Flu is often used to mean an infection with an influenza virus but in fact the word ‘flu’ describes the symptoms that may be caused by a range of respiratory viruses, including influenza. Flu-like illness presents with symptoms of fever, cough, malaise, muscle aches and may also be associated with a blocked nose, sore throat, headache and a runny nose.1 In adults the most common cause of flu is infection with the influenza virus, as adults unlike children, do not tend to suffer from fever when infected with common cold viruses. There are three types of influenza viruses; A, B and C, but it is influenza A that causes the seasonal influenza that occurs every winter and the epidemics and pandemics of influenza that move around the world infecting millions.
Infection with the influenza virus can cause a wide range of disease severity from mild common cold symptoms through to severe and life-threatening lower respiratory tract infection.
Some patients infected with the virus will show no symptoms whatsoever. The incidence of flu varies greatly from year to year depending on the viruses that are circulating in the community, and the weather. Infection with influenza viruses may only account for a quarter of patients suffering from flu-like illness and other viruses such as respiratory syncytial virus (RSV) may account for a similar number of flu cases in both adults and children.2
Seasonal flu occurs every winter. Some patients may wonder how they can be at risk of flu when they have had it the previous winter and consider themselves ‘immune’. However, the influenza A and B viruses that cause seasonal flu are able to adapt and gradually change or ‘drift’ their structure so that they are unrecognisable to the immune system and can infect again and again each year.
Sometimes influenza A viruses can undergo a more dramatic change in structure called a ‘shift’. These new virus strains often originate from the crowded cities of Asia where they may be transmitted between animals such as ducks, chickens and pigs – hence the terms ‘bird flu’ and ‘swine flu’. The busy marketplaces of Asian cities where animals are often slaughtered openly can become a massive reservoir of infection. The local population will not have developed immunity to the new virus strain and infection can spread rapidly. This can result in pandemic influenza with large numbers of the population falling ill at the same time which then inflicts a great strain on the health service, especially when health care staff are just as likely to be infected as the general population.
How to diagnose influenza
Two criteria are needed to be reasonably confident of a diagnosis of influenza. Firstly, the symptoms of fever and cough occurring together, and secondly, the confirmation by the local public health laboratories that influenza is circulating in the community. At times when influenza has been confirmed to be circulating, flu caused by infection with the influenza virus will be the diagnosis in eight out of 10 patients who present with both fever and cough.3,4
Public health laboratories monitor for the presence of influenza in the population and will produce bulletins to advise when influenza is circulating. For diagnosis of influenza it is essential that healthcare professionals are aware of this information. Without the confirmation from public health about influenza in the community the diagnosis of influenza is very uncertain, as flu-like illness with cough and fever can be caused by other common respiratory viruses, such as RSV.
Patients at risk with influenza
Most people who get flu have a mild illness and do not need any medical care, and recover in less than two weeks. However, the elderly and those with chronic respiratory disease or weakened immune systems can suffer from serious complications if infected with influenza. Those at risk are shown in the Table 1 and are the patients targeted by the annual NHS flu vaccination programme.
Patients must be vaccinated annually as the changes in structure of the viruses from year to year mean that the previous vaccination will not protect against the newer strains of virus. The World Health Organisation (WHO) recommends which strains of influenza to include in the vaccine each year.
Role of vaccination in preventing influenza
The WHO recommends three strains of influenza to be included in the annual vaccine, two A strains and a B strain. This recommendation is based on the common strains of influenza circulating in the previous flu season. In general the WHO recommendations protect against the common types of influenza but they may not protect against all types of influenza and any new types that may emerge while the vaccine is in production. Vaccination of those in the ‘at risk’ group is vital as it will protect this group against serious disease caused by the influenza types in the vaccine and these are usually the common strains. Some patients will complain that they had the vaccine and still suffered from flu. This is quite common as flu-like illness can be caused by many viruses, but the importance of vaccination is that it protects the patient against the most likely influenza infections that are circulating during the winter period.
