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Influenza

Page content: Victorian statutory requirement | Infectious agent | Identification | Incubation period | Public health significance & occurrence | Reservoir | Mode of transmission | Period of communicability | Susceptibility & resistance | Control measures | Outbreak measures | Additional sources of information

Victorian statutory requirement

Influenza (Group B disease) must be notified in writing within five days of laboratory confirmation.

School exclusion: exclude until well.

Infectious agent

Influenza virus (types A, B and C) is the causative agent.

Identification

Clinical features
Influenza is an acute respiratory disease. Symptoms include fever, headache, myalgia, lethargy, coryza, sore throat and cough. Infections in children, particularly type A and B (H1N1) may also be associated with gastrointestinal symptoms such as nausea, vomiting and diarrhoea. Croup is a common presentation in children.

Most symptoms resolve within two to seven days although the cough may persist longer. Complications of influenza include middle ear infections, secondary bacterial pneumonia and exacerbation of underlying chronic health conditions.

During influenza epidemics, patients with early influenza symptoms (fever >38°C, plus at least one systemic symptom such as myalgia, and one respiratory symptom) have a 60–70% chance of having influenza infection.

Method of diagnosis
A clinical diagnosis can be confirmed by culture or antigen testing of appropriate respiratory specimens such as nasopharyngeal aspirate or nose and throat swabs, taken within five days of onset. Or it can be confirmed by serology performed on blood specimens taken during the acute and convalescent stages.

The diagnosis can be confirmed in the laboratory by one or more of the following:

  • detection of influenza virus by culture or nucleic acid testing, most commonly polymerase chain reaction (PCR) testing
  • demonstration of a significant rise, i.e. fourfold increase in the influenza-specific antibody titre between a serum sample collected in the acute phase and another sample collected in the convalescent phase two to three weeks after onset of symptoms
  • a single high influenza-specific antibody titre of five dilutions or greater. This means a titre of 160 or greater, or 128 or greater, depending upon the titration method.

Incubation period

The incubation period is one to four days.

Public health significance & occurrence

Influenza occurs as pandemics, epidemics, outbreaks and as sporadic cases.
Severe disease and complications such as viral and bacterial pneumonia occur primarily among the elderly and those debilitated by a chronic disease.

In temperate zones outbreaks tend to occur in winter. In the tropics they often occur in the rainy season but outbreaks or sporadic cases may occur at any time.

Most human infections are caused by either type A or B influenza viruses. Type A has been associated with widespread epidemics and pandemics, while type B has been infrequently associated with regional epidemics, and type C is only rarely associated with human infection.

Influenza A is sub-typed further. It has two surface antigens (proteins) that are used for sub-typing: haemagglutinin (H) and neuraminidase (N). Since 1918 the only three influenza A sub-types known to usually cause human disease are H1N1, H2N2 and H3N2. Other subtypes such as HSN1 are very rare.

Influenza viruses are formally named according to their type (A, B or C), their sub-type antigenic characterisation and location of first isolation; for example, influenza A (H1N1) or New Caledonia.

The emergence of completely new sub-types of type A virus (antigenic shift) occurs at irregular intervals and is responsible for pandemics. Minor antigenic changes (antigenic drift) are responsible for annual epidemics and regional outbreaks.

Reservoir

Humans are the primary reservoir. Animal reservoirs are suspected as sources of new human subtypes and may occur particularly when people and livestock (for example pigs and poultry) live closely together. In 2004 an outbreak of avian influenza (influenza A H5N1) caused a number of human infections in South East Asia.

Mode of transmission

Influenza viruses are predominately transmitted by airborne spread in aerosols but can also be transferred by direct contact with droplets. Nasal inoculation after hand contamination with the virus is also an important mode of transmission.

Direct contact is important, as the virus will survive some hours in dried mucus particularly in cold and dry environments.

Period of communicability

It is probably communicable for three to five days from clinical onset in adults and up to seven days and occasionally longer in young children.

Susceptibility & resistance

When a new subtype appears, all people are susceptible except those who have lived through earlier epidemics caused by a related subtype.

Infection produces immunity to the specific infecting virus, but the duration and breadth of immunity varies widely. This is partly dependent on host factors, the degree of antigenic drift in the virus and the period of time since the previous infection.

Control measures

Preventive measures
The influenza vaccine in Australia is developed in time for the annual winter rise in ‘flu’ activity. The strains that are contained in the vaccine are based on the circulating strains in the previous couple of years as well as those circulating in the previous Northern Hemisphere winter. It normally includes representatives of both major influenza A subtypes (H1N1, H3N2) and B strain.

Influenza vaccine is recommended on the Australian Standard Vaccination Schedule annually for all persons 65 years and older.

Free annual influenza vaccine is provided and recommended for the following groups in Victoria:

  • all people aged 65 years and older
  • all Aboriginal and Torres Strait Islanders aged 50 years and older, and those aged 15–49 years who are at high risk for the complications of influenza including those with:
    • chronic disease such as diabetes, heart, lung, kidney or liver disease
    • decreased immunity
    • living in chronic care facilities
  • all public hospital staff in both outpatient and ward settings who provide direct care to patients, to protect themselves and their patients.

Annual influenza vaccination is also recommended for staff working in nursing homes and other chronic care facilities to protect themselves and their patients.

Control of case
Symptomatic treatment alone or with the addition of a neuraminidase inhibitor, if commenced within the first 36 hours of the onset of the illness, can shorten the duration by two to three days. Consult the current version of Therapeutic guidelines: antibiotic (Therapeutic Guidelines Limited).

For sporadic cases isolation is often unrealistic due to the delay in diagnosis. If cases are still symptomatic they should be advised to remain at home until well and to avoid contact with high risk persons.

Control of contacts
Control of contacts may be of benefit in high risk populations who should be advised to seek medical advice on prophylaxis and to seek early medical review if symptoms develop.
Chemoprophylaxis with amantadine or a neuraminidase inhibitor may be considered in special circumstances against influenza A strains, for example in residential institutions. The potential value of chemoprophylactic drugs must be assessed against their side effects.

Control of environment
Cases and carers should be advised about the importance of hand washing, covering the mouth when coughing, sneezing into disposable tissues, and the appropriate cleaning or disposal of contaminated objects.

Outbreak measures

The most important control measure to prevent serious morbidity and mortality from influenza epidemics is appropriate immunisation. Investigations are generally restricted to outbreaks in groups at higher risk of complications (see Special settings, below).

An influenza pandemic results when antigenic shift leads to a new highly virulent influenza subtype for which there is little or no immunity in the population. Public health action in this setting may involve a variety of measures to control spread in the community.

Special settings
Aged care facilities, health care facilities and child care centres are all special areas at high risk of influenza outbreaks.

Aged care facilities
Specific infection control measures should be implemented in the event of:

  • a laboratory-confirmed case of influenza
  • two or more cases of an acute respiratory illness consistent with influenza (’influenza-like illness’).

Infection control measures include vaccination of any unvaccinated staff and residents, exclusion of sick staff members, cohorting of resident cases, active case finding and, in some settings, the use of antiviral treatment and prophylaxis.

Health care facilities
Outbreaks of an unidentified respiratory illness in a hospital setting including outbreaks of influenza-like illness are investigated jointly by the Department of Health and the hospital’s infection control unit.

Child care centres
Outbreaks of influenza or influenza-like illness in child care require exclusion of cases and may warrant prophylaxis for high risk contacts. The Department of Health can advise on prophylaxis and infection control procedures.

Additional sources of information

Victorian Infectious Disease Reference Laboratory, The flu report (during flu season).

 


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