Avian influenza and Newcastle disease.

Avian influenza (AI) and Newcastle disease (ND) affect various avian species, especially domestic poultry , and are caused by type A orthomyxoviruses and type 1 avian paramyxoviruses, respectively. 1,2 Clinical manifestations of such infections vary with the virus strain, the host species, and the presence or absence of secondary environmental factors or exacerbating agents. In domestic poultry, infections can be subclini-cal or induce mild to severe disease syndromes, including respiratory tract disease, decreases in egg production , or a multiorgan systemic disease with a near 100% mortality rate. Severe syndromes are caused by highly virulent specific virus strains termed highly pathogenic AI and velogenic ND viruses. The highly virulent forms of AI and ND are contained on list A of Office International des Epizooties (OIE), the official international organization for animal health and sanitary standards under the World Trade Organization. These highly virulent viruses have severe global impact on poultry health and limit international trade in poultry and poultry products. Furthermore, the viruses that cause highly pathogenic AI and highly virulent ND are potential agribioterrorism agents. 3 However, the milder or low virulent forms of AI and ND are more common and endemic in domestic poultry in many parts of the world, but neither are contained on list A or B of OIE. Cases of human infection by AI and ND viruses have been documented, but are rare. AVIAN INFLUENZA Understanding the unique structural and biological features of AI viruses is important in understanding the zoonotic potential for these viruses. Avian influenza virus (family: Orthomyxoviridae, genus: Influenza virus A) is a negative-sense, single stranded RNA virus with a genome composed of 8 gene segments that code for 10 proteins (Fig 1). 2 The 2 surface glycoproteins, hemagglutinin and neuraminidase, are used for epi-demiologic study of the virus. There are 15 hemagglu-tinin (H1 to H15) and 9 neuraminidase (N1 to N9) subtypes; however, the virulence of AI viruses for chickens does not correlate with these subtypes. Most infections with AI viruses (H1 to H15 subtypes) are subclinical or induce mild disease syndromes, whereas a few isolates of H5 and H7 subtypes induce severe, highly fatal disease, termed highly pathogenic AI (historically called fowl plague or fowl pest). The high vir-ulence of H5 and H7 AI viruses in chickens does not correlate with their ability to infect and cause disease in humans. Diagnosis of AI virus infections in animal and humans has …