Arch Virol (2002) 147: 1611–1621 DOI 10.1007/s00705-002-0837-z Avian embryo susceptibility to Italian H7N1 avian influenza viruses belonging to different genetic lineages Brief Report I. Capua 1 , F. Mutinelli 1 , and M. H. Hablovarid 2 1 National Reference Laboratory for Newcastle Disease and Avian Influenza, Istituto Zooprofilattico Sperimentale delle Venezie, Legnaro, Italy 2 Razi Vaccine and Serum Institute, Karaj, Iran Received February 12, 2002; accepted April 17, 2002 Published online July 19, 2002 c  Springer-Verlag 2002 Summary. In the present paper we report of the results of an immunohistochemical investigation to assess tissue tropism and viral replication in developing chicken, turkey, Muscovy duck and mallard duck embryos, of Italian H7N1 isolates be- longing to different genetic lineages. LPAI isolates were chosen on the basis of the location in the phylogenetic tree: a progenitor strain, A/ty/Italy/977/V99, (exhibiting no additional glycosylation site, nAGS), strain A/ty/Italy/2379/V99 (AGS in position 123) and strain A/ty/Italy/3675/V99 (AGS in position 149) were selected. The latter two strains belonged to distinct lineages originating from the pool of progenitor strains. HPAI isolate A/ty/Italy/4580/V99 was also included in the study. All the embryos tested supported the growth of HPAI. The LPAI isolates replicated readily in the allantoic layer of the CAM of all the species tested, and did not grow in the developing chicken, turkey and Muscovy duck embryos. In contrast, they replicated to different extents in the respiratory tract of the developing mallard embryo, which also presented lower mortality rates than the other species. We conclude from these findings that the pathogenesis of LPAI infections in mallard embryos is different to that observed in other species, and should be investigated further. * Avian influenza (AI) viruses may be classified on the basis of the clinical condition they cause in susceptible birds. Low pathogenicity avian influenza (LPAI), may be caused by viruses belonging to all 15 haemagglutinin types (H1-H15) and is usually a mild disease in susceptible poultry. Highly pathogenic avian influenza (HPAI), which is caused by only certain viruses of the H5 and H7 subtypes, is, in