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INTRODUCTION

Coronaviruses are found in avian and mammalian species. They resemble each other in morphology and chemical structure: for example, the coronaviruses of humans and cattle are antigenically related. There is no evidence, however, that human coronaviruses can be transmitted by animals. In animals, various coronaviruses invade many different tissues and cause a variety of diseases, but in humans they are only proved to cause mild upper respiratory infections, i.e. common colds. On rare occasions, gastrointestinal coronavirus infection has been associated with outbreaks of diarrhoea in children, but these enteric viruses are not well characterized and are not discussed in this chapter.

Classification
Coronaviruses (and toroviruses) are classified together on the basis of the crown or halo-like appearance of the envelope glycoproteins, and on characteristic features of chemistry and replication. Most human coronaviruses fall into one of two serotypes: OC43-like and 229E-like.

Clinical Presentation
Coronaviruses cause acute, mild upper respiratory infection (common cold).

Structure
Spherical or pleomorphic enveloped particles containing single-stranded (positive-sense) RNA associated with a nucleoprotein within a capsid comprised of matrix protein. The envelope bears club-shaped glycoprotein projections.

Multiplication
The virus enters the host cell, and the uncoated genome is transcribed and translated. The mRNAs form a unique “nested set” sharing a common 3′ end. New virions form by budding from host cell membranes.

Pathogenesis
Transmission is usually via airborne droplets to the nasal mucosa. Virus replicates locally in cells of the ciliated epithelium, causing cell damage and inflammation.

Host Defenses
The appearance of antibody in serum and nasal secretions is followed by resolution of the infection. Immunity wanes within a year or two.

Epidemiology
Incidence peaks in the winter, taking the form of local epidemics lasting a few weeks or months. The same serotype may return to an area after several years.

Diagnosis
Colds caused by coronaviruses cannot be distinguished clinically from other colds in any one individual. Laboratory diagnosis may be made on the basis of antibody titers in paired sera. The virus is difficult to isolate. Nucleic acid hybridization tests (including PCR) are now being introduced.

Control
Treatment of common colds is symptomatic; no vaccines or specific drugs are available. Hygiene measures reduce the rate of transmission.

Clinical Manifestations
Coronaviruses invade the respiratory tract via the nose. After an incubation period of about 3 days, they cause the symptoms of a common cold, including nasal obstruction, sneezing, runny nose, and occasionally cough. The disease resolves in a few days, during which virus is shed in nasal secretions. There is some evidence that the respiratory coronaviruses can cause disease of the lower airways but it is unlikely that this is due to direct invasion. Other manifestations of disease such as multiple sclerosis have been attributed to these viruses but the evidence is not clear-cut.

Structure
Coronavirus virions are spherical to pleomorphic enveloped particles. The envelope is studded with projecting glycoproteins, and surrounds a core consisting of matrix protein enclosed within which is a single strand of positive-sense RNA (Mr 6 × 106) associated with nucleoprotein. The envelope glycoproteins are responsible for attachment to the host cell and also carry the main antigenic epitopes, particularly the epitopes recognized by neutralizing antibodies. OC43 also possesses a haemagglutin.