Molecular epizootiology of infectious bronchitis virus in
Sweden indicating the involvement of a vaccine strain.
Farsang A, Ros C, Renstrom LH, Baule C, Soos T, Belak S.
Institute for Veterinary Medicinal Products, H-1107, Budapest, Szallas utca 8,
Hungary.
To improve the detection and molecular identification of infectious bronchitis
virus (avian coronavirus), two reverse transcriptase-polymerase chain reaction
(PCR) assays were developed. As 'diagnostic#10; PCR', a set of consensus nested
primers was selected from highly conserved stretches of the nucleocapsid (N)
gene. As 'phylogeny' PCR, a fragment of the spike protein gene (S1) was
amplified and the PCR products were directly sequenced. To study the
phylogenetic relationships of the viruses from various outbreaks, studies of
molecular epizootiology were performed in Sweden, a Nordic region, where the
occurrence of natural cases of the disease is relatively low and the occasional
use of live vaccine(s) is well recorded and monitored. The disease appeared in
the region in 1994, associated with production problems among layers of various
ages. During outbreaks in 1995 and 1997, both layers and broilers were affected.
To reduce losses, a live attenuated vaccine has been applied since 1997. By
examining 12 cases between 1994 and 1998, molecular epizootiology revealed that,
before 1997, the viruses had gene sequences very similar to strains of the
Massachusetts serotype. However, comparative sequence analysis of the S1 gene
revealed that the identity was not 100% to any of the strains of this serotype
that we analysed. A virus related to the Dutch-type strain, D274, was also
identified on one farm. Surprisingly, from 1997, the year that vaccination
commenced with a live Massachusetts serotype vaccine, the majority of viruses
detected had S1 sequences identical to the live Massachusetts vaccine strain.
This genetic relation to the vaccine virus was also confirmed by N gene sequence
analysis. The studies of molecular epizootiology reveal a strong probability
that the vaccination had lead to the spread of the vaccine virus, causing
various disease manifestations and a confusing epizootiological situation in the
poultry population.
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