The prevention and treatment of viral respiratory disorders
A briefing from the Board of Science
September 2007
What is influenza?
Influenza is a highly contagious viral infection which affects epithelial cells in the upper respiratory system, typically those of the nose, throat and bronchi. It can cause fever, muscle aches, and more severe versions of the symptoms typical of the common cold (see the section on symptom comparison). Unlike other common viruses such as chickenpox or measles, there is not just one, but three types of influenza virus: A, B and C. Influenza A and C can infect multiple species, while influenza B almost exclusively infects humans. [see reference 12] Type A viruses are the most virulent human pathogens of the three. Influenza A is believed to have caused about 74 per cent of influenza since 1990 (although this has varied from 20-97% depending on the season). [see reference 13] The influenza A virus can be subdivided into different serotypes based on two of the proteins found on the virus’ outer coating (see influenza A structure). Influenza type C infection usually causes either a very mild respiratory illness or no symptoms at all; it does not cause epidemics and does not have the severe public health impact of types A and B. While influenza can be contracted at any time of the year, in the Northern hemisphere, the most common time is between December and March. [see reference 14]
Influenza A structure
The following applies for influenza A viruses, although other strains are very similar in structure.
The influenza A virus particle is 80-120nm in diameter and usually roughly spherical, although filamentous forms can occur. Influenza A contains eight pieces of segmented RNA, which encode 11 proteins. The best-characterised of these viral proteins are haemagglutinin (H) and neuraminidase (N), two large glycoproteins found on the outside of the viral particles. N is involved in the release of progeny viruses from infected cells, and H mediates binding of the virus to target cells and entry of the viral RNA to the target cell. The H and N proteins are targets for antiviral drugs, and are also recognised by antibodies. The responses of antibodies to these proteins are used to classify the different serotypes of influenza A viruses, hence the H and N in viral nomenclature (eg H5N1 – ‘avian flu’).[see reference 15]
Figure 1 Influenza Virion
Source: Chotani RA (2006) The Impact of pandemic influenza on public health. [see reference 16]
The influenza virus is able to mutate, meaning that contracting influenza once does not impact immunity.[see reference 17] This continual variation of the influenza virus means that its eradication is not thought possible. The surface of the influenza virus can mutate antigenically from year to year. This change occurs either as antigenic drift or antigenic shift:
- Antigenic drift, which is the more common, arises as a result of gene mutations that are selected for in response to immune pressure from hosts.
- Antigenic shift occurs when two different subtypes of influenza A, one from humans and the other from a different host species, co-infect a single host. Within the co-infected host, the viral genes mix to form a new subtype of the virus that has some of the properties of both parental strains. Such a subtype may be infectious in humans, having a sufficiently new surface, to which humans lack an immunity. Such a subtype is likely to cause widespread infection, possibly reaching pandemic levels, because resistance in the population would be low. [see reference 13]
Viruses need to be inside living cells in order to multiply; this can occur in any living cell within an organism. In the case of respiratory disorders this is typically the nose of humans, although they are also common in related higher primates such as chimpanzees. [see reference 18] The influenza virus can, depending on the subtype, infect pigs, horses and birds among others. [see reference 16] While they need a host in order to multiply, viruses can remain viable outside cells for differing amounts of time. They can then be transported from these surfaces back to a living cell where they can resume multiplying. Viruses such as those that cause influenza and the common cold can be spread in a number of ways. They are usually transmitted via droplets and aerosols released when an individual coughs or sneezes. These can be breathed in directly or spread via contact with water, food or objects such as clothing or surfaces where the viruses have settled, including hands, door knobs, handrails etc. Individuals are contagious for one to four days before, and approximately five days after the first onset of symptoms. [see reference 16] The problem of contagion is intensified by the fact that approximately 50 per cent of people who are contagious do not present any symptoms. [see reference 16]
The length of time that a virus can survive outside a living cell is dependent on many factors, such as the type of virus, type of surface, relative humidity, moisture content, temperature, and the presence of
antiviral, chemical or biological agents. Research into the lifespan of viruses, including respiratory viruses, such as those that cause the common cold and influenza, on different surfaces has been carried out in detail. [see references 19 and 20] A study by Bean et al [see reference 21] has reported that the influenza virus can survive on a hard non-porous surface such as plastic or stainless steel for 24-48 hours and on materials such as clothes, paper and tissues for 8-12 hours. [a] On hands, it is reported that the influenza virus can only survive for approximately 10 minutes, although this is more than enough time to allow transfer to other surfaces, such as cutlery, door handles, ATMs, hand rails and keyboards, to facilitate the spread of infection.
Research has shown that viruses can survive indefinitely without a host if conditions are optimal. In frozen Siberian ice lakes it has been found that the influenza viruses can survive for decades. Many other ancient viruses and bacteria have also been found in these lakes. This is of concern because as the ice melts, with the onset of global warming, it is feared that these micro-organisms could resurface and cause pandemic outbreaks due to a lack of resistance. [see reference 22] The Health Protection Agency (HPA) publishes an online weekly summary of influenza, [see reference 23] throughout the influenza season (October to May), which includes a summary of the clinical activity of influenza and a number of respiratory disorders in the UK.[b] These reports also provide analysis of the clinical and virological indicators that affect influenza activity in the UK.
Additional information about the influenza virus can be found online at BBC health, [see reference 24] the World Health Organisation (WHO), [see reference 25] Medline Plus, [see reference 26] NHS Direct Online [see reference 27] and the European Influenza Surveillance Scheme (EISS). [see reference 28]
[a] These survival rates were determined where the temperature was 28șC and there was a humidity level of 35-40 per cent.
[b] Data are compiled from the Royal College of General Practitioners (RCGP), the National Public Health Service for Wales (NPHS), Health Protection Scotland (HPS), Communicable Disease Surveillance Centre Northern Ireland (CSDC NI), the Office for National Statistics (ONS) and NHS Direct.