The ability to recognize cases of the new H1N1 flu and distinguish these from seasonal influenza and other respiratory illnesses is perhaps the overriding concern of primary care practitioners. Prompt and accurate identification of this entity is the key to both effective management of individual illness and effective public health measures. Here I present pertinent and up-to-date information on the clinical characteristics of H1N1 influenza to help clinicians achieve this goal.
Background. After the large initial outbreak of novel H1N1 influenza in Mexico in March and April 2009, the virus quickly spread to the United States, Canada, and many other countries. The rapid spread—via occult human-to-human transmission between unsuspecting persons—was in part facilitated by air travel, although there were also de novo non–travel-associated cases.1,2 Since last spring, the United States has experienced an unprecedented amount of influenza activity, which is largely attributable to the circulation of the H1N1 virus. A recently published document found that the pattern in the United States is similar to patterns seen in selected comparator Southern Hemisphere countries (Argentina, Australia, Chile, New Zealand, and Uruguay) with regard to virological data (including antigenic characteristics), types of at-risk affected populations, socioeconomic impact, and community interventions used.3
To better understand the clinical and epidemiological presentation of the illness caused by the new H1N1 virus, it is helpful to place the current outbreak in the context of past outbreaks of "swine flu" or H1N1 infections seen in the 20th century. The H1N1 virus that is causing the current influenza pandemic is a quadruple reassortment influenza A virus that is considered by many experts to be a "fourth-generation descendant of the 1918 virus."4 An important 2007 analysis of all previous human cases of swine-origin influenza (from 1958 to 2005) described 37 civilian cases; 22 (61%) involved exposure to pigs, and human-to-human transmission was considered possible in 5.5
An analysis of 11 cases of sporadic reassortment swine H1N1 or H1N2 influenza reported to the CDC during a more recent period (2005 to 2009) revealed that in only 7 cases had the patients reported exposure to pigs before becoming ill. Although there were no associated fatalities, all patients had cough, 9 patients (82%) had fever, 6 (55%) had headache, and about one-third had diarrhea.6
Recent reports indicate that persons older than 50 years appear to be relatively less affected by the current H1N1 virus.7 This may be because they have some immunity as a result of having been exposed to a similar strain that circulated before 1957, the year of the H2N2
Presentation. H1N1 influenza appears to have an incubation period of 1 to 4 days, with an overall range of 1 to 7 days, although the exact duration is not known and likely variable. The period of viral shedding, during which viral particles may be released, making the patient potentially contagious, extends from 1 day before symptoms develop to about a week after illness onset. Children may shed for even longer periods.9
The most common presentation of patients with H1N1 influenza includes the cardinal symptom of fever (temperature usually about 38°C [100.5°F] or higher), along with cough and/or sore throat and/or myalgias. In fact, the syndrome of influenza-like illness (ILI) is defined as "fever with cough or sore throat." Overall, the signs and symptoms of H1N1 infection and seasonal influenza are similar.10 Although it may vary according to many factors, acute onset of cough plus fever has an overall predictive value for laboratory-confirmed influenza of 79% to 88%, especially during peak periods of influenza virus circulation.11 Bear in mind, however, that clinicians are likely to see a range of scenarios, including patients with afebrile influenza (seasonal or H1N1)—especially in children and the elderly—and, conversely, patients infected with other viral pathogens who present with mild to moderate fever. A wide spectrum of illness may occur, including respiratory arrest and neurological illness (including seizures). Secondary bacterial pneumonia (caused by Streptococcus pneumoniae and Staphylococcus aureus [both susceptible and methicillin-resistant strains]) with respiratory collapse has been recently reported, especially in children. A high index of suspicion is required to diagnose and treat such bacterial complications of influenza virus infection. 12,13
Other symptoms that may be seen in H1N1 influenza include chills, headache, fatigue, and GI symptoms (such as nausea, vomiting, and diarrhea). The presence of GI symptoms is a distinctive feature of H1N1 influenza (as compared with seasonal influenza) and may be seen in elderly patients, infants, and immunocompromised persons. Presenting symptoms such as conjunctivitis would be uncommon for H1N1 or seasonal influenza.
Physical examination findings. Although of limited benefit in the diagnosis of influenza, a physical examination may guide the clinician in determining the following:
•Which relevant organ systems are involved in the clinical illness.
•Whether there are other active underlying or concurrent diseases.
•The general severity of the illness and degree of structural or functional impairment.
All of this information can assist the clinician with triage decisions, such as whether to hospitalize a patient or manage his or her illness on an outpatient basis.
It is important to carefully assess the vital signs and to use such findings as tachypnea, tachycardia, fever, hypothermia, or hypotension to determine the acuteness of the patient's illness. In addition, carefully note the patient's appearance (healthy, chronically ill, or "toxic"); significant respiratory findings (eg, pharyngeal injection/exudate, nasal discharge, evidence of head and neck syndromes [eg, otitis, sinusitis], rales, consolidation, rhonchi, pleural rubs); lymphadenopathy; abdominal findings; and any neurological abnormalities (eg, altered mental status). Signs of disease severity in children can include apnea, cyanosis, lethargy, or irritability.
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10. Novel Swine-Origin Influenza A (H1N1) Virus Investigation Team; Dawood FS, Jain S, Finelli L, et al. Emergence of a novel swine-origin influenza A (H1N1) virus in humans [published correction appears in N Engl J Med. 2009;361:102]. N Engl J Med. 2009;360:2605-2615.
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