Influenza overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Alejandro Lemor, M.D. [2]
Overview
Influenza, commonly known as flu, is an infectious disease of birds and mammals caused by RNA viruses of the biological family Orthomyxoviridae (the influenza viruses). In humans, common symptoms of influenza infection are fever, sore throat, muscle pains, severe headache, coughing, weakness and general discomfort.[1] In more serious cases, influenza causes pneumonia, which can be fatal, particularly in young children and the elderly. Sometimes confused with the common cold, influenza is a much more severe disease and is caused by a different type of virus.[2] Although nausea and vomiting can be produced, especially in children,[1] these symptoms are more characteristic of the unrelated gastroenteritis, which is sometimes called "stomach flu" or "24-hour flu."[3]
Historical Perspective
Classification
Influenza virus can be classified into 3 main types: A, B and C. Based on the surface glycoproteins, hemagglutinin (HA) and neuraminidase (NA), influenza type A is subclassified into several subtypes and can infect many species, including humans. Type B can only infect humans and type C is not that common and causes mild illness.
Pathophysiology
Influenza virus is under constant evolutionary change. These genetic changes may be small and continuous or large and abrupt. Small genetic changes happen continuously in Type A and Type B influenza as the virus makes copies of itself. This process is called antigenic drift. Drifting happens frequently enough to make new strains of virus unrecognizable to the human immune system. Type A influenza also undergoes infrequent and sudden changes known as antigenic shift. Antigenic shift occurs when two different flu strains infect the same cell and combine portions of their genetic material. The novel assortment of HA and/or NA proteins in a shifted virus may create a new influenza A subtype. Influenza viruses spread mainly through tiny droplets expelled when people with the disease cough, sneeze, or talk. These droplets can land in the mouths or noses of people who are nearby or be inhaled into the lungs.
Causes
Differentiating Influenza from other Diseases
Influenza should be differentiated from other conditions that cause fever, cough, myalgias, headache and fatigue, such as RSV infection, dengue fever, parainfluenza virus infection, ebola infection adenovirus infection, rhinovirus infection, and legionella infection.
Epidemiology and Demographics
The CDC estimates that 5 to 20 percent of Americans are affected by the flu during each flu season, which typically lasts from October to March. Children are 2 to 3 times more likely than adults to get sick with the flu, and children frequently spread the virus to others. For elderly people, newborn babies, pregnant women, and people with certain chronic illnesses, the flu and its complications can be life-threatening. Although most people recover from the illness, between 3,000 and 49,000 US citizens die from the flu and its complications every year.[4]
Risk Factors
Risk factors for influenza are for those patients that are exposed to the disease, such as healthcare workers, residents of nursing homes or children. However, there are certain groups that are at risk for severe disease and complications, such as children younger than 2 years old, adults aged 65 or older, pregnant women, patients with comorbidities, asthmatic patients, and patients with a weakened immune system.
Natural History, Complications and Prognosis
Influenza is a viral respiratory infection spread from person to person chiefly through droplet transmission. The incubation period period if 1-4 days. The uncomplicated influenza can present with constitutional symptoms and can resolve in 3-7 days while others can develop into pneumonia , ear infection, encephalitis and sepsis.
Diagnosis
History and Symptoms
Influenza illness can include several non-specific symptoms, such as fever, muscle aches, headache, lack of energy, dry cough, sore throat, and runny nose. The fever and body aches can last 3-5 days and the cough and lack of energy may last for 2 or more weeks. The symptoms of the flu are more severe than their common-cold equivalents.
Physical Examination
During the physical examination in a patient with influenza, findings can include fever, tachycardia, tachypnea, sore throat, and fatigue. Complications can worsen the patient's condition and findings in the lung examination may include dry cough, rales and focal wheezing.
Laboratory Findings
Diagnostic tests available for influenza include viral culture, serology, rapid antigen testing, polymerase chain reaction (PCR), immunofluorescence assays, and rapid molecular assays. Sensitivity and specificity of any test for influenza might vary by the laboratory that performs the test, the type of test used, and the type of specimen tested. Among respiratory specimens for viral isolation or rapid detection, nasopharyngeal specimens are typically more effective than throat swab specimens. As with any diagnostic test, results should be evaluated in the context of other clinical and epidemiologic information available to health-care providers.
Chest X Ray
Chest X-ray is not routinely done in patients with influenza. A chest X-ray is useful to assess complications of influenza infection, such as a superimposed bacterial pneumonia.
Other Diagnostic Studies
Other diagnostic test for influenza include molecular assays, such as RT-PCR. New technologies being pursued include those that examine influenza viruses at the molecular level. By examining the genetic makeup of influenza viruses, such tests could identify both the virus type and subtype simultaneously.
Treatment
Medical Therapy
Primary Prevention
Cost-Effectiveness of Therapy
Future or Investigational Therapies
References
- ↑ 1.0 1.1 Merck Manual Home Edition. "Influenza: Viral Infections".
- ↑ Eccles, R (2005). "Understanding the symptoms of the common cold and influenza". Lancet Infect Dis. 5 (11): 718–25. PMID 16253889.
- ↑ Seasonal Flu vs. Stomach Flu by Kristina Duda, R.N.; accessed March 12, 2007 (Website: "About, Inc., A part of The New York Times Company")
- ↑ "National Institute of Allergy and Infectious Diseases - Flu Overview".