Organism There are three types of influenza viruses A, B and C with H antigen (15 subtypes) and N antigen (9 subtypes), so that the virus is designated as H1N1, H1N2,..., H3N2. In addition, the site of isolation, culture number and year of isolation are used (e.g. A/Beijing/262/95) (H1N1). So far a major antigenic shift to H4 or N3 in human infection has not yet occurred. Anti-genic drift in both A and B viruses, producing new strains occurs at infrequent intervals and is responsible for most epidemics.

Clinical features Influenza presents with fever, malaise, muscle aches and upper respiratory symptoms of sudden onset. There is initially a dry cough, which can sometimes be severe and often leads to secondary infection, with the production of sputum. It is a serious infection in the elderly with high death rates. When a major antigenic shift occurs as it did in 1918, all ages are susceptible and the number of deaths can be enormous (an estimated 50 million).

Diagnosis is on clinical grounds taking care to differentiate influenza (occurring seasonally or in epidemics) from other causes of respiratory infections, especially the common cold. Direct immunofluorescence (DIF) or virus isolation from throat or nasal swabs can be made in specialist centres.

Transmission is via the airborne route through sneezing or coughing, but can also be by direct contact with mucus. Influenza is highly infectious and spreads throughout whole communities, potentiated by overcrowding and frequent social contact, such as at the work place.

Influenza also occurs in birds and pigs and these may well be the reservoir from which human infection originates. The close association of humans with domestic birds and animals, particularly in South China, is thought to be how new variants arise.

Incubation period 1-5 days with a mean of 2 days.

Period of communicability 2 days before onset of symptoms to 5 days after.

Occurrence and distribution Influenza A is responsible for pandemics and regular seasonal outbreaks, B for smaller localized outbreaks and C produces mild infections. In the tropics, epidemics tend to occur in the rainy season, while in temperate climates, influenza is nearly always a disease of the winter months. Pandemics have occurred in 1889, 1918, 1957, 1968 and 1977.

Control and prevention As influenza is highly infectious, the majority of the population becomes infected during an epidemic, but any reduction of social contact, particularly in crowded places, can reduce this likelihood. Spitting should be outlawed and people encouraged not to cough directly at people. The wearing of masks, as practised in China and Japan, is probably more effective in preventing spread from an infected person wearing a mask than in protecting the non-infected. Once the cloth mask becomes damp through exhaled breath, it ceases to be effective, but tight-fitting masks with changeable filters should offer reasonable protection.

Influenza vaccine is available for protecting at-risk persons, such as people over 65 years of age, those with chronic chest or kidney disease and the immunocomprom-ised. The difficulty of producing a vaccine is that a new one has to be produced each year, containing the expected composition of antigenic sub-units. The WHO collaboration centres, with reference laboratories throughout the world, provide advance warning to assist countries in producing vaccine, but new techniques, such as virus manipulation to anticipate natural change in the virus, could allow banks of virus to be kept in store.

Surveillance Sentinel reporting centres with an agreed case definition probably provide a better idea of the influenza situation than collecting data of variable quality from every clinic and hospital. This can be compared with laboratory-confirmed cases where facilities permit. WHO reports on the global situation in the Weekly Epidemi-ological Record (WER) from a worldwide network of reporting centres.

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