Comparison of serological tests for detection of antibodies to infectious laryngotracheitis virus.

Four serological tests i.e. ELISA, serum neutralisation (SN), fluorescent antibody (FA), and agar gel immunodiffusion (AGID) were compared for sensitivity using several criteria, for detection and titration of infectious laryngotracheitis (ILT) virus antibodies in chicken sera. In the ELISA test, sera were tested in parallel on virus positive and negative control antigens with results expressed as positive-negative difference. Non-specific binding was not a problem in this test. SN tests were performed in microtitre plates using chick embryo liver cells, while sera for FA tests were titrated on multispot slides on which were fixed ILT virus-infected chick embryo liver cell cultures. The AGID test was the standard test still widely used for serological diagnosis of ILT. The four tests were compared using (1) sera from experimentally inoculated birds bled regularly at intervals from 4 to 35 days post-inoculation, (2) convalescent sera from a natural outbreak of ILT, and (3) serial dilutions of an ILT positive serum. In all experiments the ELISA test was of slightly greater sensitivity than SN and was comparable to the FA test. In the experimentally infected birds ELISA and FA test detected sero-conversion in more birds at 7 days than SN. In tests with the serially diluted hyperimmune ILT serum, ELISA, FA and SN tests were comparable. SN however was the most useful test for quantification of ILT antibodies. ILT-SN titres in birds were never high, the highest titre recorded in experimental birds and in convalescent sera was 1/48. AGID was found to be less sensitive than ELISA, FA or SN test but was considered useful for detection of antibodies on a flock basis, since, with the convalescent sera AGID detected a significant proportion of positives.