Birnaviridae

Birnavirus: Electron Micrographs of Animal Viruses, Veterinary Sciences, The Queen's University of Belfast, kindly provided by Prof Stewart McNulty Copyright notice: This notice must accompany any copy of the images. The images must not be used for commercial purpose without the consent of the copyright owners. The images are not in the public domain. The images can be freely used for educational purposes.

Baltimore Classification

Higher order taxa

Viruses; dsRNA viruses; Birnaviridae

Genera

Aquabirnavirus, Avibirnavirus, Entomobirnavirus

Description and Significance

Birnaviruses infect animals: vertebrates, insects, molluscs and crustaceans. They are bi-RNA viruses and have double stranded RNA genomes in two components.

Genome Structure

The genome of Binyaviridae is segmented and consists of linear double-stranded RNA. Minor species of non-genomic nucleic acid are also found in virions. The complete genome is 5880-6400 nucleotides long. Segment A has been fully sequenced and the complete sequence is 3100-3200 nucleotides long while segment B is 2750-2850 nucleotides long. The genome guanine+cytosine content is 45.5-54.5 %. The 5'-end of the genome has a cap on both segments genome-linked protein (VPg). The multipartite genome is found in one type of particle only. (source ICTV dB Descriptions)

Virion Structure of a Birnaviridae

The virions of a birnaviridae have a simple construction. The virions are not enveloped and consist of a capsid. The capsid is round and exhibits icosahedral symmetry (T=13). The capsid shells of the virions are composed of a single layer. The capsids appear hexagonal in outline. The capsomer arrangement is clearly visible and there are 132 capsomers in the capsid. There are no surface projections and the capsids all have the same appearance. (source ICTV dB Descriptions)

Reproduction Cycle of a Birnaviridae in a Host Cell

The replication of birnaviruses takes place in the cytoplasm. The dsRNA serves as a template for the production of mRNA (+) and progeny genomes. (source: IVIS)

Viral Ecology & Pathology

The avibirnavirus is the causative agent of Infectious Bursal Disease. The route of infection is mainly oral, but the conjunctiva and respiratory tract can also play a role in infection, The virus is found in Kuppfer cells of the liver shortly after initial infection. The virus can also be found in macrophages and lymphoid cells of the jejunum, duodenum and cecum. The cells of the bursa of Fabricius are infected withing 12 hours. The thymus, the Harderian gland and spleen are infected after viremia. The immune response is impaired because of the depletion of the bursa.