Guidelines for the Prevention and Control of Influenza Outbreaks in Residential Care Facilities for Public Health Units in Australia
3.3 Case definition
The case definition is formulated once an outbreak of respiratory disease has been declared. The case definition should be carefully considered for each outbreak. The definition for an influenza outbreak presented here is a guide only.
Suspected case of influenza
A person from the population at risk (e.g., staff or residents from the RCF), with
onset of a respiratory illness from a defined point in time, characterized by fever (>380C);
PLUS
one or more respiratory symptoms: cough, shortness of breath, coryza (runny nose), sore throat;
PLUS
one or more systemic symptoms: fatigue (severe tiredness), myalgia (muscle ache), rigors (chills), headache.
From a disease control viewpoint, suspected cases should be deemed to be true cases (and therefore infectious) until proven otherwise (e.g., by demonstration of an alternative clinically-compatible pathogen), or until 7 days has elapsed since symptom onset.
(Note: Studies have shown that the best predictors of influenza are cough and fever).24
Confirmed case of influenza
- Isolation of influenza virus by culture from appropriate respiratory tract specimen
- Detection of influenza virus by nucleic acid testing (NAT) from appropriate respiratory tract specimen
- Detection of influenza antigen from appropriate respiratory tract specimen
- Seroconversion or significant rise in antibody level or titre to influenza virus
- Single high titre by complement fixation test (CFT)
- Positive result in a rapid point of care influenza test kit.
