Age-dependent pathogenesis of clade 2.3.4.4A H5N2 HPAIV in experimentally infected Broad Breasted White turkeys

Highly pathogenic avian influenza (HPAI) is a viral disease with devastating consequences to the poultry industry as it results in high morbidity, mortality and international trade restrictions. In the present study, we characterized age-related differences in terms of pathology in commercial white broad breasted turkeys inoculated with A/turkey/Minnesota/12582/2015 (H5N2) HPAIV clade 2.3.4.4A, a virus from the largest HPAI poultry outbreak that affected the Unites States in 2014-2015. Turkeys infected at 6-weeks of age showed inapparent to little clinical signs with rapid disease progression, reaching 100% mortality at 3 days post infection (dpi). In contrast, turkeys infected at 16-weeks of age developed ataxia and lethargy and reached 100% mortality by 5 dpi. Infection in the 6-weeks old turkeys resulted in peracute lesions consistent of extensive hemorrhages, edema and necrosis, but inflammation was not prominent. In the 16-weeks old turkeys, necrosis and hemorrhages in tissues were accompanied by a more prominent subacute inflammatory infiltrate. Both age groups showed presence of avian influenza virus (AIV) nucleoprotein (NP) in multiple cell types including neurons, glial cells, ependymal cells, respiratory epithelial cells, air capillary epithelium and pulmonary macrophages, cardiac myocytes, smooth muscle fibers, pancreatic acini and ductal cells. Cells of the vascular walls stained strongly positive for viral antigens, but no positivity was found in the endothelial cells of any organs. These findings indicate that age is a determinant factor in the progression of the disease and delay of mortality during infection with the H5N2 clade 2.3.4.4A HPAI virus in naïve white broad breasted turkeys.