Newcastle disease-A serious threat to poultry Article Abstract

Newcastle disease is a serious disease in many species of birds, both domestic and wild. Because chickens are the most susceptible birds, the disease can cause serious economic problems in the poultry industry worldwide. The virus that causes this avian disease is Newcastle disease virus (NDV) a member of the Rubulavirus genus of the subfamily Paramyxovirinae (family Paramyxoviridae, order Mononegavirales).
NDV contains a non-segmented single-stranded RNA genome of negative polarity with a size of 15186 nucleotides and contains six genes encoding the nucleocapsid protein (NP), phosphoprotein (P), matrix protein (M), fusion protein (F), haemagglutinin–neuraminidase protein (HN) and large polymerase protein (L) ..
NDV is an enveloped virus with two membrane proteins, one of which is involved in cell attachment and release (HN protein) and the other in mediating fusion of the viral envelope with cellular membranes (F protein). The F protein is synthesized as a precursor, F0, and is only fusogenic after cleavage into disulfide-linked F1 and F2 polypeptides. This cleavage is accomplished by host-cell proteases. Thus, NDV becomes infective only when the precursor glycoprotein F0 is cleaved into F1 and F2. The ability to cleave F0 varies among different strains of NDV and is the main determinant of mortality and morbidity in infected chickens. Using mean death time (MDT) determined in chicken eggs, NDV strains can be classified as highly virulent (velogenic), intermediate (mesogenic) or non-virulent (lentogenic). Within the highly virulent NDV strains, two types of virus can be distinguished. These strains are termed neurotropic velogenic or viscerotropic velogenic, based on the clinical signs induced. Phylogenetic studies at the molecular level of the NDV F0 cleavage site have determined the consensus amino acid sequence 112R/K-R-Q-R/K-R-F117 for virulent and mesogenic strains and 112G/E-K/R-Q-G/E-R-L117 for NDV strains of low virulence . However, two exceptions to the consensus amino acid sequence for virulent strains have been observed for the so-called pigeon paramyxovirus type 1 (PPMV-1) isolates. These virulent isolates show fusion protein cleavage sites of 112GRQKRF117 and 112RRKKRF117. The different F0 protein cleavage sites are the substrates for different kinds of cellular proteases.

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