There are three distinct types of influenza virus: A, B and C. The haemagglutinin (HA) and neuraminidase (NA) surface proteins are critical for protective immunity. Influenza A also infects many other animals (including birds, pigs and horses), unlike B and C, and also shows much greater antigenic variation. Antigenic variation occurs by antigenic drift (due to point mutations in the HA gene) or antigenic shift (due to acquisition of completely new HA or NA genes-possibly from animal influenza viruses by reassortment of the segmented RNA genome between viruses). Antigenic shift can produce a virus new to humans, and in the absence of pre-existing immunity this can result in influenza pandemics; there have been four in the 20th century and the first and greatest of these in 1918 caused 20 million deaths world-wide. Epidemics occur in the winter months in temperate climates, beginning abruptly, peaking after 2-3 weeks and lasting 6-10 weeks; attack rates may be 10-20% in the community and hospital admissions rise.
The virus is transmitted in respiratory secretions. The incubation period until onset of illness and virus shedding is 1-3 days and virus is shed for 3-7 days. Influenza A and B infection is characterised by the sudden onset of fever, rigors, headache, malaise and myalgias and arthralgias, with dry cough and nasal discharge. Influenza C usually only causes upper respiratory infection.
Respiratory complications include primary influenza viral pneumonia (particularly in patients with underlying lung or heart disease or immunodeficiency), secondary bacterial pneumonia (particularly due to Staph. aureus) and mixed pneumonia. Asthma and chronic bronchitis may be exacerbated. Reye's syndrome is a recognised complication in children. (Aspirin should be avoided in patients under 16 because of its association with this syndrome.) Myocarditis, pericarditis, aseptic meningitis and post-influenzal encephalitis are rare.
Diagnosis can be made by detecting virus in throat and nasal secretions and retrospectively by a rise in antibody titres.
Amantadine and rimantadine are active only against influenza A and shorten illness if given early. They may cause side-effects in the central nervous system and are rarely used in the UK. Treatment is otherwise symptomatic.
Inactivated influenza vaccines are prepared annually; they contain the most recently circulating strains of influenza A and B, and provide 60-90% protection against these strains. Vaccine should be given annually to patients over 65, and those with chronic lung, heart and renal disease, diabetes and immunodeficiency. Amantadine and rimantadine can be used to prevent influenza A.