BOD - Revision - Michaelmas - Smith - Viruses Lecture 17. INFLUENZA AND HEPATITIS VIRUSES Influenza Virus 1. Transmission · Enveloped viruses like influenza are less stable than non-enveloped ones. The lipid membrane is fragile and can be easily disrupted. The envelope contains the virus attachments for proteins such as HA, NA and M2. Therefore, the virus is transmitted via close contact · The virus is transmitted via aerosol when an infected person coughs or sneezesand inhaled . The virions initiate infection in the respiratory tract . Transmission is highly seasonal with epidemics occurring the winter months when people are closer together . The graph shows the highly seasonal incidence of influenza virus from 1989/90 to 2004/5. " The lower line is the GP consultation rate per 100,000 population, and the upper line is the number of deaths. " Note (i) the highly seasonal incidence of the disease and (ii) there are 10,000 and 15,000 deaths in an average year from influenza. 2. Why successful . Antigenic variation allows the virus to escape existing neutralising antibodies 1. Firstly, antigenic drift allows the gradual accumulation of mutations within the viral genome HA is needed for influenza binding to cells and therefore is the target for neutralising antibodies * Natural selection yields the emergence of amino acid mutations in the HA to produce a new strain. These mutations prevent the existing antibodies binding to and neutralising it For example, the Hft subtype that appeared in 1968 was very different to Hft seen in previous strains. In the diagram the black dots show the amino acid changes that occurred between 1968 and 200ft showing the vast change on the HA surface 2. Secondly, antigenic shift can cause complete change in the structure of viral proteins * A more radical change can occur by the acquisition of a completely new HA from another influenza virus # During an infection with 2 influenza viruses for example one human and one avian, both sets of RNA segments are replicated. During packaging of the copied segments into new virions reassortment can occur. * The new virion can get RNA segments from both the human and avian influenza
# A dangerous combination can occur if the human virus obtains the avian HA gene because existing antibodies to the human HA will be of no value. Thus the virus can spread and replicate unchecked * This can give rise to pandemics which have occurred several times during the last 100 years # Most devasting was 1918/19 where 50 million people died # 1957 both the HA and NA changed # 1968 only the HA subtype changed from the virus circulating previously. This showed that antibody to HA is critical in preventing influenza virus infection and antibody to NA is less important. 1977 saw the re-emergence of an H1N1 virus that was very similar to a virus that had been circulating in the 19ft0s. # 2009 saw the emergence of a new H1N1 virus that was derived from swine in