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Your Organisational Guidance

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Prévention de l’InfluenzaPublished by: Groupe de travail Développement de recommandations de première ligneLast published: 2018Preventie van influenzaPublished by: Werkgroep Ontwikkeling Richtlijnen Eerste Lijn (Worel)Last published: 2018

Seasonal influenza occurs in outbreaks mainly from December through March in the northern hemisphere and between May and September in the southern hemisphere. Influenza seasonality is less well defined in tropical countries.[80][81] Knowledge of local community disease activity is important when assessing the likelihood that a patient has influenza. The Centers for Disease Control and Prevention (CDC) publishes a weekly influenza surveillance report for the US. CDC: FluView - weekly influenza surveillance report Opens in new window

Patients at high risk of developing complications, including those with a history of chronic lung, heart, or renal disease, infants and young children, and older adults, require special attention.[16]​ Testing for influenza should be done if it will influence the decision to begin antiviral therapy, to order additional diagnostic tests, to institute infection control measures, and for community surveillance of influenza circulation.[82]​ In the US, the CDC has provided guidance on testing and treatment of influenza when influenza and SARS-CoV-2 viruses are cocirculating.[82]​ Recommendations on testing may vary in different locations and you should consult your local guidance.

Diagnosis is usually made clinically during an outbreak in the community, although testing assays are available. Diagnostic testing may be important to establish the diagnosis quickly, as in hospitalized patients with acute onset of severe pulmonary disease.

History and exam

Seasonal influenza presents most commonly as an acute respiratory illness during the winter season (in tropical countries, influenza seasonal patterns are less well defined).[80][81] After an incubation period of approximately 2 days, there is an abrupt onset of high fever, chills, headache, and myalgia. These systemic symptoms may be associated with upper and lower respiratory tract symptoms similar to a common cold, such as cough and sore throat.[83] Viral shedding in influenza peaks within 48 hours of the illness, and most uncomplicated cases resolve within 1 week.[84] Influenza does not present commonly with primary gastrointestinal symptoms such as nausea and vomiting, except in the pediatric population. Diarrhea is rare with influenza and would suggest a viral gastroenteritis. Despite not being caused by influenza, these illnesses are commonly referred to as stomach flu.

During a known influenza outbreak, any person with acute fever and respiratory symptoms should be considered to possibly have influenza. However, if the person has been exposed to influenza or a situation where influenza may spread quickly (e.g., international travel, cruise ships), the diagnosis of influenza should be considered at any time of the year.

Although there are no clear pathognomonic features of influenza, it affects the upper and lower respiratory tract in association with systemic symptoms. Fever, headache, myalgia, and fatigue are often associated with upper respiratory symptoms such as sore throat and lower respiratory symptoms of cough. Not all patients with influenza exhibit these symptoms, and those that do may not always have influenza. Manifestations of influenza infection also depend on patient age and previous history of immunization.[83][85]

With sporadic cases of influenza, it may be difficult to differentiate influenza clinically from infections caused by other respiratory viruses. In this scenario, influenza virus infection may account for only a small number of such cases.

Clinical findings are helpful, but do not confirm or exclude the diagnosis of influenza.[83] Examination may yield nonspecific findings, since physical findings generally are few in cases of uncomplicated influenza. The patient may appear hot and flushed, and the oropharynx may demonstrate hyperemia, with complaints of severe sore throat. Mild cervical lymphadenopathy may be present and is more frequent in younger patients.

Laboratory testing

Diagnostic testing is not usually required to confirm a clinical diagnosis of suspected influenza, especially when there is increased influenza activity in the local community.[86]​ In the US, testing is recommended for all hospitalized patients with signs and symptoms suggestive of influenza and is recommended for outpatients if the results of the test would influence clinical management, for example, use of antiviral treatment or antibiotic treatment, need for further diagnostic tests, consideration for home care, or considerations for those at high-risk for influenza complications.[2][82]

Diagnostic tests available for influenza include molecular assays (rapid molecular assays, reverse-transcription polymerase chain reaction [RT-PCR], other nucleic acid amplification tests [NAATs]), antigen detection tests (rapid influenza diagnostic tests, immunofluorescence assays), and viral culture.[2][82][87][88][89]

Nasopharyngeal specimens are recommended for a respiratory specimen for viral isolation.[2] They are more effective than throat swab specimens. The Infectious Diseases Society of America recommends using rapid molecular assays for detection of influenza viruses in respiratory specimens of outpatients and RT-PCR or other molecular assays for detection in respiratory specimens of hospitalized patients.[2] NAATs are the preferred method for detection of influenza virus in clinical samples because of their superior diagnostic sensitivity compared to rapid antigen tests.​[90]​​ Viral culture remains the definitive test, but is not used for initial clinical management as results may take 3 to 10 days. Rather, it is used for confirming screening tests and for public health surveillance. Culture isolates can provide specific information regarding circulating strains and subtypes of influenza viruses. Virus isolates also may provide information about the emergence of antiviral resistance and the development of novel influenza A subtypes that may potentially cause a pandemic. Some molecular assays can identify influenza virus types and specific influenza A virus subtypes.[2] Rapid molecular assays are 66% to 99% sensitive and 55% to 99% specific, depending on virus type.[2] Rapid influenza diagnostic tests (antigen detection) have poor sensitivity (50% to 75%), though specificity is high (90% to 99%).[2][90]​​

Do not order broad respiratory pathogen panels, including comprehensive viral panel testing, unless the result will directly influence management decisions or if the patient is hospitalized or considered high risk (e.g., immunocompromised).[91][92]​​ There is a lack of consistent evidence to demonstrate the impact of comprehensive viral panel results on clinical outcomes.[93] ​​Do not use serologic testing to diagnose influenza or to inform clinical decision making as results from a single serum specimen cannot be reliably interpreted, and collection of paired sera 2 to 3 weeks apart is required.[2][82]

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Pneumonia

If a patient has an underlying chronic medical condition or falls into a high-risk category, viral or bacterial pneumonia should be considered. These patients will experience persistence of their symptoms beyond the usual time frame for resolution of uncomplicated seasonal influenza. There may be high fever, cough, and dyspnea. If there is an exacerbation of fever and cough with purulent sputum, a secondary bacterial pneumonia is most likely. A chest x-ray confirms infiltrates.

Diagnosis in children

Signs and symptoms of upper and/or lower respiratory tract involvement are common, but seasonal influenza may present more variably in children, depending on age and previous exposure.

The typical symptoms of uncomplicated seasonal influenza virus infection are still often present and include the abrupt onset of fever, headache, myalgia, and malaise associated with manifestations of respiratory tract illness, such as cough, sore throat, and rhinitis.

However, young children frequently struggle to vocalize such symptoms as myalgia and headache. Cough and fever may be less prominent symptoms in young children, or they may have higher fevers than adult patients, experience febrile seizures, and have more gastrointestinal complaints (e.g., nausea and vomiting, poor appetite).[94][95][96] Respiratory symptoms may be less prominent in children at the onset of illness than in adolescents and adults.[94]

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