A SUBCLINICAL BLUETONGUE VIRUS INFECTION IN RUMINANTS WITH THREE UNIQUE AMINO ACID VARIATIONS ON VP7 CORE PROTEIN OF TAIWAN ISOLATES

Abstract

Bluetongue is an arthropod-borne disease in domestic and wild ruminants caused by bluetongue virus (BTV), and it leads to great economic loss worldwide. Previous studies showed that BTV in ruminants in Taiwan was often subclinical infection. The aim of this study was to determine the current status (years 2016–2017) of BTV infection in ruminants in Taiwan, to compare it to the results of a large-scale study conducted in the year 2003, and to investigate whether new viral strains exist. Competitive ELISA tests of serum samples for anti-BTV-VP7 group-specific antibody revealed seropositive rates of 26.7% in cattle by head, similar to 32.7% in the year 2003, suggestive of a BTV-vector-host (cattle) dynamic balance. In goats, the seropositive rate was 18.6%, slightly increased from 8.2% in the year 2003, suggestive of a slow but active infection taking place. This notion was supported by the detection of VP1 gene nucleic acid from whole blood in six out of 29 seropositive goats by reverse transcription–polymerase chain reaction. However, no new virus strain was isolated from embryonating chicken embryos (ECEs) inoculation. Alignment of VP7 amino acid sequences revealed that Taiwan and Japan isolates possessed three specific amino acids on sites No. 82 (arginine), No. 328 (aspartate), and No. 336 (glutamine), which are different from many countries. In a three-dimensional model, these amino acids were located closely on the middle lateral surface of VP7 trimers. Since VP7 is a major outer protein engaged in entry into insect cells and a strong T cell response inducer, these differences likely indicate the result of positive selection of local vectors and hosts in Taiwan.