Coronavirus

Baltimore Classification

Higher order taxa

Virus; Coronaviridae (Family); Coronavirus

Species

Human coronavirus- SARS, Bovine coronavirus, Canine coronavirus (examples)

Description and Significance

The coronavirus was first isolated from chickens in 1937. The virus infects not only man but cattle, pigs, rodents, cats, dogs and birds, causing respiratory and enteric diseases. Coronaviruses are the largest positive strand RNA viruses. Coronaviruses cause about one third of common colds and the newly recognized severe acute respiratory syndrome (SARS), making them extremely important. (source: Microbiology@Leicester: Coronaviruses)

Genome Structure

The genome of the coronavirus is not segmented and contains a single molecule of linear positive-sense, single-stranded RNA. The complete genome is 27600-31300 nucleotides long. The 5'-end of the genome has a methylated nucleotide cap while the 3'-terminus has a poly (A) tract. (source: ICTV dB Descriptions)

Virion Structure of a Coronavirus

The virions of a coronavirus consist of an envelope and a nucleocapsid that is enveloped. Virions are spherical to pleomorphic and the surface projections are evenly dispersed. They are distinctively clun-shaped peplomers that are spaced widely apart, covering evenly the surface, forming a prominent fringe comprised of SA proteins responsible for attachment to cells, hemagglutination and membrane fusion and hemagglutinin esterase proteins (HE) which form short projections in some species. Different types of proteins makeup the 12-24 nm long surface projections. The capsid is elongated with helical symmetry and is 9nm, or 11-13 nm wide. (source: ICTV dB Descriptions)

Reproductive Cycle of a Coronavirus in a Host Cell

Relatively very little is known of human coronaviruses because they do not grow in cultured cells but two strains, 229E and OC43, grow in some cell lines and have been used as a model. The replication of coronavirus is slow compared to other enveloped viruses.

The virus enters the cell by endocytosis and membrane fusion. This process is probably mediated by E2. The replication of the virus occurs in the cytoplasm. Initially, the 5' 20kb of the (+)sense genome is translated to produce a viral polymerase. This polymerase then produces a full-length (-)sense strand but very little is known of this step of the process. The (-)sense strand is used as a template to produce mRNA as a 'nester set' of transcripts, all within an identical 5' non-translated leader sequence of 72nt and coincident 3' polyadenylated ends.

Each mRNA is monocistronic and the genes at the 5' end are translated from the longest mRNA and so on. There unusual cytoplasmic structures are produced by polymerase during transcription and not by splicing. There is a repeated intergenic sequence-UCUAAAC- between each of the genes which interacts with the transcriptase and cellular factors to modify the leader sequence after transcription (splice) into the start of each ORF.

Each mRNA is monocistronic, the genes at the 5' end being translated from the longest mRNA & so on. These unusual cytoplasmic structures are produced not by splicing (post-transcriptional modification) but by the polymerase during transcription. Between each of the genes there is a repeated intergenic sequence - UCUAAAC - which interacts with the transcriptase plus cellular factors to 'splice' the leader sequence onto the start of each ORF.

The assembly occurs by budding into golgi apparatus and the particles are transported to the surface of the cell by the secretory nature of this organelle and then released. (From Microbiology@Leicester: Coronaviruses)

Viral Ecology & Pathology

Anti-coronaravirus antibodies are present in most people but reinfection is common. This indicates that there are many circulating serotypes of of the virus in the human population. So far, animal reservoirs have not been found for those viruses that infect humans.

Cornavirus causes cold more often in the winter, as is typical with most respiratory infections, because of closer contact. major outbreaks occur every few years with a cycle that depends on the type of virus that is involved.

The symptoms of coronavirus colds are similar to those of rhinovirus colds which include runny nose, sore throat, cough, headache, fever and chills. The incubation period is about 3 days. The immune response of most patients limits the viral spread but this immunity is short-lived. Although it varies considerably between patients, symptoms may last about a week. The patient might shed infectious virus even when there are no apparent symptoms.

Coronaviruses are rather unstable and transmission of the virus is by transfer of nasal secretions caused by sneezes, such as in aerosols. Viruses that infect epithelial cells of the enteric tract cause diarrhea. This is most common in young animals where the infection can be fatal. Although local, coronary infections can spread as well.

Most infections cause a mild, self-limited disease but there may be rare neurological compliations as well. SARS is a form of viral pneumonia where infection encompasses the lower respiratory tract. (From University of South Carolina: Department of Pathology, Microbiology and Immunology)