Influenza: the Disease

Influenza Transmission

Influenza is a contagious viral illness, primarily spread from person to person via droplets formed during coughing and sneezing. The droplets also infect surfaces that can then transmit infection through contact - usually from the hand touching an infected surface and then touching their mouth, nose or eyes from which site the virus can enter the body.

Is there airborne transmission?

The role of airborne transmission is controversial. Droplets produced do not remain airborne as they are too large. However, a droplet that is <5 micrometres in diameter (also called a micro-droplet) is small enough to remain airborne. Thus the distinction between droplet spread and airborne spread relates to the size of the droplet produced by an infected person. In other words, it is likely that a combination of host, environment conditions, as well as viral properties affect whether there is airborne transmission. Certain medical procedures (eg, nebulisation, intubation) will generate aerosols that can then lead to airborne transmission.

Period of Transmissibility?

Virus particles are inhaled and cause infection in the respiratory tract, although infection can also occur through the mucous membranes of the eyes, nose and mouth. The incubation period for seasonal influenza is short, mostly between 1-3 days but can range up to 7 days. There is very limited information on whether influenza can be transmitted prior to, or without, the development of clinical illness. Peak viral shedding occurs after one to three days. Children may shed virus for considerably longer. NB Evidence of virus shedding does not necessarily equate to transmissibility; there is only one report of spreading infection before full blown symptoms.

Ref: Sheat K. An investigation into an explosive outbreak of influenza - New Plymouth. Communicable Disease New Zealand 1992;92:18-19. http://bmj.com/cgi/eletters/331/7525/1145#122682

Influenza Symptoms:

The classic symptoms of influenza are sudden onset of high fever, chills, headaches and body aches. Fever (38-40°C) peaks within 24 h of onset and lasts 1-5 days. As the body symptoms improve, nasal discharge and dry cough become more apparent.

Although several of the symptoms of influenza are common to all age groups, a review of published reports of influenza in children, adults and elderly adults show that the proportion of patients in whom these complaints are noted varies by age (refer Table below). Infants and young children can present with a non-specific febrile illness, or with a respiratory illness such as croup, bronchiolitis, or bronchitis.

Influenza signs and symptoms: Mean (range) % of children and adults reported in different studies

Source: Cox NJ, Subbarao K. Influenza. Lancet 1999; 354: 1277-82.

For patient information on influenza, avian influenza and pandemic influenza  refer "Information about influenza" pdf from Ministry of Health Website

Diagnosis of Influenza

Influenza cannot be reliably diagnosed on clinical features. The presence of fever and cough or a standard case definition for 'influenza-like illness' are poorly related to laboratory-confirmed influenza particularly if influenza virus is not circulating in the community. Many cases with laboratory confirmed influenza do not meet the case definitions; even more cases who have the clinical symptoms do not have laboratory-confirmed infection.

Complications

Influenza can cause a primary viral pneumonia, or lead to a secondary bacterial pneumonia. In some people influenza can exacerbate underlying medical conditions such as cardiac, metabolic or respiratory conditions. Some of the many reported complications associated with influenza include encephalopathy, Reye's syndrome, myositis, myocarditis and pericarditis

Ref: Ministry of Health Immunisation Handbook 2002. Wellington, NZ

Clinical Guidelines

Guidelines around the clinical assessment and management of influenza, see Clinical Guidelines Dr T Blackmore CCH

Difference between Influenza and a 'Cold':

Patient Handout from NISG on the Ministry of Health website

> Case Definition for Avian Influenza