Notes
History & examination
Key diagnostic factors
- fever and chills (common)
-
- Fever is an early sign and is defined as a temperature of ≥37.8°C (≥100°F). It can reach as high as 40°C (104°F), or more in children. [51] It is particularly frequent among patients admitted to the hospital with 2009 H1N1 influenza, and it is part of the case definition for influenza-like illness. [11] Patients often report chills.
- cough (common)
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- Particularly frequent among patients admitted to the hospital with 2009 H1N1 influenza, and part of the case definition for influenza-like illness. [11]
- sore throat (common)
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- Part of the case definition for influenza-like illness
Other diagnostic factors
- runny nose (common)
-
- Commonly seen in patients with H1N1 swine influenza.
- age (increased risk of complications) (common)
-
- Influenza can affect patients of any age. In general, patients <5 years and those 65 years of age or older are at increased risk for complications. However, with the 2009 H1N1 influenza, the highest rates of admission to the hospital have been in patients <24 years of age, and the highest mortality has been among those between 25 and 60 years of age. [11] [12] [52] However, although older adults (age >65) were less likely to contract 2009 H1N1 influenza, the ones who did had high rates of complications, including death. [12] [13] [52] [53]
- comorbidities (increased risk of complications) (common)
-
- Comorbidities leading to increased risk of complications from influenza include chronic pulmonary, cardiovascular, hepatic, haematological, neurological/neuromuscular, metabolic (e.g., diabetes), or immunosuppressive (e.g., malignancy, HIV) conditions.
- immunosuppressive medication (common)
-
- Medication lists should be evaluated for the presence of immunosuppressive medication that would compromise the immune response against influenza infection.
- pregnancy (increased risk of complications) (common)
-
- Pregnant women seem at increased risk for complications of 2009 H1N1 influenza; [14] women of childbearing age should be asked whether they are pregnant.
- smoking (increased risk of complications) (common)
-
- People who smoke are at increased risk of complications.
- residence in a nursing home (increased risk of complications) (common)
-
- People resident in a nursing home or another chronic-care facility are at increased risk of complications.
- presence of ill contacts, or residence in or travel to pandemic areas (common)
-
- People with recent contacts with influenza or who have had prolonged stay in areas where 2009 H1N1 influenza is known to be circulating are at increased risk for developing infection.
- constitutional symptoms (fatigue, headache) (common)
-
- Non-specific symptoms such as fatigue and headache are common in patients with H1N1 swine influenza.
- myalgia and arthralgia (common)
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- Commonly seen in patients with H1N1 swine influenza. Arthralgia was present in about half of patients in an outbreak of H1N1 swine influenza in a school in New York City. [51]
- tachypnoea (common)
-
- Often present, and more severe in complicated cases.
- nausea, abdominal pain, diarrhoea (common)
- extreme irritability (young children and infants) (common)
-
- Young children and infants can have atypical presentations and may present without respiratory symptoms.
- altered mental status/lethargy (young children and infants) (common)
-
- Young children and infants can have atypical presentations. Altered mental status and lethargy may be the presenting symptoms. In adults, these findings may be present in complicated cases.
- seizure (uncommon)
- severe shortness of breath (uncommon)
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- Present in complicated cases. Secondary bacterial pneumonia should be ruled out.
- hypotension (uncommon)
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- Low BP, particularly orthostatic hypotension, can be present in patients with volume depletion. Frank shock with evidence of end-organ hypoperfusion is very uncommon and is a sign of severe illness.
- wheezing (uncommon)
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- Sometimes heard in patients with H1N1 swine influenza.
- crackles (uncommon)
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- A sign of lower respiratory tract infection. Secondary bacterial pneumonia should also be considered.
Risk factors
Strong
- close contacts with influenza (H1N1) infection
-
- Anyone who presents with influenza-like symptoms with recent close contact with influenza-infected persons must be presumed to have influenza.
- Although some data suggest that 2009 H1N1 influenza had a higher secondary attack rate than seasonal influenza, [9] [10] data collected in the US indicate that the secondary household attack rate among patients with respiratory infection ranges from 8% to 19%, similar to that observed with seasonal influenza. [11]
- pregnancy
-
- Pregnant women seem to be at increased risk for complications of 2009 H1N1 influenza, as with seasonal influenza. [12] Pregnant women are 4 to 5 times more likely to develop severe disease, with the greatest risk of death during the third trimester. [1] [13] Data collected in the US soon after the start of the 2009 pandemic suggested that pregnant women have higher rates of admission to the hospital, severe pneumonia, and death. Half of the reported deaths among pregnant women occurred in previously healthy people. [14]
- age <65 years
-
- During the pandemic, unlike seasonal influenza, the novel H1N1 swine influenza virus appeared to disproportionately affect age groups <60 years of age, presumably due to pre-existing immunity among older individuals. In the US, the overwhelming majority of cases, hospitalisations, and deaths from the 2009 pandemic influenza virus occurred in individuals <65 years of age. [15] [16] [17] Similar reports have emerged from other countries. [12] [18] [19] [20]
- Individuals <14 years of age appear to have prolonged viral shedding in respiratory secretions. [20] However, patients who are 50 years and older and are hospitalised for H1N1 influenza infection have the highest case fatality rate.
- comorbidities
Weak
- obesity
-
- Obesity (BMI >30) also seems to be an important risk factor for severe illness from 2009 H1N1 influenza. [22] Among adults ≥20 years, obesity appeared to be associated with hospitalisation and death due to pandemic H1N1 influenza infection. [23] In addition, obesity was also associated with longer hospital and ICU stays as well as increased use of mechanical ventilation. [24] Extreme obesity (BMI >40) has been associated with increased risk of death. [25]
Last updated: Dec 30, 2011
