Newcastle disease (ND) is a highly contagious disease. The present paper deals with classification of ND virus (NDV), clinical signs and pathology, virus strain classification and molecular backgrounds for the pathogenicity. Major emphasis is reviewing immunity and vaccination. Clinical forms of the disease vary depending on many factors, but mainly on the virulence of Newcastle disease virus (NDV) strains. Virulent strains are considered List A pathogens by the ‘Office International des Epizooties’ (OIE). The virulence has been traditionally determined using in vivo pathogenicity tests to distinguish between highly, moderately and low virulent isolates. More recently, molecular biological techniques like polymerase chain reaction and sequencing have been described to differentiate virulent from non- virulent strains.
The systemic and mucosal immune systems are considered to function more or less independently. Systemic antibodies are essential elements in protection against ND, whereas the local antibodies limit multiplication of NDV at the site of entry. Cytotoxic T lymphocytes specific against NDV have been detected in the spleen of vaccinated birds, however, their contribution to protection remains to be elucidated.An increase of the number of various leukocyte subsets was noticed in the respiratory tract and the Harderian gland (HG), which favours involvement of local cellular immunity in the defence against NDV infection. It is tempting to speculate that the local lymphoid infiltrates are involved in first defence and that cytolytic cells clear virus by directly lysing infected target cells at the site of NDV inoculation. Secondly, various cell types, mainly T-lymphocytes and macrophages, may be equipped to produce a range of cytokines with antiviral activity and cytokines that stimulate B-lymphocytes to proliferate and differentiate into antibody-forming cells responsible for the local antibody production against NDV.