Influenza

Influenza
Influenza viruses are unique among the respiratory viruses with respect to their variability and impact on the general population. 'Spanish Flu' killed more people in the 1918-1919 world-wide pandemic than did the First World War. The most recent large epidemic in the United Kingdom was in 1989-1990, when an estimated 26,000 people died in association with influenza. Between epidemic periods influenza circulates annually within the population, and its spread across the country is monitored by clinical surveillance schemes including sentinel general practitioners, emergency bed admissions and medical officers of schools. Nevertheless, monitoring the impact and control of the virus remains difficult due to its unpredictable behaviour.

The influenza viruses

There are three types of influenza virus, A, B and C. Influenza A viruses are found in humans and animals, whereas B and C are found only in man. Influenza A virus possesses two surface proteins, haemagglutinin (H) and neuraminidase (N), which are used to classify the virus into subtypes (e.g. H1N1 etc.). The natural reservoir for influenza A is considered to be aquatic birds, as all influenza A subtypes are found in these birds, whereas only certain subtypes are found in mammals. These two surface proteins may undergo change over time. Minor changes are termed antigenic drift, while major changes in one or both of the surface proteins, is called antigenic shift. The importance of these changes is that antibodies in the blood, which are responsible for protection against repeated infection with the virus, are less effective if the virus surface protein changes; the greater the change, the less effective is the antibody. Only influenza A can undergo this dramatic variation in which a virus of a new subtype can suddenly appear in the human population; influenza B and C viruses appear to be more stable than influenza A. It is this antigenic shift that can be the catalyst for a nation-wide epidemic, or more seriously a world-wide pandemic of influenza, such as occurred in 1918, 1957 and 1968 (Spanish, Asian and Hong Kong Flu respectively). The latest example of an emerging new subtype is the H5N1 'bird flu', first isolated in Hong Kong in May 1997, which previously had only been found in birds and was not associated with disease in humans.

Transmission of the illness

Influenza viruses spread from person to person by tiny droplets produced by coughing and sneezing. The initial site of infection is the lining of the respiratory tract, and the infection has a short incubation period of up to five days. The virus is shed in respiratory secretions starting about one day before the onset of the illness and lasting for about three to five days.

The influenza syndrome

The classic symptoms of influenza include symptoms of fever, malaise (a feeling of being generally unwell), headache, aches and pains in the muscles and joints, and a characteristic dry cough and sore throat. The acute illness usually lasts for three to five days but recovery may be slow, and cough and tiredness may persist for two to four weeks post infection. Complications may occur in groups of patients who are particularly at risk (e.g. those with underlying lung disease or those with defective immune systems), and usually affect the lungs and the heart. Upper and lower respiratory tract infections are common and subsequent invasion of the lungs by bacteria may result in the development of pneumonia. Deaths associated with influenza are unusual, but there is evidence that during an epidemic many additional deaths occur which are not directly attributable to influenza, but are related to it.

A serious complication, Reye's syndrome, is occasionally seen in children who develop an abnormality of brain function and liver damage, with vomiting, drowsiness and sometimes convulsions. This is a rare condition (approximately one case per million children aged 16 or under) and is believed to be associated with high doses of aspirin, which should not be given to children to relieve the symptoms of influenza. However, children may have paracetamol.

Immunity

Infection with influenza produces an immune response with the production of antibodies in the blood which neutralise the virus. If the person encounters the same influenza virus again while the antibodies are still present, the body is protected and the person should not develop the infection. However, these antibodies will not protect individuals against an influenza virus in which the surface proteins have undergone a significant change since the previous infection.

Prevention

Current influenza vaccines contain either inactivated virus, treated so that it cannot cause infection, or virus components. Recent vaccines contain a mixture of two influenza A subtypes, H3N2 and H1N1, and one influenza B virus. The vaccines are very effective and have been shown to reduce hospital admissions for pneumonia. All viruses used in vaccine production are currently grown in chicken eggs, and experimental vaccines made in mammalian cells are likely to be the next development in vaccine production.

Clearly an effective vaccine must be of the same type which is thought likely to be common in the following influenza season. Changes in the virus due to antigenic drift or antigenic shift mean that the vaccine must also change to cover the prevailing strains of the virus; last year's vaccine may not protect against this year's virus. A group of experts advises national health authorities and pharmaceutical companies in the selection of virus strains that should be used to produce vaccines. The composition of the influenza vaccine for the next season is co-ordinated by the World Health Organisation. However, this recommendation is based on evidence from the previous season and cannot, therefore, be totally accurate. The ability of influenza continually to undergo antigenic change ensures that there is always a possibility that new variants not covered by the vaccine will appear, which may give rise to serious epidemics. This means that making a completely accurate prediction of likely influenza activity for the oncoming season is very difficult. It should also be remembered that during the annual winter influenza season, other 'flu-like' viruses also circulate, against which influenza vaccine offers no protection.

Influenza vaccine is currently recommended for certain groups of patients including this with chronic heart or lung disease (including asthma), chronic renal failure, diabetes and this with deficient immune systems, including patients who have lost their spleen. It is also recommended for residents of nursing and residential homes and similar long-stay facilities. The vaccine is not recommended for routine use in healthy children and adults, including health care workers, but the final decision rests with the patients' doctors. The vaccine should not be given to people who are allergic to egg products.

Influenza A infections can be prevented to a certain extent with drugs amantadine and rimantadine which have been found to be 70-90% effective in preventing illness. The combination of vaccination with prophylaxis using the above drugs offers the highest level of protection for high-risk patients.

Treatment

Amantadine is of proven effectiveness in the treatment of influenza but is not indicated in most uncomplicated cases. New types of anti-influenza drugs are currently under development. Patients should rest, keep warm and take plenty of non-alcoholic fluids plus any medication recommended by their doctor to relieve symptoms. Antibiotics are of little value except in patients in whom bacterial pneumonia occurs.