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November 11, 2019

ePub www.aafp.org/afp American Family Physician 1

Published online November 11, 2019

Influenza is an acute respiratory infection caused by a negative-strand RNA virus of the Orthomyxoviridae fam- ily. T ere are three distinct types of in f uenza viruses that infect humans: infuenza A, B, and C. Infuenza A infects multiple species, including humans, swine, equines, and birds. It is more su sceptible to anti genic variation and, hence, is the cause of major pandemics. 1,2 T e surface of the virion envelope is covered with proteins hemaggluti- nin (HA), neuraminidase (NA), and matrix 2. 1

Antigenic

variation is associated with changes in the HA or NA sur- face proteins and is generally classi f ed as antigenic dri f or shi f 1

Antigenic dri

f involves small, gradual amino acid substitutions of the HA or NA proteins that can result in smaller outbreaks. Antigenic shi f occurs when there are signi f cant changes in the HA or NA proteins that create novel in f uenza subtypes with the potential to cause wide- spread pandemics. 1,3

Each in

f uenza A subtype is charac- terized by numbering both the HA and NA proteins (e.g.,

H3N2, H5N1).

Epidemiology

From 2010 to 2018, estimated seasonal in

f uenza activity in the United States ranged from 9.3 million to 49 million cases and 12,000 to 79,000 deaths per year. 4 T e 2017-2018 in f uenza season was the third most severe since 2003-2004 and was characterized by high severity of disease in all age groups. 5

Most cases were secondary to the A/H3N2 subtype,

and low vaccine e f ectiveness was a signi f cant contributing

Influenza: Diagnosis and Treatment

David Y. Gaitonde, MD; Cpt. Faith C. Moore, USA, MC; and Maj. Mackenzie K. Morgan, USA, MC Dwight D. Eisenhower Army Medical Center, Fort Gordon, Georgia

Influenza is an acute viral respiratory infection that causes significant morbidity and mortality worldwide. Three types of influ-

enza cause disease in humans. Influenza A is the type most responsible for causing pandemics because of its high susceptibility

to antigenic variation. Influenza is highly contagious, and the hallmark of infection is abrupt onset of fever, cough, chills or

sweats, myalgias, and malaise. For most patients in the outpatient setting, the diagnosis is made clinically, and laboratory con-

firmation is not necessary. Laboratory testing may be useful in hospitalized patients with suspected influenza and in patients

for whom a confirmed diagnosis will change treatment decisions. Rapid molecular assays are the preferred diagnostic tests

because they can be done at the point of care, are highly accurate, and have fast results. Treatment with one of four approved

anti-influenza drugs may be considered if the patient presents within 48 hours of symptom onset. The benefit of treatment is

greatest when antiviral therapy is started within 24 hours of symptom onset. These drugs decrease the duration of illness

by about 24 hours in otherwise healthy patients and may decrease the risk of serious complications. No anti-influenza drug

has been proven superior. Annual influenza vaccination is recommended for all people six months and older who do not

have contraindications. (Am Fam Physician. 2019; 100:online. Copyright © 2019 American Academy of Family Physicians.)

BEST PRACTICES IN INFECTIOUS DISEASE

Recommendations from the Choosing

Wisely Campaign

RecommendationSponsoring organization

Do not routinely avoid

influenza vaccination in egg-allergic patients.

American Academy of Allergy,

Asthma, and Immunology

Source: For more information on the Choosing Wisely Campaign, see https:// www.choosingwisely.org. For supporting citations and to search Choosing Wisely recommendations relevant to primary care, see https:// www.aafp.org/afp/recommendations/search.htm. See related Practice Guideline at https://www.aafp.org/ afp/2019/1015/p505.html. Author disclosure: No relevant financial afliations. Patient information: Handouts on this topic are available at https:// family doctor.org/preventing-the-flu and https:// family doctor.org/flu-myths.

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2 American Family Physician www.aafp.org/afp ePub

November 11, 2019

factor. 5

Based on public health laboratory specimens, the

predominant in f uenza A subtype for the 2018-2019 season was (H1N1)pdm09 (56.6% of positive specimens), followed by the A/H3N2 subtype (43.6%) and in f uenza B (4%). 6

Seasonal in

f uenza occurs primarily in the colder months in temperate regions. T e primary mode of transmission is via inhalation of infectious respiratory particles (large drop- let transmission) when an infected person coughs or sneezes. T ere is also evidence of airborne (small particles transmit- ted by tal king or exhalation) and fomite tr ansmissi on. 6,7 T e typical incubation period is 24 to 48 hours. Patients are infectious one to two days before symptom onset and for f ve to seven days a f erward. Children and immunosuppressed people may exhibit prolonged viral shedding. 8,9 In 2009, a novel H1N1 subtype (pH1N1) emerged from a quadruple reassortment of two swine viruses, one avian virus, and one human virus. 10

First identi

f ed in Mexico in 2009, it caused millions of cases worldwide and an esti- mated 18,500 laboratory-con f rmed deaths. 11 Since 2011, outbreaks of a swine-origin H3N2 virus have been reported in the United States. Cases predominantly involved young children exposed to swine, o f en at state fairs. 12,13

Additionally, the H5N1 avian in

f uenza virus has become a global concern. First identi f ed solely in wild geese populations in China in 1997, the virus quickly spread to domestic poultry and ultim ately to human populatio ns. T ere are now avian in f uenza cases reported worldwide, although poultry-to-human and human-to-human trans- mission remains relatively low. Despite low transmissibility, the reported fatality rate is high (approximately 60%). 14

Prevention

T e Centers for Disease Control and Prevention's (CDC's) Advisory Committee on Immu nization Practices (ACIP) and the American Academy of Family Physicians (AAFP) recommend annual in f uenza vaccination for all people six months and older who do not have contraindications. 15,16

Vaccination e

f orts should target people at increased risk of complicated or severe in f uenza (Table 1 17-19 ) and those who care for or live with high-risk individuals, including health care professionals. 15

T wo previou s FPM ar ticles provided

communication strategies and tools for increasing in f uenza vaccination rates in practice. 20,21

Multiple formulations of the in

f uenza vaccine are avail- able, including inactivated in f uenza vaccines (IIV); a recom- binant inactivated vaccine (RIV); an d a live, a tten uated in f uenza vaccine (LAIV). For physicians and patients con- cerned about egg allergy, quadrivalent RIV and cell culture- based quadrivalent IIV are completely egg-free. 15 An age-app ropriate IIV or RIV is suitable for most patients; LA IV is contraind icated i n pregnant women, immunosuppressed people, children younger than tw oquotesdbs_dbs26.pdfusesText_32

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