eMedicine Specialties > Pediatrics: General Medicine > Infectious Disease
Influenza
Updated: Aug 11, 2009
Introduction
Background
Influenza is the one of the most significant acute upper respiratory tract infections. Influenza viruses cause a broad array of respiratory illnesses responsible for significant morbidity and mortality in children. Influenza viruses cause epidemic disease (influenza virus types A and B) and sporadic disease (type C) in humans.
Pathophysiology
Influenza is an acute infection of the respiratory tract in the nose, throat, and, sometimes, the lungs. Following respiratory transmission, the virus attaches to and penetrates respiratory epithelial cells in the trachea and bronchi. Viral replication occurs, which results in the destruction of the host cell. Viremia does not occur. The virus is shed in respiratory secretions for 5-10 days.
Influenza occurs as sporadic illness, epidemics, or pandemics. Epidemic disease occurs annually, especially in the winter months.
Influenza viruses cause global pandemics, in part because of the high degree of transmissibility and the emergence of an influenza virus with a major antigenic shift (major antigenic variations on the hemagglutinin surface protein) in a nonimmune population. The most recent pandemics included the 1889 pandemic, the 1918-1919 Spanish pandemic (influenza virus subtype H1), the 1957 pandemic (subtype H2N2), the 1968-1969 pandemic (Hong Kong subtype H3N2), and, to a lesser extent, the Russian pandemic in 1977 (subtype H1N1). Approximately 21 million persons died worldwide in the 1918-1919 influenza pandemic, with 549,000 deaths in the United States.
On April 26th, 2009, the US Department of Health and Human Services issued a nationwide public health emergency regarding human cases of swine influenza A (H1N1) virus (swine flu).1 In the preceding weeks, an outbreak of the virus was reported in Mexico. The first cases in the United States were confirmed by the Centers for Disease Control and Prevention (CDC); the outbreak is due to a new strain of influenza virus that contains a combination of swine, avian, and human influenza virus genes. By May 19, 2009, 5,123 cases of H1N1 influenza had been confirmed in nearly all states within the United States. As of May 19, 2009, 5 deaths had been attributed to H1N1 flu in the United States.2,3,4
For an updated tally and case counts in specific states, see the CDC's H1N1 (Swine Flu) Web page.
As of early June, 2009, H1N1 influenza (swine flu) had infected more than 27,500 people in 73 countries, and 141 deaths were confirmed to have been caused by the disease. On June 11, 2009, the World Health Organization (WHO) raised the pandemic alert level to phase 6, indicating a global pandemic, because of widespread infection beyond North America to Australia, the United Kingdom, Chile, Spain, and Japan.
If swine influenza (swine flu) is suspected, clinicians should obtain a respiratory swab for swine influenza (swine flu) testing and place it in a refrigerator (not a freezer). Once collected, the clinician should contact their state or local health department to facilitate transport and timely diagnosis at a state public health laboratory.5
The new virus is resistant to the antiviral agents amantadine and rimantadine but sensitive to oseltamivir (Tamiflu) and zanamivir (Relenza). Initiation of antiviral agents within 48 hours of symptom onset is imperative to provide treatment efficacy against influenza virus. The usual vaccine for influenza administered at the beginning of the influenza season is not effective for this viral strain.
Initial symptoms of swine influenza (swine flu) include high fever, myalgias, rhinorrhea, and sore throat. Nausea, diarrhea, and vomiting have also been reported. Infection control precautions (ie, handwashing, covering mouth with tissue when sneezing or coughing) are encouraged. If suspected swine influenza occurs, isolation is recommended for infected individuals and household contacts. For more information, see updated information from the CDC. For guidance in managing suspected cases, see Medscape's H1N1 influenza algorithm adaptation.
Frequency
United States
Approximately 250,000-500,000 new cases of influenza occur each year in the United States.
Mortality/Morbidity
Influenza viruses cause 20,000 deaths and 200,000 hospitalizations each year in the United States.
Race
No difference based on race has been identified.
Sex
No difference based on sex has been identified.
