Surveillance and control of influenza in pigs

Influenza viruses are members of the family Orthomyxoviridae. They are enveloped, single stranded ribonucleic acid (RNA) viruses with a segmented genome and are grouped into 3 types, designated A, B and C (Wright and Webster 2001). Influenza viruses of the C type are found exclusively in humans and are not considered a public health concern. Influenza B viruses cause sporadic outbreaks of mild respiratory disease in humans. They have also been isolated from pigs (Takátsy et al. 1967) but they are of no veterinary interest. Of greater importance are influenza A viruses. They have a spherical or filamentous morphology and their size ranges from 80 to 120 nm (Wright and Webster 2001). Their genome consists of 8 RNA segments, which encode 10 proteins (Table 1). These include two transmembrane “spike-like” glycoproteins: the haemagglutinin (HA) and the neuramindase (NA), a third transmembrane protein referred to as matrix protein M2, the underlying matrix protein M1, which forms a layer below the lipid envelope and gives structure to the virus, two non structural proteins, the NS1 and NS2 and the ribonucleoprotein (RNP) complexes, which consist of four additional proteins, the nucleoprotein (NP) and the three polymerases: PA, PB1 and PB2 (Figure 1). The HA and NA are of particular interest because (a) they facilitate the entry and release of the virus into and out of the host cell and (b) they are the primary targets recognized by the immune system after infection or vaccination. So far, 16 different types of haemagglutinin (H1 to H16) and 9 of neuraminidase (N1 to N9) have been recognized (Webster and Bean 1998, Fouchier et al. 2005). Influenza A viruses are named using the following convention: A/species of origin/location of isolation/isolate number/year of isolation (in the case of human isolates the species is not mentioned), i.e. A/Swine/Belgium/1/98 or A/New York/55/04 (WHO 1980). Furthermore, influenza A viruses are subtyped based on the nature of the HA and NA, their combination defines their subtype accordingly: H1N1, H3N2, H5N1. Because of their segmented single stranded RNA genome, influenza viruses have a high mutation rate (genetic drift) and the possibility to undergo reassortment. Reassortment may occur when more than one virus co-infect the same cell, exchange genes and provide a novel influenza virus, which combines gene segments from the original viruses (genetic shift) (Scholtissek 1998, Wright and Webster 2001)