Quantitative estimation of Newcastle disease virus antibody levels in chickens and Turkeys by Elisa

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Abstract
A quantitative single‐well ELISA for estimation of Newcastle disease (ND) virus antibodies in chickens and turkeys was developed using purified antigen from PMV‐1/Chicken/Ulster 2C/71. The test was standardised using sera from 20‐week‐old chickens or 20‐ 30‐week‐old turkeys. Absorbance values for negative sera in chickens increased with the age of the birds but overall was lower than the cut‐off for the test. ND haemagglutination inhibition (HI) positive field sera were always positive by ELISA and the mean was significantly higher than that of the negative population. Standard antisera to four of seven of the other PMV serotypes (including PMV‐3) gave positive reactions in ELISA and three were also positive at low level by PMV‐1 HI test. Absorbance values remained negative in turkeys given two inoculations of PMV‐3 vaccine in spite of good PMV‐3 HI responses. Doubling dilutions of chicken and turkey sera were tested by ELISA and end‐point titres calculated. Standard curves relating ELISA titre and absorbance of each sample at 1/100 dilution were constructed and used to determine titres of test samples from single well absorbance values. A significant positive correlation between ELISA titre and HI titre for chicken and turkey sera was demonstrated. Sensitivity of the test was investigated using birds experimentally infected with PMV‐1/Chicken/Ulster 2C/71 or a pigeon PMV‐1 isolate. Seroconversion was detected at the same time by ELISA and HI. In experiments to estimate the ELISA titre required to protect birds against virulent ND, five groups of chickens were vaccinated twice with one of four commercial ND vaccines (three inactivated; one live) on two occasions and challenged with a virulent ND strain (PMV‐1/ Chicken/Antrim/73). Two of the groups vaccinated with inactivated vaccines were protected against challenge. In another group also given inactivated vaccine, clinical signs were seen in one bird and ELISA proved a better indicator of immune status than HI. In the groups given living vaccine, no signs of ND were seen and ELISA indicated a much higher level of vaccinal antibody than HI test. In turkeys given two inoculations with inactivated vaccine, antibody levels were boosted to acceptable levels by ELISA and HI indicating that vaccinal antibody levels were adequate.