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Influenza Report 2006 is a medical textbook that provides a comprehensive overview of epidemic and pandemic influenza. The download of the PDF is free. Influenza Report has also been published in Chinese, Croatian, German, Indonesian, Mongolian, Serbian, and Slovenian.

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The Viruses

Influenza viruses are spherically or longitudinally shaped enveloped particles with an up to eight-fold segmented, single-stranded RNA genome of negative polarity. Influenza viruses hold generic status in the Orthomyxoviridae family and are classi Þed into types A, B or C based on antigenic differences of their nucleo- and matrix proteins. Avian influenza viruses (AIV) belong to type A. Excellent reviews on the structure and replication strategy of influenza viruses have been published recently (e.g. Sidoronko and Reichl 2005).

The main antigenic determinants of influenza A and B viruses are the haemagglutinin (H or HA) and the neuraminidase (N or NA) transmembrane glycoproteins, capable of eliciting subtype-speciÞc and immune responses which are fully protective within, but only partially protective across, different subtypes. On the basis of the antigenicity of these glycoproteins, influenza A viruses currently cluster into sixteen H (H1 ? H16) and nine N (N1 ? N9) subtypes. These clusters are substantiated when phylogenetically analysing the nucleotide and deduced amino acid sequences of the HA and NA genes, respectively (Fouchier 2005).

The conventional nomenclature for influenza virus isolates requires connotation of the influenza virus type, the host species (omitted in the case of human origin), the geographical site, serial number, and year of isolation. For influenza virus type A, the haemagglutinin and neuraminidase subtypes are added in brackets. One of the parental avian strains of the current outbreaks of H5N1 of Asian lineage was isolated from a goose in the Chinese province, Guangdong: accordingly, it is designated A/goose/Guangdong/1/96 (H5N1) (Xu 1999) while the isolate originating from the Þrst-documented human case of Asian lineage H5N1 infection from Hong Kong (Claas 1998) is referred to as A/HK/156/97 (H5N1).

The haemagglutinin, a glycosylated and acylated protein consisting of 562 ? 566 amino acids, is incorporated in the viral envelope. The globular head of its membrane- distal, knob-like external domain is associated with binding to cellular receptors composed of oligosaccharides which terminally carry derivates of neuraminic acid (Watowich 1994). The exodomain of the second transmembrane glycoprotein, the neuraminidase (NA), exerts sialolytic enzymatic activity and liberates virus progeny captured at the surface of infected cells during egress. This function prevents viral aggregation during egress, and possibly also facilitates the drifting of the virus through the mucus layers of the targeted epithelial tissues leading to viral attachment (Matrosovich 2004a). This renders the neuraminidase an interesting target of antiviral agents (Garman and Laver 2004). Mutually attuned and coordinated actions of the antagonistic glycoprotein species HA and NA of a viral strain are pivotal for effective attachment and release processes of the virions (Wagner 2002).

Attachment to cell surface proteins of inßuenza A virions is achieved through mature trimerised viral HA glycoproteins. Attachment is stratiÞed by recognition of distinct terminal sialic acid species (N-acetyl- or N-glycolylneuraminic acid), the type of glycosidic linkage to penultimate galactose (a2-3 or a2-6) and the composition of further inner fragments of sialyloligosaccharides present at the cell surface (Herrler 1995, Gambaryan 2005). A variety of different sialyloligosaccharides are expressed with restriction to tissue and species origin in the different hosts of influenza viruses. Adaptation in both the viral HA and the NA glycoprotein to the speci Þc receptor type(s) of a certain host species is a prerequisite for efficient replication (Ito 1999, Banks 2001, Matrosovich 1999+2001, Suzuki 2000, Gambaryan 2004). This implies a re-shaping of the receptor binding units of the HA protein following interspecies transmission (Gambaryan 2006). A mechanistic overview of the diverse receptor types is given in Þgure 1. Avian influenza viruses generally show the highest affinities for a2-3 linked sialic acid as this is the dominating receptor type in epithelial tissues of endodermic origin (gut, lung) in those birds that are targeted by these viruses (Gambaryan 2005a, Kim 2005). Human-adapted influenza viruses, in contrast, primarily access 2-6 linked residues which predominate on non-ciliated epithelial cells of the human airway. These receptor predilections define part of a species barrier preventing hassle-free transmission of avian viruses to humans (Suzuki 2000, Suzuki 2005). Yet recently, it has been shown that there is a population of ciliated epithelial cells in the human trachea which also carry avian receptor-like glycoconjugates at lower densities (Matrosovitch 2004b), and also chicken cells carry human-type sialyl receptors at low concentrations (Kim 2005).

This might explain why humans are not entirely refractory towards infection with certain avian strains (Beare and Webster 1991). In pigs, and also in quails, both receptor types are present at higher densities which renders these species putative mixing vessels for avian and human strains (Kida 1994, Ito 1998, Scholtissek 1998, Peiris 2001, Perez 2003, Wan and Perez 2005). Once successfully attached to a suitable receptor, the virion

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