S1 gene sequence analysis of a nephropathogenic strain of avian infectious bronchitis virus in Egypt

Abstract

Background: Infectious bronchitis is highly contagious and constitutes one of the most common and difficult poultry diseases to control. IBV is endemic in probably all countries that raise chickens. It exists as dozens of serotypes/genotypes. Only a few amino acid differences in the S1 protein of vaccine and challenge strains of IBV may result in poor protection. Tropism of IBV includes the respiratory tract tissues, proventriculus and caecal tonsils of the alimentary tract, the oviduct and the kidney.

Results: Infectious bronchitis virus (IBV) strain closely related to Massachusetts (Mass) serotype was isolated from broiler chickens suffering from severe renal and respiratory distresses. The isolate was serologically identified by Dot-ELISA and further characterized by RT-PCR then genotyped using S1 gene sequence analysis. Alignment of the S1 sequence of the isolate with 16 IBV strains revealed high homology to isolates related to Mass serotype. Inoculation with the strain reproduced the disease in experimental 1-day-old chickens and resulted in 20% mortality, severe renal and moderate respiratory distresses. Marked histopathological changes in both kidney and trachea were observed in experimentally infected chickens. A protection study using the H120 live attenuated vaccine showed low protection rate in spite of high S1 sequence homology (97%). Protection based criteria were: virus re-isolation attempts from trachea, tracheal and renal histopathology as well as IBV antigens detection by immunofluorescent antibody technique in kidney sections.

Conclusion: Periodical evaluation of cross-protective capabilities of IBV vaccine(s) versus recently recovered field isolates should be performed to ensure optimum control of IBV.

Figure 1

Dot-ELISA shows positive reaction in tested (chorioallantoic membrane homogenate) and control positive sample.

Figure 2

IBV S1 gene sequence relationships expressed as a phylogenetic tree of Egypt/F/03 isolate and selected IBV reference strains.

Figure 3

Nucleotides identities of Egypt/F/03 with commonly used vaccine strains sequences. Dots indicate residues identical to Egypt/F/03. Bold letters denotes codon areas. Shaded letters denote sites of differences.

Figure 4

Amino acid identities of Egypt/F/03 with commonly used vaccine strains sequences. Dots indicate residues identical to Egypt/F/03. Potential glycosylation sites (NXS or NXT, except where X = P) are underlined. Shaded letters denote sites of differences. A:Alanine, C:Cysteine, D:Aspartic acid, E:Glutamic acid F:Pheny-lalanine, G:Glycine, H:Histidine, I:Isoleucine, K:Lysine, L:Leucine, M:Methionine, N:Asparagine, P:Proline, Q:Glutamine, R:Arginine, S:Serine, T:Threonine, V:Valine, W:Tryptophan, Y:tyrosine.

Figure 5

Trachea and kidney histopathology following experimental infection of 1-day old chickens with Egypt/F/03. Trachea and kidney stained with H & E: a. Trachea of chickens 5 d P.I with Egypt/F/03 showed hyperplasia, lymphcytic infiltration and oedema (40 ×). b. Trachea of chickens 7 d P.I with Egypt/F/03 showed diffuse lymphocytic aggregation, degeneration of the epithelium mucus, and haemorrhages (20 ×). c. Kidney of chickens 5 d P.I with Egypt/F/03 showed focal lymphocytic aggregation in the interstitium and in the glomeruli, as well as degenerative changes in tubular epithelium (40 ×). d. Kidney of chickens 7 d P.I with Egypt/F/03 showed massive renal haemorrhages and degeneration renal tubular epithelium (20 ×).