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Acute, typically self-limited, febrile infection caused by orthomyxovirus influenza types A and B
Marked by inflammation of nasal mucosa, pharynx, conjunctiva, and respiratory tract
Outbreaks have varying degrees of severity and generally peak in winter.
Influenza virus can undergo antigenic shift (abrupt change) leading to strains of virus to which little immunologic resistance exists in a population, potentially resulting in pandemic outbreak. Minor seasonal variations are called antigenic drift.
System(s) affected: typical cases: head/eyes/ears/nose/throat; pulmonary; complicated cases: cardiac and CNS involvement
Synonym(s): flu; grippe; acute catarrhal fever
Predominant age: children (3 months to 16 years) and young adults
Predominant sex: male = female
Seasonal influenza in preuniversal vaccination: 95 million cases per year, typically fall/winter
Attack rates in healthy children: 10-40% each year, prior to routine influenza vaccination
Weekly reports are available: http://www.cdc.gov/flu/weekly
Incubation is 1 to 4 days; infected persons are most contagious during peak symptoms.
Spread by aerosolized droplets or contact with respiratory secretions
Hemagglutinin binds to columnar respiratory epithelium where replication occurs, and neuraminidase protein facilitates spread along respiratory epithelium.
For contracting disease
Vaccination: All persons >6 months should be vaccinated annually, with few exceptions:
IIV recommended annually for the following:
LAIV is not recommended for the 2016 to 2017 influenza season.
IIV-HD: high-dose quadrivalent IIV
Antiviral prophylaxis depends on current resistance patterns each year; see http://www.cdc.gov/flu/ for current patterns or check with local health department.
Vaccinate children 6 to 23 months old with IIV.
Either IIV or LAIV in healthy children ages 2 to 18 years
For prophylaxis, oseltamivir dosage varies by weight and is recommended by the CDC for prophylaxis for children ≥3 months; zanamivir is approved for prophylaxis for children ≥5 years of age at a dosage of 2 inhalations per day. For prophylaxis, the dosage of amantadine and of rimantadine is 5 mg/kg/day up to 150 mg in 2 divided doses. Currently, amantadine and rimantadine are not recommended due to resistance.
The CDC recommends vaccinating all women who will be pregnant during influenza season.
If unvaccinated at the time of flu season, pregnant women should receive IIV.
Oseltamivir, zanamivir, peramivir, rimantadine, and amantadine are pregnancy Category C.
Not being able to cope with daily activities
Being confined to bed
Fever (37.7-40.0ºC), especially if combined with presenting within 3 days of illness onset
Chills, sweats, malaise, myalgia, arthralgia
Rhinorrhea, nasal congestion
Physical exam is not specific for influenza.
Physical examination should exclude complications such as otitis media, pneumonia, sinusitis, and tracheobronchitis.
Respiratory viral infections including respiratory syncytial virus, parainfluenza, adenovirus, enterovirus (“influenza-like illness”)
Viral or streptococcal tonsillitis
Atypical mycoplasmal pneumonia
Less likely possibilities include severe acute respiratory syndrome, primary HIV infection, acute myeloid leukemia, tuberculosis, anthrax, and malaria.
During influenza season, diagnosis is based solely on clinical findings. If additional testing is needed
Symptomatic treatment is typically all that is required (saline nasal spray, analgesic gargle, antipyretics, analgesics).
Cool-mist or ultrasonic humidifier to increase moisture of inspired air
Droplet precautions: see http://www.cdc.gov/HAI/settings/outpatient/basic-infection-control-prevention-plan-2011/transmission-based-precautions.html#c
5 days is the average period of viral shedding in immunocompetent hosts.
Hospitalized patients may require oxygen or ventilatory support.
Antiviral treatment depends on yearly resistance patterns; check http://www.cdc.gov/flu/ or with local health department. Antivirals are most effective if administered within first 48 hours in laboratory-confirmed (or highly suspected based on clinical findings) influenza cases.
Antivirals within 48 hours of symptom onset are recommended for patients at risk of complications (i.e., diabetes, CHD, COPD, asthma, etc.) 1[A].
Antivirals are recommended if hospitalized 2[A].
Antivirals include amantadine, rimantadine, oseltamivir, zanamivir, and peramivir.
Antivirals may be considered for persons not at increased risk of complications from influenza whose onset of symptoms is within the past 48 hours and who wish to shorten the duration of illness and further reduce their relatively low risk of complications 1[A].
Symptomatic treatment is preferred for those patients without risk factors and without signs of lower respiratory tract infection 2.
Effect is 24-hour reduction of symptoms and a reduction in complication rates.
Peramivir dose: 600 mg IV infusion over 15 to 30 minutes for adults ≥18 years of age
Amantadine and rimantadine are currently not recommended due to resistance.
Decrease dose of certain antivirals if creatinine clearance ≪30 mL/min.
Ibuprofen or other NSAIDs for symptomatic relief
Aspirin: should not be used in children ≪16 years due to risk of Reye syndrome
Outpatient treatment is sufficient except for cases with severe complications or in high-risk groups.
Mild cases: Usually, no follow-up is required.
Moderate or severe cases: Follow up until symptoms and any secondary sequelae resolve.
Pneumonia (primary viral or secondary bacterial)
Apnea in neonates
COPD or CHF exacerbation
Centers for Disease Control and Prevention. Influenza (flu). http://www.cdc.gov/flu/index.htm. Accessed October 14, 2016.
Ebell MH, White LL, Casault T. A systematic review of the history and physical examination to diagnose influenza. J Am Board Fam Pract. 2004;17(1):1–5.
Grohskopf LA, Sokolow LZ, Broder KR, et al. Prevention and control of seasonal influenza with vaccines: recommendations of the Advisory Committee on Immunization Practices—United States, 2016-17 influenza season. MMWR Recomm Rep. 2016;65(5):1–54.
Osterholm MT, Kelley NS, Sommer A, et al. Efficacy and effectiveness of influenza vaccines: a systematic review and meta-analysis. Lancet Infect Dis. 2012;12(1):36–44.
J11.1 Influenza due to unidentified influenza virus with other respiratory manifestations
J10.1 Flu due to oth ident influenza virus w oth resp manifest
J11.00 Flu due to unidentified flu virus w unsp type of pneumonia
J11.89 Influenza due to unidentified influenza virus w oth manifest
J09.X2 Flu due to ident novel influenza A virus w oth resp manifest
487.1 Influenza with other respiratory manifestations
488.12 Influenza due to identified 2009 H1N1 influenza virus with other respiratory manifestations
487.0 Influenza with pneumonia
488.19 Influenza due to identified 2009 H1N1 influenza virus with other manifestations
488.82 Influenza due to identified novel influenza A virus with other respiratory manifestations
488.11 Influenza due to identified 2009 H1N1 influenza virus with pneumonia
488.81 Influenza due to identified novel influenza A virus with pneumonia
488.89 Influenza due to identified novel influenza A virus with other manifestations
487.8 Influenza with other manifestations
6142004 Influenza (disorder)
231000124101 Influenza A virus subtype H1 2009 pandemic strain present (finding)
195878008 Pneumonia and influenza (disorder)
441345003 Influenza B virus present (finding)
63039003 Influenza with respiratory manifestation other than pneumonia (disorder)
Influenza is an acute, (typically) self-limited, febrile infection caused by influenza virus types A and B.
With rare exceptions, all persons >6 months should be vaccinated against influenza on an annual basis.
Complications from influenza are most common in the very young, very old, and individuals with comorbid disease.
Hand hygiene either with soap and water (slightly superior) or with alcohol-based hand rubs and covering coughs are simple ways to reduce the spread of influenza.