Influenza virus.Swine zoonosis. In the late 1990’s, after the emergence of a triple reassortant H3N2 strain of swine influenza virus, the epidemiology of swine influenza virus (SIV) in North America has changed (Olsen, 2002). The triple-reassortant H3N2 viruses (containing genes related to corresponding genes of human, swine and avian influenza viruses) weredocumented on swine premises from different regions of North America (Karasin et al, 2000c; Zhou et al, 1999). As expected, multiple reassortant SIV variants between the classical swine H1N1 and the triple-reassortant H3N2 virus (and other influenza viruses) emerged as a result of recombination. First, a reassortant H1N2 was detected in multiple herds and regions of North America (Karasin et al, 2000b; Karasin et al, 2002), followed by reassortant H3N1 (Lekcharoensuk et al, 2006; Ma etal, 2006). Although the antigenic drift in the classical swine H1N1 was documented in the early 2000’s (Olsen et al, 2000); more recently, a reassortant swine H1N1 (containing PA and PB2 genes corresponding to genes of avian influenza viruses) has been found to be the predominant H1N1 strain isolated from the US swine population (Janke, 2004; Ma et al, 2006). Reassortant swine H1N1 (containing human influenza virus PB1 gene) have also been identified in Ontario swine (Karasin et al, 2000a). In addition, different influenza virus subtypes of wholly avian (H1N1, H3N3, H4N6) and human lineages (H3N2, H1N2) were identified from clinical cases in pigs in Canada (Karasin et al, 2000a; Karasin et al, 2004; Karasin et al, 2006). Although the spatial distribution ofall aforementioned subtypes and variants does not necessarily overlap, and different variants are not necessarily effectively transmitted; the intensive trade between regions in North America provides opportunity for mixing pigs from different sources. Consequently, the number of possible recombinations is high which could represent challenges from diagnostic and clinical perspectives, and concern for public health. Currently swine influenza virus is considered tobe an occupational hazard. Although being of clinical importance in swine and in human population, direct transmission of swine influenza viruses to people is London Swine Conference – Tools of the Trade 1-2 April 2009 100 reported at a surprisingly low level. Despite this apparently low occurrence, cases of human influenza due to exposure to swine influenza viruses are always concerning. This is because swine harbor receptors that could interact withhuman and avian influenza viruses. Thus, the concern is that a virulent virus, such as highly pathogenic H5N1 could emerge in avian population, infect swine and experience changes that could lead to more efficient transmission in people. From surveys of Ontario herds prior to 2005 we concluded that the sow population was likely free from the selected H3N2 strains (Table 1). In early 2005, an outbreakof triple-reassortant H3N2 SIV in Ontario swine herds was documented (Carman and Ojkic, 2005). Using sera collected in 2004, based on Ontario Swine Sentinel Project, 919 sera from 46 finisher herds were tested for H1N1 and 920 sera from 46 herds for H3N2. In 2005, 978 sera from 49 herds were tested for both SIV subtypes. At the pig-level, prevalence of antibodies against H1N1 SIV was 13.4% and 14.9% in 2004, respectively. Prevalence of antibodies against H3N2 SIV varied between 2.7% and 25.9% in 2004 and 2005, respectively.
Posted on: Wed, 12 Jun 2013 20:27:12 +0000
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