MVSc

The present study has been undertaken to assess the prevalence of Newcastle disease (ND), infectious bronchitis (IB) and egg drop syndrome-76 (EDS-76) viruses in apparently healthy layer chicken in Kerala by seroprevalence studies and virus isolation trials and to associate these viruses with low egg production. Sera, cloacal and tracheal swab samples were collected from two to five per cent of total population of a farm to carry out the study. Samples were collected from Regional Poultry Farms (RPF) of Kerala, University Poultry Farm, Mannuthy and also from birds brought to the department of microbiology for disease investigation. To assess the production performance of the layers data pertaining to the egg production were collected, for the past one to five years depending upon the availability.

The haemagglutination inhibition (HI) test was used to study the seroprevalence of ND. Out of 615 samples tested, 548 samples were found to be positive. Newcastle disease vaccination was carried out regularly in all the organized farms and the titre is to be considered as an effect of vaccination. Seven virus isolates were recovered from cloacal swab samples and was identified as NDV by HI test. The isolates were subjected to conventional characterization by mean death time, intracerebral pathogenicity index, intravenous pathogenicity index, and stability of haemagglutinins at 56º C, agglutination of mammalian erythrocytes, and adsorption of haemagglutinins by chick embryo brain cells. Results of these tests identified all the seven isolates as lentogenic. The isolates produced mild cytopathic effects (CPE) like syncytia, pyknosis of nuclei and vacuolation of nuclei in chick embryo fibroblast (CEF) monolayer culture. Immunofluorescence test (IFT) was carried out in CEF monolayer using NDV antiserum and specific fluorescence was noticed in the cytoplasm. All the isolates were subjected to reverse transcriptase polymerase chain reaction (RT-PCR) for amplifying 254 bp fusion protein cleavage site (FPCS) sequence gene.

Sera samples were tested by ELISA for detecting IB antibodies due to the erratic results of HI test. Out of 418 sera samples screened, 311 (74.40 per cent) samples were found to be positive for IB. Since IB vaccination was not followed in any of the farms in Kerala, the antibodies might be due to past/subclinical infection. One IBV isolate was recovered from tissue samples of ailing broiler bird and was identified by AGID test. The ciliostatic effect of IBV was studied in tracheal organ culture (TOC) and the virus was identified in TOC by IFT and immunoperoxidase test (IPT). The IB virus was confirmed by RT-PCR using primers for a 464 bp nucleotide sequence of S1gene.

The antibodies against EDS-76 virus were identified by HI test. Out of 615 chicken sera samples tested, 58 (9.43 per cent) were found to be positive. None of the swab and tissue samples yielded EDS-76 virus.

By correlating the production performance of chicken with seroprevalence and isolation trials ND, IB and EDS-76 viruses, this study revealed the possible involvement of IB and EDS-76 viruses in low egg production of chicken in Kerala, because the birds are well protected against ND by vaccination.