Pathogenicity of a new Italian genotype of infectious bursal disease virus in SPF chickens.

Infectious bursal disease virus (IBDV) genotype ITA was first detected in 2011 in Italy in IBD- live vaccinated broilers. Full genome caracterization confirmed ITA to be a genetically distinctive IBDV genotype. In particular peculiar molecular characteristics were observed in key positions of the VP2 hypervariable region, which affect charged or potentially glycosylated amino acids virtually associated with important changes in virus properties. The aim of this study was to determine the pathogenicity of ITA genotype in SPF chickens. 84 Birds were housed in poultry isolators and inoculated at 35 days of life with ITA IBDV strain Italy/1829/2012 (40 birds) or a classical reference strain (40 birds). 28 controls were contextually mock inoculated with sterile water. All groups were daily observed for clinical signs up to 28 days post-infection (dpi). At 0, 7, 14, 21 and 28 dpi birds were bled for IBDV antibody detection by a commercial ELISA Kit. At 2, 4, 7, 14, 21 e 28 dpi 5 birds from each of the inoculated groups, and 3 from the control group, were euthanized and subjected to post-mortem examination. Bursal and Thymus indexes were calculated; histopathology of the Bursas of Fabricious was carried out and viral tissue distribution determined by qRT-PCR in the following tissues: bursa of Fabricious, thymus, kidney, cecal tonsils, spleen, proventriculus, harder gland and bone marrow. No clinical signs, nor mortality were recorded in any group during the study. Bursas of Fabricious of both IBDV inoculated groups showed enlargement and oedema in the acute phase of the infection (2 dpi), followed by atrophy, which lasted until the end of the trial. Atrophy of the Bursas appeared earlier and was more substantial in birds inoculated with the ITA strain compared to the ones inoculated with the Classical strain. Histopatology of the Bursas showed lymphocyte depletion, cystic cavities and poor regeneration process. Viral RNA was persistently detected until the end of the trial in bursal tissues and in most of the tissues collected. The study showed that ITA genotype, though it has a subclinical course, causes a severe and persistent damage of Bursa tissues. Therefore its circulation in broilers might be a threat for the poultry industry.