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H1N1 (Swine Flu) 2009 : Section 3BY: JGlassFNP-BC | Category: Health and Fitness | Post Date: 2009-11-14
BY: James Glass FNP, Wellmont Health System, Holston Valley Medical Center Part 1 | Part 2 | Part 3 | Additional Information on CDC Website DEFINITIONS Case definitions — The following case definitions have been provided by the United States Centers for Disease Control and Prevention: Influenza-like illness (ILI) is defined as fever (temperature of 100ºF [37.8ºC] or greater) with cough or sore throat in the absence of a known cause other than influenza. A confirmed case of pandemic H1N1 influenza A is defined as an individual with an ILI with laboratory-confirmed H1N1 influenza A virus detection by real-time reverse transcriptase (rRT)-PCR or culture. Pandemic H1N1 influenza A may be suspected in an individual who does not meet the definition of confirmed pandemic H1N1 influenza A, but has an ILI and an epidemiologic link. Full case definitions can be found at the CDC's website Mortality — Seasonal influenza results in higher mortality rates among patients with certain chronic medical conditions, as well as in pregnant women and those at the extremes of age. During the current H1N1 influenza A pandemic there have been higher mortality rates among patients with certain underlying medical conditions and in pregnant women. Since April 2009, 129 children in the US have died from laboratory-confirmed H1N1 influenza infection, and another 15 children have died with laboratory-confirmed influenza infection that was not subtyped. Few elderly individuals have been infected, which may be due to some degree of preexisting immunity in older individuals against antigenically similar influenza viruses. However, based on past experience, elderly individuals who are infected are still thought to be at increased risk for complications. In a surveillance study of 1088 probable or confirmed cases of pandemic H1N1 influenza A resulting in hospitalization or death in California, individuals ≥50 years of age had the highest mortality rates; this may have been the result of comorbidities, since 80 percent of patients in this age group had underlying medical conditions. One-third of deaths have occurred in individuals aged 25 to 49 in the US, which is a higher percentage than occurs with seasonal influenza. As of November 1, 2009, there have been over 482,300 laboratory-confirmed cases of pandemic H1N1 influenza A worldwide, including at least 6071 deaths. Most deaths have been related to respiratory failure resulting from severe pneumonia with multifocal infiltrates and acute respiratory distress syndrome. In the surveillance study of probable or confirmed cases of pandemic H1N1 influenza A resulting in hospitalization or death described above, 11 percent were fatal. Of 45 fatal cases in Mexico, 24 (54 percent) occurred in previously healthy individuals. In contrast, most of the deaths outside of Mexico occurred in individuals with underlying health problems. Approximately 6 percent of deaths caused by pandemic H1N1 influenza A virus in the US have occurred in pregnant women, although only 1 percent of the population is pregnant at any given time. In a cohort study of patients in Australia and New Zealand admitted to the ICU with confirmed pandemic H1N1 influenza A infection, older age, the presence of preexisting conditions, and a requirement for mechanical ventilation were independently associated with mortality. Variable severity of human infections with swine influenza viruses isolated before the 2009 pandemic has been reported in several studies, as illustrated by the following findings: In a review of 37 cases of human infections with swine influenza virus reported between 1958 and 2005, six cases (17 percent) resulted in death, all of which were due to pneumonia. Influenza virus was the only pathogen identified from the lungs in four patients; in one individual, Streptococcus viridans, Neisseria spp, and Klebsiella spp were also identified in addition to influenza virus. Between 2005 and January 2009, 12 cases of human infection with swine influenza virus were detected in the United States with no fatalities. An outbreak occurred among soldiers in Fort Dix, New Jersey in 1976, which involved up to 230 individuals but resulted in only one death. A fatal human infection with swine influenza virus complicated by pneumonia was also reported in a previously healthy pregnant woman in 1988. DIAGNOSIS — Guidelines for the diagnosis of pandemic H1N1 influenza A virus have been released by the United States Centers for Disease Control and Prevention (CDC). Updated recommendations can be found at the CDC's website . Clinicians in other countries should consult their individual health ministries for information about recommended diagnostic testing. Whom to test — Most patients with an uncomplicated influenza-like illness who reside in areas where influenza viruses are known to be circulating do not need to be tested for influenza infection. Recommendations regarding whom to test may differ by state or community. Patients in whom influenza testing should be considered include: Hospitalized patients with suspected influenza infection. Patients for whom a diagnosis of influenza will affect decisions regarding clinical care, infection control, or management of close contacts. In addition, influenza testing should be considered when conducting autopsies of individuals who died of acute illness in whom influenza was suspected. Specimens — To establish the diagnosis of pandemic H1N1 influenza A, an upper or lower respiratory sample should be collected. Appropriate specimens