Checking of immunity against Newcastle disease by ELISA

A total of 284 seven- to twelve-week-old Tetra SL chickens were assigned to 18 groups and immunized with a full or fractional dose of monovalent or multivalent inactivated vaccines subcutaneously or intramuscularly. Sera were taken from the birds at the time of challenge and tested for antibodies to Newcastle disease virus (NDV) by enzyme-linked immunosorbent assay (ELISA). The vaccinated birds were challenged with 10(6) ELD50 of virulent NDV subcutaneously. A close positive correlation (r = 0.954) was found between the protection percentage of the different groups and the group's arithmetical mean net extinction percentage (NE%) calculated from the net extinction values obtained for sera of birds belonging to the given group. NE% is easy to calculate and a good indicator of the flocks' immunity status. In our case: if NE% exceeded 80, the protection percentage of the flock was between 93.3 and 100%; if NE% was between 60 and 80, the protection percentage was between 75 and 87%, while with an NE% less than 60 the protection percentage ranged between 6.7 and 64.3%.