Virus-Host Cell Interactions

VIRUS–HOST CELL INTERACTIONS

Viruses need to interact with a host cell as they cannot reproduce on their own. They have no metabolism and cannot synthesize their own proteins, lipids or nucleic acids. Thus viruses can be considered as true intracellular parasites that grow within living cells and use their energy and synthetic machinery to produce viral components. The production and excision of viruses from the host cell will result in cell death, although this might not be immediate. Following the replication of one virus within the host cell, hundreds of new viruses (virus progeny or virions) can be released and infect adjacent cells (within a tissue). The propagation from one infected cell to new cells, and the subsequent destruction of tissue or cells, provides signs of the viral disease.

On the basis of host specificity, three major viral groups can be distinguished: (1) viruses of bacteria and blue-green algae, (2) plant viruses and (3) animal (including insect) viruses. Viruses are usually very specific and rarely cross species barriers, although there are some exceptions, such as rabies and influenza that can cause diseases in both animals and humans. Viruses can also be asymptomatic in certain hosts where they do not cause an infection; the host becomes a reservoir and can transmit the virus to a susceptible recipient (e.g. transmission of yellow fever to humans by mosquitoes).

Viruses can interact with the host cell in five different ways:

·        multiplication of the virus and destruction of the host cell upon release of the viral progeny.

·        multiplication of the virus and release of the virions without the immediate destruction of the host cell

·        survival of the virus in a latent stage without noticeable changes to the infected cell.

·        survival of the infected cell in a dramatically altered or transformed state (e.g. transformation of a normal cell to one having the properties of a cancerous cell).

·        incorporation of the viral nucleic acid in the host cell genome without noticeable changes to the infected cell.

The interaction with the host cell will vary between viruses, but will generally follow one of these five routes.

There is a great diversity in viral infections and viral diseases. Many viral infections are asymptomatic or ‘silent’ whereby the virus replicates within the host but does not produce symptoms of a disease. Other common infections produce some mild symptoms, such as a low  grade fever and a ‘runny nose’. This is the case of the common cold, caused by rhinoviruses, from which patients make a full recovery within a few days. At the other end of the spectrum, some viruses kill their host very quickly following infection, as in the case of haemorrhagic viruses such as the Ebola virus. On other occasions a range of symptoms can be observed in different hosts. This has been the case recently with ‘swine flu’, which produced a range of symptoms, from a slight fever to full influenza including a high fever, vomiting, and dizziness. Other problematic viruses might not cause immediate symptoms, but following the systematic destruction of host cells will lead to an incurable disease; e.g. HIV and oncogenic (tumour) viruses.