Influenza B viruses in pigs, Taiwan

Abstract

Background: Influenza B viruses (IBVs) have never been isolated from natural-infected pigs in clinical cases, although the susceptibility of domestic pigs to experimental IBV infections had been confirmed as well as IBV-specific antibodies were detected from pigs under natural and experimental conditions.

Objectives: We aimed to assess and investigate the activities for infection and circulation of IBVs in pigs.

Methods: Annual active surveys for influenza have been implemented on swine populations in Taiwan since July 1998. Nasal swabs, trachea, lungs, and blood from pigs were tested using virological and serological assays for influenza. Gene sequences of influenza viral isolates were determined and characterized. Preliminary sero-epidemiological data for influenza virus were investigated.

Results: Three strains of IBV were isolated and identified from natural-infected pigs in 2014. Genetic characterization revealed the highest identities (>99%) of molecular sequence with the contemporary IBVs belonged to the B/Brisbane/60/2008 genetic clade of Victoria lineage in the phylogenetic trees for all 8 genes. IBV-specific antibodies were detected in 31 (0.2%; 95%CI: 0.1%-0.2%) of 15 983 swine serum samples from 29 (2.8%; 95%CI: 1.9%-3.9%) of 1039 farm visits under annual active surveys from 2007 through 2017. Seropositive cases have been found sparsely in 1-5 of test prefectures every year except 2015 and 2017 as well as scattered loosely over 26 townships/districts of 11 prefectures in Taiwan cumulatively in 11 years.

Conclusions: Influenza B viruse infections from humans to pigs remained sporadic and accidental currently in Taiwan but might have paved potential avenues for newly emerging zoonotic influenza in the future.

Keywords: Taiwan; Zoonosis; emerging disease; influenza virus; surveys; swine.

Figure 1

Molecular phylogenetic analyses of corresponding genes between swine and human strains of influenza B virus. Evolutionary analyses were conducted in MEGA7.17 In addition to corresponding genes of B/Lee/1940, B/Victoria/02/1987, B/Yamagata/16/1988, 4 vaccine strains recommended by WHO and implemented in Taiwan from 2009 through 2016 (Yam denotes Yamagata and Vic denotes Victoria),16 3 sIBVs (denoted as ▲) and one of the BLAST‐searched best‐matching viruses (denoted as ●), 122 HA genes (panel A), 151 NA genes (panel B), 31 PB1 genes (panel C), 31 PB2 genes (panel D) and 32 PA genes (panel E), 32 NP genes (panel F), 32 M genes (panel G) and 49 NS genes (panel H) of human IBVs isolated from 2009 through 2016 in Taiwan were involved, respectively. The numbers of nucleotide analyzed in corresponding genes are indicated on panels. The evolutionary history was inferred using the maximum‐likelihood method based on the Hasegawa‐Kishino‐Yano model with a gamma distribution (HKY + G) for HA genes and the Tamura 3‐parameter model with gamma distribution (T92 + G) for NA and 6 internal protein genes, respectively. Reliability of the tree topology was assessed by bootstrap analysis with 1000 replications.18 Only branches with bootstrap values >75% are indicated on phylogenetic trees. Scale bar indicates nucleotide substitutions per site. Accession numbers are listed in parentheses