Outbreak of swine influenza in Argentina reveals a non-contemporary human H3N2 virus highly transmissible among pigs

Abstract

Sporadic outbreaks of human H3N2 influenza A virus (IAV) infections in swine populations have been reported in Asia, Europe and North America since 1970. In South America, serological surveys in pigs indicate that IAVs of the H3 and H1 subtypes are currently in circulation; however, neither virus isolation nor characterization has been reported. In November 2008, an outbreak of respiratory disease in pigs consistent with swine influenza virus (SIV) infection was detected in Argentina. The current study describes the clinical epidemiology, pathology, and molecular and biological characteristics of the virus. Phylogenetic analysis revealed that the virus isolate shared nucleotide identities of 96-98 % with H3N2 IAVs that circulated in humans from 2000 to 2003. Antigenically, sera from experimentally inoculated animals cross-reacted mainly with non-contemporary human-origin H3N2 influenza viruses. In an experimental infection in a commercial swine breed, the virus was of low virulence but was transmitted efficiently to contact pigs and caused severe disease when an infected animal acquired a secondary bacterial infection. This is the first report of a wholly human H3N2 IAV associated with clinical disease in pigs in South America. These studies highlight the importance of two-way transmission of IAVs and SIVs between pigs and humans, and call for enhanced influenza surveillance in the pig population worldwide.

Fig. 1.

(a, b) Cranioventral consolidation. Note the distinctive chessboard-like distribution over the lung in (b). (c, d) Histopathology of the case shown in (a). Medium-sized bronchioles were lined with immature flat epithelial cells (c), whilst small bronchioles showed necrosis and sloughed-off epithelium (d). The lumen was filled with detached necrotic epithelial cells and neutrophils. (e) Immunohistochemical staining showed IAV antigen on the epithelial cells in bronchioles that showed slight inflammatory changes and a few scattered mononuclear cells within the alveoli. Magnification, ×40 (c–e).

Fig. 2.

Gross and microscopic pathology of pigs directly infected with A2/08. Groups of pigs were directly inoculated intratracheally with 10⁸ TCID₅₀ A2/08 per pig and the lungs were collected at 5 days p.i. to evaluate lung pathology. Tissues were processed routinely for histopathology and stained with haematoxylin and eosin (H&E). Macroscopic examination of the lungs with pneumonia showed distinct rib imprints on the surface of the lungs (upper row). Microscopic examination of the trachea, bronchi and bronchioli are shown in the lower three rows. Each column represents an individual animal. The lung of animal 303 shows dark red area of lung consolidation in the middle right lobe typical of swine influenza pneumonia. Representative pictures from a control pig inoculated with PBS are show on the right.

Fig. 3.

(a) Replication of A2/08 in swine lungs. BALF was collected from DI pigs at 5 days p.i. and viral titres were determined as TCID₅₀ on MDCK cells. Values are shown as the log₁₀ TCID₅₀ ml⁻¹ for each infected pig. The pig ID corresponds to that shown in Fig. 2. (b) Pulmonary cytokine response induced by A2/08 in pigs. The levels of nine porcine (po) cytokines in BALF samples were determined by multiplex sandwich ELISA. TNF-α, Tumour necrosis factor alpha. (c) Virus shedding of A2/08. Groups of pigs were directly inoculated intratracheally with 10⁸ TCID₅₀ A2/08 per pig and three DC pigs were co-housed with the DI pigs on 1 day p.i. Nasal swabs were collected daily and titrated as TCID₅₀ on MDCK cells. Values in (b) and (c) are shown as mean±sem.