Abstract

Marek's disease (MD) remains to be one of the major problems in the production of poultry despite widespread vaccination. Recent study suggests that vaccinated flocks are breaking out, with emerging virulent strains and decreased vaccine efficacy being considered a major concern. There is limited data are available on MD prevalence in southern Iraq. This study examined a case of an outbreak in immunized commercial flocks of layers (White and Brown Lohmann) by clinical, pathological, histopathological, serological, and molecular analyses.

300 chickens were studied including 100 clinically affected White Lohmann, 100 affected Brown Lohmann and 100 control White Lohmann. Clinical symptoms consisted of asymmetric paralysis, emaciation, drooping of the wings and a mortality rate of 7.5 in Brown Lohmann and 4.2 in White Lohmann. On gross examination, hepatosplenomegaly, kidney enlargement, and swelling of the sciatic nerve were seen. Diffuse infiltration of proventriculus, lungs, heart, liver and spleen with lymphoid tumors, replacing normal tissues, was observed in 85-95 per cent of affected birds by histopathology. ELISA showed a significant decrease in (P < 0.05) NDV and IBV antibody in the infected birds as opposed to controls. The results of PCR confirmed that the Meq gene was positive in 100 percent of infected birds and negative in controls which had no lesions or PCR results.

The outbreak is the visceral variant of MDV in vaccinated Basra layer flocks, which indicates the existence of the extremely virulent (vv) or vv+ strains of MDV overcoming the existing vaccines. The extreme cases of immunosuppression observed emphasize the necessity of the better vaccination strategies, strict biosecurity approaches, and the constant monitoring by the means of molecular monitoring to restrain MD in large-scale poultry farms.