Basic structure of viruses
• All viruses have the same basic structure - they are made out of a basic coat of protein and enclose the genetic material (either DNA or RNA) inside.
• The protein coat protects the genetic material and allows the virus to get into other cells
• The genetic material is made out of a small number of genes (up to 12) which codes for all the proteins the virus makes.
Importance
• They cause some devastating human diseases and are the reason for various epidemic and
pandemics (e.g. the ebola virus)
• They cause diseases to crops and animals • They can be easily modified and might be a
source of bioterrorism in the future
HIV
• The spread of HIV is considered to be a pandemic • HIV causes the disease AIDS (acquired immune
deficiency disease virus).
• By 2006 the WHO estimates that AIDSs had killed 25 million people since when it was first
recognized as a virus in 1981
HIV transmission
• It is spread by blood, semen, vaginal fluid and breast milk
• The major ways that the virus can be spread are sex, blood transfusions, IV drug users sharing
contaminated needles, breast milk and during birth
• The heterosexual transmission rate has increased relative to the MSM (men who have sex with
How does HIV cause AIDS
• HIV infects particular types of white blood cells, particularly a type of cell known as a CD4+ T helper
cell.
• HIV infection leads to low levels of CD4+ T cells as
the HIV virus causes them to die
• When the numbers of CD4+ T cells fall below a
AIDS
• AIDS is caused by the body not having enough white blood cells to fight infection.
• About 9 out of every 10 persons with HIV will progress to AIDS after 10-15 years.
• After the diagnosis of AIDS is made, the current average survival time with antiretroviral therapy (as of 2005) is estimated to be more than 5 years. • Without antiretroviral therapy, death normally
occurs within a year.
HIV Structure and how HIV enters CD4+ T
helper cells
• Enveloped retrovirus.
• Retroviruses transcribe RNA to DNA.
• Two viral strands of RNA
found in core surrounded by protein outer coat.
• Outer envelope contains a lipid matrix within which specific viral glycoproteins are imbedded.
How the virus replicates
• Once inside the T helper cell the viral RNA is used as a copy to produce viral DNA by an enzyme
called reverse transcriptase
• The viral DNA is then incorporated into the host cell’s genome by the action of the enzyme
integrase;
The course of HIV infection
• These viruses then infect further T helper cells. These T helper cells may then be destroyed by T killer cells or when the cells lyse to release more viruses
• The reduction in T helper cells reduces immunity to other disease as T helper cells are involved in the immune response e.g. producing cytokines and activating B cells and killer T cells
• Death is caused by e.g. opportunistic disease,
Acute Phase
• Short, flu-like illness – occurs 1 to 6 weeks after infection. Mild symptoms: fever, headache,
tiredness. Some people have no symptoms at all.
• During this time there is rapid replication of the virus and loss to T helper cells
• Infected person can infect other people
Chronic (latent) phase
• The virus continues to reproduce rapidly but the numbers are kept in check by the immune
system.
• There are no symptoms but there can be an increasing tendency to suffer colds or other
infections. Dormant diseases like TB and shingles can reactivate.
Disease phase: AIDS
• The primary immune response is weakened as T helper cells CD4 cells decrease below 200/mm3 of blood
• Severe symptoms appear such as major weight loss,
dementia, cancers such as Kaposi’s sarcoma and serious opportunistic infections such as TB and cryptococcal
meningitis.
• Worldwide increase of HIV/ AIDS is the main cause of the worldwide increase in cases of TB.
Treating AIDS
• No cure but there are two types of anti-retroviral drugs that can slow down progression:
1. Reverse transcriptase inhibitors – prevent the viral RNA from making DNA for integration into the host genome.
HIV virus shows antigenic
variation
• Many different strains of HIV virus with different antigens will evolve in a patient through mutations
• Some strains will become resistant to a particular drug • Those strains that are resistant survive and reproduce
increasing in number so new drugs need to be prescribed to the patient to keep the HIV replication rate low
• HIV mutates very quickly and can replicate very quickly which is why it is particularly difficult to keep its
