Experiment : Chickens of group 1 were inoculated with 0.2 ml (2*105 EID50) of the H7N1 HPAIV strain, equally divided between the intranasal and intratracheal route. In the same way, animals of group 2 received 2*105 EID50 of the H7N1 LPAIV strain. Sampling: Six chickens from each group were sacrificed just before infection (controls) and at 4, 8, 16 and 24 h.p.i. From all sacrificed chickens, gross pathology of the organs was studied. Lung, trachea, ileum and brains were collected. For RNA isolation, organs were snap-frozen in liquid nitrogen and stored at -80C until use. For immonohistochemistry, samples were fixed immediately by immersion in a zinc salt-based fixative, containing 0.5 % zinc chloride, 0.5% zinc acetate in 0.1 M Tris base buffer containing 0.05 % calcium acetate, pH 7.4 for 48 h. After fixation, the tissues were dehydrated and embedded in paraffin wax. All HPAIV experiments were performed in Biosafety Level 3 facilities.