Coronaviruses in Avian Species – Review with Focus on Epidemiology and Diagnosis in Wild Birds

Milek, Justyna; Blicharz-Domanska, Katarzyna

Information about the role of virus proteins in the course of infection comes from studies on IBV and TCoV. The crucial stage in the virus life cycle relies on interaction of a viral attachment protein with a particular host cell receptor and then release of the genome inside the cell through the fusion with this cell membrane. The key player in both stages is the S protein, which is therefore recognised as a determinant of tissue and cell tropism and pathogenesis. This protein consists of two subunits: the N-terminal S1 subunit which forms a globular head structure and the C-terminal S2 subunit which is a transmembrane stalk. The S1 subunit is responsible for recognising and binding the receptor cell to the host, and the S2 domain specialises in the fusion process (44) . Coronaviruses identify their various specific receptors and co-receptors, which may be proteins and sugars. The IBV has a primary affinity for the respiratory systems of chickens, but its variants could have tropism also to other organs such as kidneys, oviduct, testes, bursa of Fabricius, caecal tonsils, or the alimentary system (7). The main attachment factor for respiratory IBV is α2,3-linked sialic acid glycan, widely distributed on host tissue, and this explains why such strains can also have affinity to other organs. Additionally, the diversity of the S1 domain sequence as high as 20%-30% among different IBV variants could further contribute to the binding capacity of these viruses. It is suspected that nephropathogenic IBV strains could use a different or additional receptor for tissue binding. Studies on enteric coronaviruses (TCoV and GfCoV) also revealed their different receptor specificity, as binding of their S protein to the host glycan receptor is independent of the presence of sialic acid residues and recognises poly-LacNAcon complextype N-glycans. The difference in receptor binding observed between respiratory IBVs and enteric gammaCoVs could be concluded from the difference between their S1 coding regions which is as high as 64% (45) . The function of the S protein in avian deltaCoV seems to be analogous to IBV or TCoV, but nothing is known about their receptor specificity.