Influenza

Published on 07/03/2015 by admin

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Last modified 07/03/2015

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Chapter 39 Influenza

Christopher Grace, MD, FACP

1 What is influenza?

Influenza is a respiratory illness caused by an RNA virus that comes in two forms: influenza A and influenza B. Although generally self-limited, influenza can cause significant morbidity and mortality especially in those at risk (see answer 4 below). Influenza is most common during the fall and winter months because of increased indoor crowding and low humidity, though illness can continue through April and May in the Northern hemisphere.

2 How are influenza A strains designated?

Influenza A is described by the surface glycoproteins: hemagglutinin (H) and neuraminidase (N). The hemagglutinin, of which there are 16 structurally different types, allows attachment to host respiratory epithelium. The neuraminidase, of which there are nine different types, acts as an enzyme that facilitates release of newly replicated viruses from the infected cell. Humans are most often infected with influenza viruses having H1, H2, or H3 and N1 or N2.

The H and N terminology is also used to name the influenza strains spreading yearly. Generally each year one or two influenza A strains and an influenza B strain circulate. During the 2010-2011 season, the circulating strains included A H1N1, A H3N2, and influenza B.

3 What are the symptoms of influenza?

Influenza is characterized by the abrupt onset of fever, nonproductive cough, sore throat, rhinitis, headache, myalgia, and fatigue. Children may also have otitis media. Symptoms generally resolve within 5 to 7 days though the cough may persist for several weeks. During the flu season, an otherwise healthy adult with fever and cough has an 80% to 90% chance of having influenza. Such a simple case definition though may not accurately diagnose influenza in young children, the elderly, or those with comorbid cardiopulmonary illness.

4 Who is at risk for more severe or complicated influenza?

The Centers for Disease Control and Prevention (CDC) has warned that influenza can be especially serious and life threatening for patients with the risk factors as follows:

image Aged 6 months to 4 years

image Aged 50 years and older

image Chronic pulmonary (including asthma), cardiovascular (except hypertension), renal, hepatic, neurologic, hematologic, or metabolic disorders (including diabetes mellitus)

image Immunosuppression caused by medications, organ transplantation, malignancy, or HIV

image Pregnancy during the influenza season

image Aged 6 months to 18 years and receiving long-term aspirin therapy (Reye syndrome)

image Residents of nursing homes and other long-term–care facilities

image American Indians and Alaska Natives

image Morbidly obese (body mass index is 40 or greater)

5 What complications can occur from influenza?

According to the CDC on average in excess of 36,000 deaths and 226,000 hospitalizations are attributable to influenza during each yearly flu season. Influenza can cause a primary viral hemorrhagic pneumonia characterized by progressive dyspnea and leukocytosis, potentially progressing to respiratory failure and acute respiratory disease syndrome. A secondary bacterial pneumonia may develop in older patients and those with chronic cardiopulmonary illness. After a period of improvement the patient appears to worsen with signs and symptoms of bacterial pneumonia, most often caused by Streptococcus pneumoniae, Haemophilus influenzae, or Staphylococcus aureus.

In addition, patients may have exacerbations of chronic cardiopulmonary illnesses. Less-common complications include myositis, myocarditis, pericarditis, encephalitis, a toxic shock–like illness, Guillain-Barré syndrome, and Reye syndrome.

6 What other infections can mimic influenza?

The symptoms of influenza are very nonspecific and can be caused by a large array of viruses and bacteria. Because these pathogens can cause an illness similar to influenza, any febrile respiratory illness may be referred to as influenza-like illness (ILI). Causes of ILI, in addition to influenza, include respiratory syncytial virus, parainfluenza, rhinovirus, and coronavirus (agents of the common cold); adenovirus; metapneumovirus; group A streptococcus; mycoplasma; chlamydia; and Bordetella pertussis.

7 How do you diagnose influenza?

Influenza is often a clinical diagnosis. For those without risks for complications or requiring hospitalization, no further diagnostics are required. The most accurate way to confirm that a patient does or does not have influenza is to obtain a nasopharyngeal swab. The swab is the same one used to collect and transport bacterial culture samples. The swab needs to be inserted through the nose to the pharynx. Testing by polymerase chain reaction (PCR) is most accurate. Testing by rapid influenza diagnostic tests (RIDT) is not very sensitive (50%-70%) but is reasonably specific (90%-95%). A negative RIDT is not helpful. A positive RIDT can be helpful, and most people with a positive RIDT have influenza A.

8 What is the approach to the patient with an ILI?

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