Introduction (Word count: 3910)
Emerging infectious diseases continue to be a major threat to public health. Most emerging or re-emerging infectious diseases are caused by RNA viruses and are zoonotic in origin, arising from domestic livestock or wildlife. Understanding viral transmission dynamics in livestock production and trade pathways can identify potential critical control points that may be amenable to disease control interventions (Leung, 2012).
Swine are an intermediate host in the pathway of pandemic emergence because they express receptors for both avian and human influenza virus in their upper respiratory tract (Ito, 1998). The 2009 H1N1 pandemic virus arose in Mexico through reassortment of gene segments from Eurasian avian-like H1N1 and North American Triple Reassortant swine viruses (Smith, 2009). The two precursor viruses were introduced into Mexico via importation of swine, possibly as breeder-stock. Swine husbandry, trading, marketing and slaughter provide opportunities for the emergence of novel reassortants and zoonotic infections through exposure of humans to swine influenza viruses. An example was H3N2 variant (H3N2v) influenza viruses causing over 300 infections in humans with exposure to swine at agricultural fairs in USA (Jhung, 2013). In Asia, the high densities and close proximity between swine, humans, poultry and wild birds provides an even greater threat for interspecies transmission.
In Hong Kong, over 90% of the swine slaughtered at its abattoirs are imported from mainland China with an annual importation of about 1.45 million swine in 2017 (Centre for Food Safety, 2019). Live pigs for consumption are transported in trucks from the source farms in multiple provinces to a transfer house at the Shenzhen-Hong Kong border where they are held for inspection overnight, and are then transported by trucks or lorries to a centralized abattoir in Hong Kong. Along this transport chain, there is opportunity for virus transmission between consignments of pigs originating from different farms, facilitated by the stress of transport and the densely packed environment. Pigs may also be held for hours to days in holding pens at the abattoir, providing further opportunities for cross infection (details in supporting information, Supplementary figure 1).
A previous study on the transmission dynamics of H1-subtype swine influenza viruses (SIVs) in China (Strelioff, 2013) analyzed virologic and serological data to infer the transmission risk over time. However, the interaction and cross-reactivity between different lineages of H1 SIVs may have affected interpretation of the serology results from the hemagglutination inhibition (HAI) test. There was a lack of epidemiological information of imported swine population, such as source farm or province and length of stay at abattoir.
Multiple swine influenza viruses (SwIV) are enzootic in swine in China, including classical swine (CS), Eurasian avian-like swine (EA), triple-reassortant swine (TRIG) and pandemic H1N1/2009 (H1N1pdm09) lineage (Vijaykrishna, 2011). Infection with one H1 virus lineage could broaden cross-reactive antibody to other H1-subtype SIVs (Perera, 2011) and hence confound interpretation of sero-epidemiology of H1 swine viruses. There was only one monophyletic H3 lineage (Binh Duong (BD)-like H3N2 lineage) circulating in China since 2010 allowing serology to accurately estimate past infection (Liang, 2014).
The objective of the current study was to understand the transmission dynamics of swine H3N2 viruses at the farm, transport and abattoir stage of the production chain. Focusing on swine H3N2 viruses allowed us to interpret serological data in the context of molecular epidemiology to infer transmission dynamics within the swine production system supplying swine for slaughter in Hong Kong. We compared the risk of infection during transportation and in the farm setting by estimating the force of infection (FOI) and identified the role of factors, such as transport distance and the length of stay at slaughterhouse.