Christina Martin Influenza is an acute infection of the respiratory tract caused by influenza virus type A or B. It is usually a self-limited disease that occurs in outbreaks, primarily during the winter months in temperate climates; it may occur year-round in the tropics. Influenza is highly contagious and occurs in all age groups. The rate of infection is highest among children; the rate of serious illness and death is highest among people 65 years old or older or those with underlying chronic medical conditions.1 Influenza tends to occur in outbreaks, which can rapidly affect 10% to 40% of the population.2 In 2009, the novel H1N1 (swine flu) virus emerged, infecting humans in pandemic proportion. It is now a regular human influenza virus that continues to circulate worldwide. Influenza is transmitted from person to person through respiratory secretions that contain virus. These respiratory secretions are spread in the form of droplets that are produced when a person talks, coughs, or sneezes. Virus is detectable and may be shed in respiratory secretions up to 24 hours before the onset of symptoms. Once virus reaches the epithelium cells of the respiratory tract, it penetrates the cells and begins replication. This viral replication leads to cell death, which releases large amounts of virus that can infect adjacent cells. This quickly causes desquamation of the ciliated epithelium. Onset of the acute symptoms coincides with this desquamation.2 Influenza viruses evolve frequently owing to point mutations or recombination events that occur during viral replication. This is called antigenic drift. Previous exposure or vaccine may not confer immunity to these new virus variants, thus the reason for seasonal epidemics and the need for adjustment of vaccine every season.1 After an incubation period of 1 or 2 days, there is an abrupt onset of symptoms. These symptoms include fever, chills, headache, malaise, myalgia, and loss of appetite. Respiratory symptoms are also present but are usually overshadowed by the severity of the systemic symptoms. Respiratory symptoms include dry cough, nasal congestion with clear discharge, and sore throat. The cough is usually the most prominent of these respiratory symptoms. The patient’s temperature rises rapidly after onset, peaking at 37.7° C to 40° C (100° F to 104° F) in about 12 hours. The fever typically begins to decline on the second or third day but may last as long as 4 to 8 days. Systemic symptoms become less prominent as the fever decreases. A convalescent phase of 1 to 2 weeks follows the acute febrile stage. Cough, malaise, and fatigue, often extreme, are seen during the convalescent phase.2 Some patients have mild illness that resembles the common cold. Older adults, children under the age of 2 years, people with underlying chronic medical conditions, and pregnant women may experience a rapidly worsening course of influenza. Symptoms caused by H1N1 are similar to those caused by seasonal influenza A or B.3 Uncomplicated cases of influenza have minimal physical findings despite the severity of clinical complaints. At the onset of symptoms, the patient’s face is often flushed, and the eyes may be watery and red. There may be fever. The skin may be hot and moist. Nasal passages can be inflamed, but pharyngeal erythema and exudates are not common.4 Cervical lymph nodes may be enlarged and tender. The findings on chest examination are usually normal. Virus can be isolated from nose and throat specimens by nasal swabs or washings, sputum, and throat swabs. Of these, nasopharyngeal specimens are the preferred source. Ideally, samples should be collected within 12 to 36 hours of onset of illness. The gold standard for influenza diagnosis is either conventional viral culture or reverse transcriptase polymerase chain reaction (RT-PCR) assays. Virus can be detected in cell cultures in 2 to 7 days. In situations wherein early diagnosis is crucial, there are several techniques to detect the presence of viral antigens in nose and throat specimens that yield results rapidly, in as little as 1 hour. These rapid tests may not be as sensitive as cell culture; however, they may be useful in the management of individual patients at high risk for serious complications from influenza, who may benefit most from early hospitalization, appropriate antiviral treatment, or prophylaxis for contacts.5,6 Infection with influenza virus can also be confirmed by at least a fourfold rise in antibody titer in convalescent serum taken 10 to 20 days after an acute serum sample. Clinical diagnosis in the setting of a confirmed influenza outbreak is very accurate. In the absence of a known outbreak of influenza, it may be difficult to distinguish an individual case of influenza from many of the other upper respiratory viral infections, such as the common cold or illness caused by respiratory syncytial virus. Other conditions to consider are Mycoplasma pneumoniae infection, bacterial pneumonia, and severe streptococcal pharyngitis. Differential Diagnosis Influenza Infection
Influenza
Definition and Epidemiology
Pathophysiology
Clinical Presentation
Physical Examination
Diagnostics
Differential Diagnosis
Influenza
Chapter 231