Age
The infection rate of influenza viruses is high in all age groups. The infection rate and the frequency of isolation of influenza viruses are highest in young children. The infection rate in healthy children is 10-30% annually.
Clinical
History
- Typical symptoms of influenza begin 2-3 days after exposure to the virus.
- Influenza produces an acute febrile respiratory illness with cough, headache, and myalgia for 3-4 days, with symptoms that may persist for up to 2 weeks.
- Patients may present with sudden onset of the following:
- High fever
- Chills
- Myalgia
- Headache
- Fatigue
- Subsequent respiratory symptoms include the following:
- Sore throat/pharyngitis
- Nasal congestion
- Rhinitis
- Nonproductive cough
- Cervical lymphadenopathy
- Conjunctivitis
- Conjunctivitis, rhinitis, and GI tract symptoms are more common in infants and young children than in adults.
- In young infants, influenza may produce a sepsislike picture with shock; occasionally, influenza viruses can cause croup or pneumonia.
- Similar symptoms can be seen in close contacts or family members.
Causes
- Influenza is an acute infection caused by any of 3 types of viruses (A, B, C). Types A and B cause epidemic disease, and type C causes sporadic disease. Type A is the most common.
- Influenza is highly contagious. The virus is spread when an individual inhales infected air-borne droplets (following coughing or sneezing by an infected person) or comes in direct contact with an infected person's secretions (eg, kissing, sharing of handkerchiefs and other items, sharing of objects such as spoons and forks). Viruses may also be transmitted via touching of smooth surfaces, such as doorknobs, handles, and telephones.
- Influenza virus types A and B usually occur in the winter and spring.
- At-risk groups include elderly persons; individuals with chronic respiratory disease, chronic cardiac disease, chronic renal failure, diabetes mellitus, immunosuppression; and persons living in residential care homes and long-stay facilities.
- Severe acute respiratory syndrome (SARS)
- SARS is a serious, infectious, pulmonary illness that is spreading through many countries in Asia, with suspected cases in Europe, Australia, Canada, and the United States. The main symptoms include a high fever, cough, and shortness of breath or other breathing difficulties.
- On March 24, 2003, scientists at the CDC and in Hong Kong announced that a new coronavirus had been isolated from patients with SARS. Over the next 2 weeks, the machinery to discover and characterize the pathogen was set in full motion by scientists at the CDC and in 10 other WHO –collaborating laboratories. Coronavirus has not been proven to be the cause of SARS, but strong supportive evidence is accumulating. For more information, see the eMedicine article Severe Acute Respiratory Syndrome (SARS).
More on Influenza |
 Overview: Influenza |
| Differential Diagnoses & Workup: Influenza |
| Treatment & Medication: Influenza |
| Follow-up: Influenza |
| References |
| Next Page » |
References
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Swine Influenza (Flu). Centers for Disease Control and Prevention. Available at http://www.cdc.gov/swineflu/. Accessed April 27, 2009.
Emergence of a Novel Swine-Origin Influenza A (H1N1) Virus in Humans. N Engl J Med. Jun 3 2009;[Medline].
Update: Novel Influenza A (H1N1) Virus Infection-- Worldwide, May 6, 2009. MMWR. May 2009;58:453-8.
Guidance for Clinicians and Public Health Professionals. Centers for Disease Control and Prevention. Available at http://www.cdc.gov/swineflu/guidance/. Accessed April 27, 2009.
[Best Evidence] Tappenden P, Jackson R, Cooper K, et al. Amantadine, oseltamivir and zanamivir for the prophylaxis of influenza (including a review of existing guidance no. 67): a systematic review and economic evaluation. Health Technol Assess. Feb 2009;13(11):iii, ix-xii, 1-246. [Medline].
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Further Reading
Keywords
influenza, flu, swine flu, swine influenza, H1N1, grip, grippe, acute catarrhal fever, respiratory infection, upper respiratory tract infection, viral infection, severe acute respiratory syndrome, SARS, pharyngitis, rhinitis, cervical lymphadenopathy, conjunctivitis, coup, pneumonia, chronic respiratory disease, chronic cardiac disease, chronic renal failure, diabetes mellitus, immunosuppression, treatment, diagnosis
