Option D7: Antivirals

D.7.1 State how viruses differ from bacteria

Viruses are such small and simple structures that there is debate about whether they can be classified as living organisms in their own right. They contain only the two components protein and nucleic acid (either RNA or DNA), have no cellular structure and are only capable of reproducing inside another living cell. In these ways, they are different from bacteria with their more complex cellular structure and ability to survive and reproduce independently from other living cells

Viruses are in fact the original hijackers - they literally take over the functioning of another cell (the host cell) and use it to carry out their own reproduction. The host cell's components are used in the assembly of new viral particles and in the process the cell eventually dies, releasing thousands of viral participle into the body.

D.7.2 Describe the different ways in which antiviral drugs work

Treating viral infections is a challenge because the viruses live within cells and so cannot be easily targeted. Lacking the cell structure of bacteria, they are not attacked by antibiotics. Another problem is the speed at which they can multiply, which means that they have often spread throughout the body by the time that symptoms appear, In addition, virus particles have a tendency to mutate rapidly and this changes their susceptibility to drugs. This is why, for example, different types of flu vaccine are developed each year according to the most abundant strain of virus around

Some antivirals work by altering the cell's genetic material (DNA) so that the virus cannot use it to multiply. Others block enzyme activity within the host cell, which prevents the virus from reproducing. In this case, the progression of the disease will be halted and there will be relief from symptoms, but not that the virus is not completely eradicated.

One reasonably effective antiviral drug is amadntadine, which causes changes in the cell membrane that prevent the entry of the virus into the cell. It is therefore used as a prophylactic treatment or given before the infection has spread widely.

D.7.3 Discuss the difficulties associated with solving the AIDs problem

The condition known as AIDs, caused by the human immunodeficiency virus (HIV), was first diagnosed in humans in 1981, AIDs is characterized by a failure of the immune system, so that the body falls prey to life-threatening opportunistic infections such as pneumonia and forms of cancer. The infection has spread at an alarming rate through the global population and it is estimated that 40 million people are currently HIV positive with a likelihood of developing AIDs.

There are three main reasons why HIV is proving even more challenging than other viruses to defeat.

• The virus destroys T-helper cells, the very cells in the immune system that should be defending the body against the virus
• The virus tends to mutate very rapidly even within a patient. It is thought that there is more variation in HIV in a single patient than in influenza virus worldwide in a year. These variations mean that the virus 'escapes' the immune response, because the patient has to make a response to the new virus
• The virus often lies dormant within host cells so the immune system has nothing to respond to

Drugs to help the fight against HIV, known as antiretroviral drugs, act at different stages in the HIV lifecycle. One target is to inhibit the enzyme reverse transcriptase, as this is specific to the virus and does not affect the host cell. The drug AZT, also known as zidovudine, works in this way and was the first antiretroviral drug approved for use in AIDs treatment.