Emergence of novel strains of avian infectious bronchitis virus in Sweden

Abstract

Infectious bronchitis virus (IBV) causes avian infectious bronchitis, an important disease that produces severe economic losses in the poultry industry worldwide. Recent IBV infections in Sweden have been associated with poor growth in broilers, drop in egg production and thin egg shells in layers. The complete spike gene of selected isolates from IBV cases was amplified and sequenced using conventional RT-PCR. Nucleotide and amino acid sequence comparisons have shown that the recent isolates bear 98.97% genetic similarity with strains of the QX-like genotype. The phylogenetic analysis revealed that strains predominant in the nineties, which were of the Massachusetts type, have been replaced by D388/QX-like strains, however the evolutionary link could not be established. The homology between the two genotypes was 79 and 81%. Remarkably, a strong positive selection pressure was determined, mostly involving the S1 subunit of the S gene. This strong selective pressure resulted in recombination events, insertions and deletions in the S gene. Two new isolates generated from recombination were found with nucleotide sequence diverging 1.7-2.4% from the D388/QX-like branch, indicating the emergence of a new lineage. The study demonstrates a constant evolution of IBV that might be in relation to increased poultry farming, trade and vaccine pressure. The findings underscore the importance of continuous monitoring to control spread of infections, as well as to timely adjust diagnostic methods, molecular epidemiological studies, development and use of vaccines that are adapted to the changing disease scenario.

Fig. 1

The selective pressure analysis in the spike glycoprotein of IBV. The graph illustrates the S1 part of the spike glycoprotein was mostly subject to positive selective pressure. In S2, the selective pressure was mostly negative.

Fig. 2

Phylogenetic tree based on partial S1 gene showing the relationship of different IBV isolates. The isolates sequenced in this study are indicated in green and grouped with Mass and the QX-like genotypes. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

Fig. 3

Phylogenetic tree based on the complete spike gene showing the relationship between different IBV strains.