by deduction) at the C-terminus of the F2 protein and phenylalanine at residue 117, which is the N-terminus of the F1 protein. The term
