Genetic relationships of infectious bronchitis virus isolates from Mississippi broilers.

A 582-base pair segment located in the nucleocapsid protein terminal part of the S1 gene of 26 Arkansas (Ark)-type infectious bronchitis virus (IBV) isolates from Mississippi broilers was amplified and sequenced. Reverse transcription-polymerase chain reaction and cycle sequencing techniques were used to elucidate the genetic and deduced amino acid relationships among the isolates. Analysis suggested that the nucleotide and amino acid sequences of the isolates were highly conserved, with greater than 99.4% nucleotide and 98.4% amino acid homology among the Mississippi isolates. The Mississippi isolates had less than 2.3% nucleotide and 5.2% amino acid variation when compared with the vaccine strain Ark DPI and less than 3.0% nucleotide and 5.2% amino acid variation when compared with the Ark-type Georgia variant. This study suggests that the 26 IBV isolates that are genetically closely related to the vaccine strain may be derived from the vaccine strain by point mutation. The results also indicate that within a 1-yr period Ark-type IBV in Mississippi was spread with little or no change in its genetic sequence. The 21 identical isolates appeared in different farms and companies, suggesting that poor biosecurity was practiced and contributed to the spreading of the disease.