Influenza A virus transmission in swine farms and during transport in the swine supply chain

Abstract

The last influenza pandemic in 2009 emerged from swine and surveillance of swine influenza is important for pandemic preparedness. Movement of swine during husbandry, trade or marketing for slaughter provide opportunities for transfer and possible genetic reassortment of swine influenza viruses. Over 90% of the swine slaughtered at the central swine abattoir in Hong Kong are imported from farms located in multiple provinces in mainland China. There is opportunity for virus cross-infection during this transport and slaughter process. Of the 26,980 swabs collected in the slaughterhouse in Hong Kong from 5 January 2012 to 15 December 2016, we analyzed sequence data on influenza A (H3N2) virus isolates (n = 174) in conjunction with date of sampling and originating farm. Molecular epidemiology provided evidence of virus cross-infection between swine originating from different farms during transport and also evidence of a virus lineage persisting in a swine farm for over 2 years. We used virus serology and isolation data from 4,226 paired pig serum and nasal swabs collected from swine originating from Guangdong Province to compare the force of infection (FOI) during transport and within farms. The mean weekly FOI during transport was λ = 0.0286 (95% CI = 0.0211-0.0391) while the weekly FOI in farms was λ = 0.0089 (95% CI = 0.0084-0.0095), assuming a duration of stay in farm of 28 weeks, suggesting increased force of infection during the transport process. Potential risk factors for infection including the duration in transport, length of stay at slaughterhouse and farm-level seroprevalence were also assessed by multivariable logistic regression analysis. Transport may increase virus cross-infection rates and provide opportunities for virus reassortment potentially increasing zoonotic risk to those involved in the transportation and slaughtering processes.