Newcastle disease virus (NDV) is caused by avian Paramyxovirus (APMV-1), an RNA virus that is highly contagious among numerous avian species. There is only one serotype, but there are nine serogroups of the avian Paramyxoviruses (APMV-1 to APMV-9). Beard and Hanson (1984) summarized a classification of NDV based on their clinical signs or pathotypes: 1) Doyle’s form or velogenic viscerotropic (vvNDV) characterized by an acute lethal infection in all ages and intestinal hemorrhages; 2) Beach’s form or velogenic neurotropic (vnNDV) characterized by respiratory and neurological signs with high mortality; 3) Beaudette’s form or Mesogenic characterized by respiratory disease with mortality in young birds; 4) Hitchner’s form or Lentogenic characterized by mild respiratory infections (mostly used as vaccines); and 5) Finally asymptomatic enteric pathotype of NDV that replicates in the intestine without causing signs of disease.

In young fully susceptible birds affected by lentogenic strains, serious respiratory disease problems can be observed, often resulting in mortality following infection with the more pathogenic La Sota strain. We have selected La Sota lentoenic NDV strain for the challenge experiment as this strain can generate a wide range of phenotype response in a population, which is required for mapping genetic markers associated with ND resistance. In chickens the pathogenicity of NDV is also affected by the route of administration. Natural routes of infection such as nasal, ocular, and oral appear to emphasize the respiratory nature of the disease while intramuscular, intravenous and intracerebral routes emphasize the appearance of neurologic signs. The nasal and ocular inoculation routes will be used in this project, as the most appropriate model for the naturally occurring infection. The incubation period for NDV in chickens averages 5 to 6 days. This virus replicates in the respiratory tract, mainly in the trachea, in the Harderian gland giving us the opportunity of retrieving virus from the tears, and also systemically in spleen.