Influenza, like the common cold, spreads rapidly in crowded areas such as schools and nurseries and the NHS is now recommending that the influenza vaccine formulated as a nasal spray be offered to healthy children aged two to four and children in school years one and two, and soon all children ages between two-16 will be offered annual vaccinations. The practice of vaccinating healthy children is routine in the USA and has been shown to not only protect the vaccinated children but also to protect the community at large by slowing the spread of influenza through the population.2,6
Treatment of influenza
For the vast majority of people influenza is little more than a nuisance and can be safely self-managed at home. Otherwise healthy adults and children who are not in the at-risk categories above can be advised to take rest, fluids and if necessary, over the counter analgesics and simple linctus cough syrups. Reassurance such as symptoms are likely to settle within two weeks can be offered.
For those patients at risk the best treatment is prevention by vaccination, but in some cases if the patient presents within 36-48 hours of symptom onset it may be possible to prescribe antiviral medicines that work solely against influenza, such as oral Oseltamivir or inhaled Zanamivir.
How does influenza spread?
Influenza is mainly spread in droplets of mucus coughed and sneezed by infected persons but it can also be spread on contaminated surfaces such as door handles and finger to nose contact. The incubation period is around 48 hours and the disease then typically has a sudden onset with tiredness, fever and cough. When influenza is circulating in the community it is very difficult to avoid exposure to the disease, but for those patients at risk, if possible, they should avoid crowded places such as schools and cinemas, and wash their hands after being in any public areas. Soap and water is all that is needed for hand cleansing and hand wipes are also useful when travelling.
The NHS has, in pandemic scares, asked for to population to ‘catch it, bin it and kill it’ meaning to catch sneezes and coughs in paper tissues and then dispose of them in the nearest bin and this is good advice to slow and prevent the spread of influenza in the community. Patients with flu should be advised if possible to avoid contact with those patients who are at risk of complications, for example elderly relatives, until symptoms have resolved.
Flu should be suspected in patients presenting with a fever and cough at times when the influenza virus is known to be circulating in the community. Flu vaccination offers protection against the common strains and is available on the NHS to both patients who are at higher risk of complications, and healthcare workers who are likely to be in contact with infected individuals. The influenza virus alters itself from year to year and a previous episode of flu does not guarantee total protection from further infection.
Healthy patients who are infected with flu can be managed at home with reassurance and supportive measures. Patients who are at a higher risk of serious infection and complications should be identified and vaccinated at the beginning of the flu season. Where flu is suspected in a vulnerable patient a clinical review would be advised to screen for complications and possibly prescribe antiviral medications if appropriate.
1. Eccles R. Understanding the symptoms of the common cold and influenza. The Lancet Infectious Diseases 2005;5:718-725.
2. Zambon MC, Stockton JD, Clewley JP, Fleming DM. Contribution of influenza and respiratory syncytial virus to community cases of influenza-like illness: an observational study. The Lancet 2001;358(9291):1410-1416.
3. Monto AS, Gravenstein S, Elliott M, Colopy M, Schweinle J. Clinical signs and symptoms predicting influenza infection. Archive of Internal Medicine 2000;160:3243-3247.
4. Zambon M, Hays J, Webster A, Newman R, Keene O. Diagnosis of influenza in the community: relationship of clinical diagnosis to confirmed virological, serologic, or molecular detection of influenza. Archive of Internal Medicine 2001;16:2116-2122.
5. Piedra PA, Gaglani MJ, Kozinetz CA, Herschler G, Riggs M, Griffith M, et al. Herd immunity in adults against influenza-related illnesses with use of the trivalent-live attenuated influenza vaccine (CAIV-T) in children. Vaccine 2005;23:1540-1548.
6. Gaglani MJ. Editorial commentary: school-located influenza vaccination: why worth the effort? Clinical Infectious Diseases 2014;59:333-335.
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