Virulence of Newcastle Disease Virus and Diagnostic Challenges

The clinical signs and symptoms vary widely in the birds infected with NDV and depends on factors such as the virus strain, host species, age of the birds, presence of other pathogen, environmental stress and the immunity status of the host.In chickens different strains of NDV cause sudden death with 100% mortality to sub clinical infection and the influence of species may be equally marked, such as the viruses causing severe disease in chickens and turkeys may cause few signs of disease in geese and ducks.Clinical signs that may be associated with ND are respiratory Review ArticleAbstract | Newcastle disease (ND) is economically most important poultry disease and distributed worldwide causing devastating loses in poultry industry.Newcastle disease virus (NDV) has a wide host range and has been reported to infect more than 240 species of birds.In chickens virulence ranges from non-virulent, associated with asymptomatic enteric infections or unapparent or mild disease of respiratory tract (lentogenic strains), mild respiratory disease and moderate mortality rates (mesogenic strains) to severe disease with high mortality rates up to 100% (velogenic strains).Virulence of NDV strains depend greatly on the host response.Virulence of ND viruses is measured by scoring disease incidence after Intracerebral inoculation of day-old chickens.Highly virulent strains of NDV can be discriminated from low virulent strains by the presence of multibasic amino acid motif at the proteolytic cleavage site of the fusion (F) protein.Genetic classification has divided NDV into 2 classes (I and II), with class I composed of only 1 genotype (class I, genotype I) and with class II divided into 18 genotypes (class II, genotypes I-XVIII).Genotypes V, VI, and VII are virulent viruses and predominant genotypes circulating worldwide.Out of these, genotype VII is particularly important because it is associated with many or the most recent outbreaks in Asia, Africa and Middle East.For rapid and accurate diagnosis real time RT-PCR tests are equally or more sensitive than virus isolation and are always faster than virus isolation.Vaccination against ND is widely practiced.However, ND is still recognised to be endemic in many parts of the world, particularly in developing countries.