The biological properties and pathogenicity of seven Newcastle disease virus field isolates were studied. These isolates were recovered from different outbreaks in Sudan (5 from chickens and 2 from pigeons) during 2008–2013. Based on intracerebral pathogenicity index, four NDV isolates were characterized as velogenic (their ICPI ranged 2.0–1.6) and three isolates were characterized as mesogenic (ICPI ranged 1.2–1.3). The mean death time for all isolates ranged from 54 to 76.8 hours. The elution time of the viruses from chicken erythrocytes and the ability to haemagglutinate mammalian red blood cells differed considerably in their reactions. 1. Introduction Newcastle disease (ND) is still considered as an economically important disease which is highly contagious infection for many avian species. ND is caused by virulent strains of avian paramyxovirus type 1 (APMV-1) of the genus Avulavirus belonging to the family Paramyxoviridae [1]. According to the World Organization for Animal Health (OIE, 2012), ND is an OIE notifiable disease when it meets certain criteria of virulence. Newcastle disease affects a wide range of domestic and wild avian species; however, the severity of the disease varies greatly, spanning from peracute disease with almost 100% mortality to subclinical disease with no lesions. Such variability makes it impossible to pinpoint ND as a single clinicopathologic entity [2]. Based on severity of clinical disease, the strains of NDV were originally classified into 3 groups based on their virulence such as lentogenic, mesogenic, and velogenic. Lentogenic strains, especially in adult chickens, may cause minimal or no clinical signs. However, the disease produced by mesogenic strains may cause mortality that can reach 25% and those by in velogenic strains may reach up to 100% [3] Differences in virulence of the virus occur during any disease outbreak, so determining virulence is essential for the effective control of the disease. ND was first reported in Sudan in 1951 [4] and still constituted a major problem facing the poultry industry. Newcastle disease virus (NDV) exhibits a wide range of pathogenicity and virulence. Assessment of the virulence of NDV is necessary in order to limit the outbreaks and to minimize their impact. So the aim of this study was to evaluate the pathogenicity of some NDV field strains isolated 2008–2013. 2. Materials and Methods 2.1. Viruses Seven NDV viruses were isolated from different hosts, including chickens (layers and broilers) and pigeons from several regions in Sudan during 2008 to 2013 from cases submitted
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