General morphology virus

It is a complex and highly organized entity which gives form to the virus. Subunits called protomere s aggregate to form capsomeres which in turn aggregate to form the capsid. Envelope -this is an amorphous structure composed of lipid, protein and carbohydrate which lies to the outside of the capsid. It contains a mosaic of antigens from the host and the virus.

A naked virus is one without an envelope. They arise from the envelope and are highly antigenic. Icosahedral -The protomeres aggregate in groups of five or six to form the capsomere.

In electron micrographs, capsomeres are recognized as regularly spaced rings with a central hole. The shape and dimensions of the icosahedron depends on characteristics of its protomeres. All icosahedral capsids have 12 corners each occupied by a penton capsomere and 20 triangular faces, each containing the same number of hexon capsomeres.

Icosahedral symmetry is identical to cubic symmetry. From Jawetz, R. Melnick, and E. Adelberg, Review of Medical Microbiology, 16th Edition, pp. Reproduced with permission. Helical -The protomeres are not grouped in capsomeres, but are bound to each other so as to form a ribbon-like structure. This structure folds into a helix because the protomeres are thicker at one end than at the other.

The diameter of the helical capsid is determined by characteristics of its protomeres, while its length is determined by the length of the nucleic acid it encloses. Complex -e.

This group comprises all those viruses which do not fit into either of the above two groups. Adsorption -Viruses can enter cells via phagocytosis, viropexis or adsorption.

Adsorption is the most common process and the most highly specific process. It requires the interaction of a unique protein on the surface of the virus with a highly specific receptor site on the surface of the cell. Penetration -This occurs by one or more processes. Uncoating -During this stage cellular proteolytic enzymes digest the capsid away from the nucleic acid.

This always occurs in the cytoplasm of the host cell. The period of the replication cycle between the end of the uncoating stage and maturation of new viral particles is termed the eclipse.

Thus during the eclipse stage, no complete viral particles can be viewed within the cell. For these viruses, attachment is a requirement for later penetration of the cell membrane, allowing them to complete their replication inside the cell. The receptors that viruses use are molecules that are normally found on cell surfaces and have their own physiological functions.

Viruses have simply evolved to make use of these molecules for their own replication. Overall, the shape of the virion and the presence or absence of an envelope tell us little about what disease the virus may cause or what species it might infect, but they are still useful means to begin viral classification.

Among the most complex virions known, the T4 bacteriophage, which infects the Escherichia coli bacterium, has a tail structure that the virus uses to attach to host cells and a head structure that houses its DNA. Adenovirus, a non-enveloped animal virus that causes respiratory illnesses in humans, uses glycoprotein spikes protruding from its capsomeres to attach to host cells. Non-enveloped viruses also include those that cause polio poliovirus , plantar warts papillomavirus , and hepatitis A hepatitis A virus.

Enveloped virions like HIV consist of nucleic acid and capsid proteins surrounded by a phospholipid bilayer envelope and its associated proteins. Glycoproteins embedded in the viral envelope are used to attach to host cells.

Other envelope proteins include the matrix proteins that stabilize the envelope and often play a role in the assembly of progeny virions. Chicken pox, influenza, and mumps are examples of diseases caused by viruses with envelopes. Because of the fragility of the envelope, non-enveloped viruses are more resistant to changes in temperature, pH, and some disinfectants than are enveloped viruses.

The virus core contains the genome or total genetic content of the virus. Viral genomes tend to be small, containing only those genes that encode proteins that the virus cannot obtain from the host cell. This genetic material may be single- or double-stranded. It may also be linear or circular. Because coronavirus infections are common, many individuals have specific antibodies in their nasal secretions, and these antibodies can protect against infection. Most of these antibodies are directed against the surface projections and neutralize the infectivity of the virus.

Cell-mediated immunity and allergy have been little studied, but may play a role. The epidemiology of coronavirus colds has been little studied.

Waves of infection pass through communities during the winter months, and often cause small outbreaks in families, schools, etc. Immunity does not persist, and subjects may be re-infected, sometimes within a year. The pattern thus differs from that of rhinovirus infections, which peak in the fall and spring and generally elicit long-lasting immunity.

About one in five colds is due to coronaviruses. The rate of transmission of coronavirus infections has not been studied in detail.

The virus is usually transmitted via inhalation of contaminated droplets, but it may also be transmitted by the hands to the mucosa of the nose or eyes. There is no reliable clinical method to distinguish coronavirus colds from colds caused by rhinoviruses or less common agents. For research purposes, virus can be cultured from nasal swabs or washings by inoculating organ cultures of human fetal or nasal tracheal epithelium.

The virus in these cultures is detected by electron microscopy or other methods. The most useful method for laboratory diagnosis is to collect paired sera from the acute and convalescent phases of the disease and to test by ELISA for a rise in antibodies against OC43 and E. Complement fixation tests are insensitive; other tests are inconvenient and can be used only for one serotype. Direct hybridization and polymerase chain reaction tests for viral nucleic acid have been developed and, particularly with the latter, are the most sensitive assays currently available for detecting virus.

Although antiviral therapy has been attempted, the treatment of coronavirus colds remains symptomatic. The likelihood of transmission can be reduced by practising hygienic measures.

Vaccines are not currently available. Turn recording back on. National Center for Biotechnology Information , U. Show details Baron S, editor. Search term. Chapter 60 Coronaviruses David A. General Concepts 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. 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.

Multiplication The virus enters the host cell, and the uncoated genome is transcribed and translated. Pathogenesis Transmission is usually via airborne droplets to the nasal mucosa. Host Defenses The appearance of antibody in serum and nasal secretions is followed by resolution of the infection. Epidemiology Incidence peaks in the winter, taking the form of local epidemics lasting a few weeks or months.

Diagnosis Colds caused by coronaviruses cannot be distinguished clinically from other colds in any one individual. Control Treatment of common colds is symptomatic; no vaccines or specific drugs are available. Introduction Coronaviruses are found in avian and mammalian species. Clinical Manifestations Coronaviruses invade the respiratory tract via the nose. Figure Clinical manifestations and pathogenesis of coronavirus infections.

Figure Immunopathogenesis of coronavirus infections. Structure Coronavirus virions are spherical to pleomorphic enveloped particles Fig. Figure Electron micrograph showing human coronavirus E. Classification and Antigenic Types The coronaviruses were originally grouped into the family Coronaviridae on the basis of the crown or halo-like appearance given by the glycoprotein-studded envelope on electron microscopy.

Multiplication It is thought that human coronaviruses enter cells, predominantly, by specific receptors. Pathogenesis Studies in both organ cultures and human volunteers show that coronaviruses are extremely fastidious and grow only in differentiated respiratory epithelial cells. Host Defenses Although mucociliary activity is designed to clear the airways of particulate material, coronaviruses can successfully infect the superficial cells of the ciliated epithelium.

Figure Seasonal incidence of coronavirus infections. Epidemiology The epidemiology of coronavirus colds has been little studied. Diagnosis There is no reliable clinical method to distinguish coronavirus colds from colds caused by rhinoviruses or less common agents.