Description of Swine Flu from the CDC

Clinical Features of H1N1 Influenza

Typical Signs and Symptoms

The incubation period for H1N1 influenza is 1-4 days, possibly as long as 7 days. The clinical features of influenza are well known and include:

• Sudden onset of fever (usually high);
• Headache;
• Extreme tiredness;
• Dry cough;
• Sore throat;
• Runny nose; and
• Muscle aches and stomach symptoms — more common in children.

(CDC. Interim guidance for clinicians on identifying and caring for patients with swine-origin influenza A (H1N1) virus infection. June 2009. Available at: http://www.cdc.gov/h1n1flu/identifyingpatients.htm Accessed September 16, 2009.)

The symptoms of pandemic H1N1 influenza of 2009 are essentially the same as the seasonal flu, although some have noted an increased frequency of gastrointestinal symptoms, including vomiting and diarrhea, and others have noted the absence of fever in a significant number with virologically proven cases.

The CDC defines cases as influenza-like illness (ILI) if there is fever of ≥100° F (37.8° C) plus cough and/or sore throat in the absence of a known cause other than influenza. Another category is acute respiratory illness (ARI), defined by the presence of 2 of the following 4 symptoms: fever, cough, sore throat, or rhinorrhea. In the outbreak of pandemic influenza in New York City, 95% of virologically proven cases satisfied the ILI definition. (CDC. Swine-origin influenza A (H1N1) virus infections in a school — New York City, April 2009. MMWR Morb Mortal Wkly Rep Dispatch. 2009;58:1-3. Available at: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm58d0430a1.htm Accessed September 25, 2009.)

Patients with 2009 influenza A H1N1 infections have higher rates of gastrointestinal symptoms and lack of fever compared with those who have seasonal flu. Most patients have mild symptoms, but a small subset of previously healthy young adults have severe pulmonary disease that progresses to acute respiratory distress syndrome (ARDS); this may occur with or without underlying conditions.

Symptoms in virologically confirmed cases. During an outbreak of H1N1 in a New York City high school, a sample of New York City school students (median age, 15 years) with virologically confirmed cases were interviewed about their symptoms by telephone. They reported:

• Cough (98%);
• Subjective fever (96%);
• Fatigue (89%);
• Headache (82%);
• Sore throat (82%);
• Abdominal pain (50%);
• Diarrhea (48%);
• Dyspnea (48%); and
• Joint pain (46%).

The measured mean peak fever in this group was 102.2° F. (CDC. Swine-origin influenza A (H1N1) virus infections in a school — New York City, April 2009. MMWR Morb Mortal Wkly Rep Dispatch. 2009;58:1-3. Available at: http://www.cdc.gov/mmwr/preview/mmwrhtml/mm58d0430a1.htm Accessed September 25, 2009.)

Case Definitions for H1N1 Influenza

(CDC. Interim guidance for clinicians on identifying and caring for patients with swine-origin influenza A (H1N1) virus infection. June 2009. Available at: http://www.cdc.gov/h1n1flu/identifyingpatients.htm Accessed September 16, 2009.)

• Confirmed case: Patient with ILI plus laboratory evidence confirmed by real-time RT-PCR or viral culture;
• Probable case: ILI plus laboratory test positive for influenza A and negative for human H1 and H3 by RT-PCR; and
• Optional: ILI without negative H1N1 test and (1) previously healthy person > 65 years hospitalized for ILI; (2) epidemiologic link to confirmed or probable case in past 7 days; or (3) ILI plus travel to a state or country with confirmed or probable cases.

Complications of H1N1 Influenza

• Exacerbation of underlying chronic disease;
• Complications related to the upper airways, including sinusitis or otitis;
• Pulmonary complications, including bronchitis, asthma (sometimes with status asthmaticus), and acute exacerbations of chronic bronchitis; and
• Miscellaneous conditions, including cardiac (myocarditis and pericarditis), myositis, rhabdomyolysis, central nervous system complications (encephalopathy, encephalitis, seizures), toxic shock syndrome, and secondary bacterial pneumonia.

Severe complications of H1N1 Influenza. In June 2009, the University of Michigan reported severe pulmonary complications of 2009 H1N1 influenza infection in 10 patients with a median age of 49 years. All 10 patients were referred for severe hypoxemia, ARDS, and inability to oxygenate with conventional ventilation methods. All had severe multilobar pneumonia on x-ray, none had evidence of bacterial pneumonia, and 4 had CT scan-confirmed pulmonary embolism. Lab findings included leukocytosis in 5 (median WBC 9500/mm³), elevated AST levels (41-109 IU/L) in all 10, and elevated CPK levels (51-6572 IU/L) in 6; none had evidence of disseminated intravascular coagulation. The major risk factor was obesity in 9 and morbid obesity (BMI > 40) in 7. All 10 required advanced mechanical ventilation with high-frequency oscillatory or bilevel ventilation with mean airway pressures of 32-55 cm H₂O. Two required veno-venous extracorporeal membrane oxygenation (ECMO) support and 6 required dialysis. At the time of the report, 3 had died, 1 was still on ECMO, 1 was still on mechanical ventilation, and 5 had been transferred back to referring institutions. (CDC. Intensive care patients with severe novel influenza A (H1N1) virus infection — Michigan, June, 2009. MMWR Morb Mortal Wkly Rep. 2009;58:749-752.)

Neurologic complications. Neurologic complications were reported in 4 children ages 7-17 years with 2009 H1N1 influenza A. Findings included seizures in 2 children, encephalitis in 2, and ataxia in 1. All recovered without neurologic sequelae. The editorial comment in this report noted that the neurologic disease in these 4 patients was less severe than what has been described in previous reports of seasonal flu. (CDC. Neurological complications associated with novel influenza A (H1N1) infection in children — Dallas, Texas, May 2009. MMWR Morb Mortal Wkly Rep. 2009;58:773-778.; Maricich SM, Neuf JL, Lotze TE, et al. Neurologic complications association with influenza A in children during the 2003-2004 influenza season in Houston, Texas. Pediatrics. 2004;114:e626-e633.; Morishima T, Togashi T, Yokota S, et al. Encephalitis and encephalopathy associated with an influenza epidemic in Japan. Clin Infect Dis. 2002;35:512-517.)

Related Risk for Infection, Hospitalization, and Lethal Outcome

Age-related risk. These data are shown in Table 1.

Table 1. Rates for H1N1 for May-July 2009 by Age

^a % of total deaths. Age data not available for 15%.
Rate expressed /100,000 population

US age data

(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. N Engl J Med. 2009;360:2605-2615.)

• Median age of confirmed cases: 12 years
• Median age of hospitalized cases: 20 years
• Median age of lethal cases: 37 years