Novel reassortment of Eurasian avian-like and pandemic/2009 influenza viruses in swine: infectious potential for humans

Abstract

Pigs are considered to be intermediate hosts and "mixing vessels," facilitating the genesis of pandemic influenza viruses, as demonstrated by the emergence of the 2009 H1N1 pandemic (pdm/09) virus. The prevalence and repeated introduction of the pdm/09 virus into pigs raises the possibility of generating novel swine influenza viruses with the potential to infect humans. To address this, an active influenza surveillance program was conducted with slaughtered pigs in abattoirs in southern China. Over 50% of the pigs tested were found to be seropositive for one or more H1 influenza viruses, most commonly pdm/09-like viruses. Out of 36 virus isolates detected, one group of novel reassortants had Eurasian avian-like swine H1N1 surface genes and pdm/09 internal genes. Animal experiments showed that this virus transmitted effectively from pig to pig and from pig to ferret, and it could also replicate in ex vivo human lung tissue. Immunization against the 2009 pandemic virus gave only partial protection to ferrets. The continuing prevalence of the pdm/09 virus in pigs could lead to the genesis of novel swine reassortant viruses with the potential to infect humans.

Fig. 1.

Genotypes of the swine influenza viruses identified. The name of a representative virus and the numbers of each variant isolated are given (left). Dates of sampling occasions A to G are as follows: 25 December 2009 (A), 31 December 2009 (B), 8 January 2010 (C), 29 January 2010 (D), 27 February 2010 (E), 7 May 2010 (F), and 28 May 2010 (G).

Fig. 2.

Virus shedding of the infected pigs and contact animals from the nasal route. TCID₅₀ values in MDCK cells from daily nasal swabs (pigs) or washes (ferrets) are shown. Codes for each animal are given in Fig. S3 in the supplemental material.

Fig. 3.

Representative pathology and virus replication in tissues from Sw/GD1361-infected pigs. Viral antigen reactions (with nucleoprotein shown as brown) in epithelial cells of a physical contact pig (pig P4, 5 dpc) are as follows: turbinate (A), trachea (B), and bronchiolar epithelial cells with intraluminal cellular debris (C). An H&E-stained lung section was taken from a physical contact pig (pig P4, 5 dpc) (D) and a naive control pig (mock infected with PBS) (E).

Fig. 4.

Viral infectivity in ex vivo human lung tissue. Human ex vivo lung tissue cubes were inoculated with the virus CA4 (A/California/04/2009 pandemic H1N1 2009), Sw/GD1361 (A/Swine/Guangdong/1361/2010), or WU95 (A/Wuhan/359/1995 seasonal H3N2). (A) Immunohistochemical detection of viral antigen (nucleoprotein) in ex vivo human lung tissue sections (36 hpi). NP-positive cells are shown by brown staining. (B) Virus titers in the ex vivo human lung tissue cubes. The values are means (± standard deviation) for five replicate tissue cubes from three independent inoculations.