**2. Avian influenza transmission**

#### **2.1 The virus**

Influenza viruses are segmented negative-sense single-stranded RNA, belonging to the family *Orthomyxoviridae* [8]. This family is classified into influenza viruses' types A, B, and C, *Thogotovirus*, and *Isavirus* [9]. Only influenza type A viruses infect poultry [8], and avian influenza viruses are known to be a diverse group of viruses in the *Orthomyxoviridae* family, genus *Influenza A virus*, and have been categorized into subtypes based on the two surface glycoproteins, the hemagglutinin (H) and neuraminidase (N) [10]. There are now 18 H and 11 N surface glycoproteins [11]. Previously, HA1-16 and NA1-9 have been identified to occur naturally in avian host, mostly waterfowls where they exist in benign form (low pathogenic) [12]. Subtypes HA 17 and 18 and NA 10 and 11 were identified in bats [13] recently. Avian influenza viruses have also been further classified into two different pathotypes (low pathogenicity [LP] and high pathogenicity [HP]), based on the ability to produce disease and death in the major domestic poultry species, the chicken (*Gallus gallus domesticus*) [14]. HPAI has been shown to be caused in susceptible host species by only the avian influenza viruses of H5 and H7 subtypes, which contain multiple basic amino acids at the cleavage site of the hemagglutinin molecule, [15]. However, not all H5 and H7 avian influenza viruses are highly pathogenic. It has been shown that HPAI viruses emerge in domestic poultry from LPAI progenitors of the H5 and H7 subtypes [16–18].

#### **2.2 Transmission**

A typical flow of avian influenza virus in poultry mostly begins with the introduction of the virus to a region or country where the virus has not been known to be present. This introduction has been known to occur by way of contact of resident water fowls and/or backyard poultry with an infected migratory wild bird

**63**

*Features of Highly Pathogenic Avian Influenza (HPAI) H5N1 in Domestic Poultry*

[19–21] which may carry any of the subtype combinations of hemagglutinin (H) and neuraminidase (N) glycoproteins. This subtype in the case of an infection with HPAI will definitely have H5 or H7 combination. This contact may occur at wetland

In addition, there could be direct introduction of infected bird into poultry. **Figure 1** illustrates the possibilities of the virus flow in poultry which may begin with local ducks or mixed species flock having contact (direct) with the virus by way of resident water fowls and or migratory wild birds. The virus can move by human vectors (indirect) especially when poultry attendants take care of the mixed species flock, after which a contact of free-ranging ducks and/or chickens with semi-intensive or backyard poultry may take place. At this point, the virus enters

Experience with farmers/poultry owners and attendants at the Central Diagnostic Laboratory of the National Veterinary Research Institute in Vom, Nigeria, revealed that poultry owners hurriedly sell off supposedly infected chickens to live bird and wet bird markets, from where unsuspecting public and local peasant poultry keepers buy these infected poultry for introduction into local

flocks. The circle of infection can continue unabated for a very long time.

Nigeria experienced an outbreak of HPAI H5N1 virus in both commercial and local poultry populations during January 2006 as an extension of the global outbreaks [7]. The index HPAI-H5N1 was confirmed in Nigeria at a commercial poultry farm in Kaduna State, and by the end of the initial outbreak, over 46,000 poultry

*DOI: http://dx.doi.org/10.5772/intechopen.86098*

[22, 23] areas or agroecological regions [24].

**3. The Nigerian scenario**

**3.1 Index outbreak**

**Figure 1.**

into intensive poultry directly or indirectly causing deaths.

*A flow chart of the typical origin and transmission of avian influenza in poultry.*

*Features of Highly Pathogenic Avian Influenza (HPAI) H5N1 in Domestic Poultry DOI: http://dx.doi.org/10.5772/intechopen.86098*

**Figure 1.** *A flow chart of the typical origin and transmission of avian influenza in poultry.*

[19–21] which may carry any of the subtype combinations of hemagglutinin (H) and neuraminidase (N) glycoproteins. This subtype in the case of an infection with HPAI will definitely have H5 or H7 combination. This contact may occur at wetland [22, 23] areas or agroecological regions [24].

In addition, there could be direct introduction of infected bird into poultry. **Figure 1** illustrates the possibilities of the virus flow in poultry which may begin with local ducks or mixed species flock having contact (direct) with the virus by way of resident water fowls and or migratory wild birds. The virus can move by human vectors (indirect) especially when poultry attendants take care of the mixed species flock, after which a contact of free-ranging ducks and/or chickens with semi-intensive or backyard poultry may take place. At this point, the virus enters into intensive poultry directly or indirectly causing deaths.

Experience with farmers/poultry owners and attendants at the Central Diagnostic Laboratory of the National Veterinary Research Institute in Vom, Nigeria, revealed that poultry owners hurriedly sell off supposedly infected chickens to live bird and wet bird markets, from where unsuspecting public and local peasant poultry keepers buy these infected poultry for introduction into local flocks. The circle of infection can continue unabated for a very long time.
