APV infections are associated with significant levels of morbidity and mortality in domestic and wild bird populations [6, 60] . Most of the investigations and reported cases are based on single APV isolates, which makes it difficult to address the pathogenicity of different APVs in different bird species. Chickens are commonly used to determine the pathogenicity of new isolates, but chickens may not be the ideal host, since APVs from wild birds may not multiply in chickens. In an attempt to identify and characterize the pathogenicity of APVs, Tripathy and others [4] found that wild isolates of Hawaiian crowpox virus had a generally mild pathogenicity in domestic chickens, characterized by relatively minor lesions of short duration at the sites of inoculation, which were in contrast to the general ability of FWPV strains to produce extensive proliferative lesions [4] . In another experimental study, two APV isolates obtained from endangered Hawaiian wild birds, the Hawaiian Goose (Branta sandvicensis) and the Palila (Loxioides bailleui), were compared with FWPV in specific-pathogen-free chickens. Immune responses were measured by ELISA before and after immunization with Hawaiian APVs and after challenge with FWPV. Both isolates from Hawaiian birds developed only a localized lesion of short duration at the site of inoculation in chickens and did not provide protection against subsequent challenge with virulent FWPV, in which severe lesions were observed. In contrast to high antibody response in chickens immunized with FWPV, birds immunized with either of the two Hawaiian isolates developed low to moderate antibody responses against viral antigens [61] . Pathogenicity studies of APVs in parrots [62] , turkeys, pigeons and canaries have also been reported. Canaries were highly susceptible to CNPV, but showed resistance to turkeypox virus, FWPV and pigeonpox virus [4, 63, 64] . A poxvirus from a Canada goose (Branta canadensis) was transmissible to domestic goose, but not to chickens or domestic ducks [15] . Pigeonpox virus produced mild infection in chickens and turkeys, but was more pathogenic for pigeons [62] . Poxvirus isolates from magpies (Pica pica) and great tits (Parus major) did not infect young chickens [16] , however, poxvirus isolated from blackbacked magpie (Gymnorhina tibicen) produced lesions in chickens. These studies were based on clinical manifestations in the chickens and suggest host specificity and pathogenicity.