Prologue: Avian Influenza - An Overview from Endemic to Pandemic

*Muhammad Abubakar, Abdullah Iqbal, Shumaila Manzoor and Kashif Saleemi*

### **1. Introduction**

Avian influenza also called as bird flu is an acute, contagious disease caused by Influenza A virus. Birds of order *Anseriformes* and *Charadriiformes are reservoir hosts of* influenza A viruses [1]. These viruses have the ability to infect many mammal species such as horses, pigs, cats, dogs, and humans. Influenza A viruses are negative-sense single-stranded RNA viruses with enveloped structure [2]. Influenza A viruses have shown high antigenic variability due to antigenic drift and shift phenomena. These mutation methods result in the emergence of new variants. These new variants can have different host ranges, mortality rates, morbidity rates, and patterns of infection [3].

Influenza A viruses are further divided into highly pathogenic avian influenza (HPAI) or low pathogenic avian influenza (LPAI). Up till now, only some H5 and H7 subtypes have been confirmed as HPAI viruses. However, it is reported that some LPAI H5 and H7 subtypes can mutate to HPAI due to antigenic shift and drift phenomena [4].

Some influenza A viruses have a zoonotic potential and can cause a pandemic hazard to human population. Outbreaks of HPAI in poultry have affected the poultry industry worldwide [5]. In 2002, due to HPAI outbreaks, Chile suffered a loss of almost \$31 million USD. In 2003, the HPAI outbreak resulted in the loss of more than \$3387 million USD in Indonesia and \$55 million USD in Vietnam. Similarly, in 2012, Mexico suffered an economic loss of approximately \$475 million USD due to HPAI outbreaks [1].

### **2. Influenza A virus**

Influenza A viruses belong to family *Orthomyxoviridae*. Influenza A viruses are single-stranded negative-sense RNA viruses having genome with 8 segmentation that encode for 10 viral proteins: hemagglutinin (HA), neuraminidase (NA), matrix proteins (M1 and M2), nucleocapsid protein (NP), nonstructural proteins (NS1 and NS2), and polymerase proteins (PB1, PB2, and PA) [6].

Influenza viruses are further classified into subtypes on the basis of their hemagglutinin (HA) and neuraminidase (NA) genes [7]. A total of 18 HA and 11 NA types has been identified up till now. On the basis of phylogenetic analysis and sequence homology of the HA gene, these viruses are clustered into virus clades [4].

A nomenclature system for influenza viruses has been established by the World Health Organization (WHO) and FAO. This system contains the type of influenza, the first place of isolation of virus, strain, and year of isolation. For example, A/ Hong Kong/156/97 would be interpreted as influenza type A, isolated in Hong Kong, strain 156, and isolated in 1997 [1].

Hemagglutinin mediates binding of the virus to sialic acid receptors at the cell surface. Neuraminidase plays its role in the detachment of the virus from the cell surface [8]. A specific type of hemagglutinin binds with a specific type of cell surface that further clarifies the difference host range for different influenza virus types [9]. Mutation of genes coding hemagglutinin and neuraminidase affects the transmission of the virus along with the alteration of the host range [10].