include: - Nasopharyngeal swab - Nasal aspirate, wash, or swab - Endotracheal aspirate (in intubated patients) - Bronchoalveolar lavage (BAL) fluid - Combined nasopharyngeal or nasal swab with oropharyngeal swab In patients with severe pneumonia who are suspected of being infected with influenza and who are intubated or undergoing bronchoscopy, lower respiratory samples (endotracheal aspirate or BAL fluid) should be obtained and tested for influenza infection. For proper specimen collection, instructions in the test's package insert should be followed. Furthermore, specimens should be obtained as soon as possible following the onset of symptoms. Swabs with a synthetic tip (eg, polyester or Dacron) and an aluminum or plastic shaft should be used. Swabs with cotton tips and wooden shafts are not recommended. Swabs made of calcium alginate are not acceptable. The collection vial in which the swab is placed should contain 1 to 3 mL of viral transport media. Specimens should be placed in viral transport media and placed on ice (4ºC) or refrigerated immediately for transportation to the laboratory. Once the samples arrive in the laboratory, they should be stored either in a refrigerator at 4ºC or in a -70ºC freezer. If a -70ºC freezer is not available, they should be kept refrigerated. Refrigerated samples should ideally be processed within 24 hours, and should not be stored for >72 hours. Diagnostic tests — Real-time reverse transcriptase (rRT)-PCR is the most sensitive and specific test for the diagnosis of pandemic H1N1 influenza A virus infection. Isolation of pandemic H1N1 influenza A virus using culture is also diagnostic, but culture is usually too slow to help guide clinical management. A negative viral culture does not exclude pandemic H1N1 influenza A infection. Several rapid antigen and immunofluorescent antibody tests are available for the diagnosis of influenza virus infection. However, the sensitivity of these tests varies widely, and although some assays are able to distinguish between influenza A and B viruses, they are not able to distinguish between pandemic and seasonal strains of H1N1 influenza A. Polymerase chain reaction — Nucleic acid amplification tests, such as real-time reverse transcriptase (rRT)-PCR, are the most sensitive and specific tests for the diagnosis of influenza virus infection. However, they may not be readily available and/or may require several days for processing since many hospitals and clinics must send samples to be processed at public health or commercial laboratories. Test performance depends on the individual rRT-PCR assay used. The United States Food and Drug Administration has authorized several rRT-PCR assays for the diagnosis of pandemic H1N1 influenza A infection under an Emergency Use Authorization. Rapid antigen tests — Clinicians may consider using rapid influenza antigen tests as part of their evaluation of patients suspected of having pandemic H1N1 influenza A, but results should be interpreted with caution. Certain rapid influenza antigen tests that are commercially available can distinguish between influenza A and B viruses, but cannot distinguish among different subtypes of influenza A (eg, pandemic H1N1 influenza A versus seasonal H1N1 or H3N2 influenza A). Confirmation of pandemic H1N1 influenza A infection can only be made by real-time reverse-transcriptase (rRT)-PCR or culture. The sensitivity of rapid antigen testing for pandemic H1N1 influenza A virus infection has ranged from 10 to 70 percent compared with rRT-PCR. Thus, a negative result does not rule out infection. The specificity of rapid antigen testing has generally been >95 percent, although in one study it was only 86 percent. Among 39 patients with pandemic H1N1 influenza A confirmed by rRT-PCR, 20 had a positive rapid antigen test using the QuickVue Influenza A+B (Quidel) assay (sensitivity 51 percent). Twelve of 19 patients who had seasonal H1N1 influenza confirmed by rRT-PCR had a positive rapid antigen test (sensitivity 63 percent). In the same study, the specificity of rapid antigen testing was 99 percent for patients with either the pandemic strain or a seasonal strain of H1N1 influenza A. Immunofluorescent antibody testing — Direct or indirect immunofluorescent antibody testing (DFA or IFA) can distinguish between influenza A and B, but does not distinguish among different influenza A subtypes. In one study, among 42 samples that were positive for pandemic H1N1 influenza A by real-time reverse-transcriptase polymerase chain reaction, 39 were positive by direct fluorescent antibody testing. However, a negative DFA or IFA does not exclude pandemic H1N1 influenza A infection since larger studies are required to define the sensitivity to detect this virus. Choice of test — As discussed above, most patients with an uncomplicated influenza-like illness who reside in areas where influenza viruses are known to be circulating do not need to be tested for influenza infection. However, among patients for whom a diagnosis of influenza will affect decisions regarding clinical care, infection control, or management of close contacts, it is reasonable to use a rapid antigen or immunofluorescence antibody test. In regions where the majority of circulating influenza viruses are known to be pandemic H1N1 influenza A, a positive result using one of these assays can be presumed to indicate infection with pandemic H1N1 influenza A. If identification of pandemic H1N1 influenza A is required, such as in pregnant patients and those with severe immunosuppression, then real-time reverse transcriptase polymerase chain reaction (rRT-PCR) testing should be performed. In addition, rRT-PCR testing should be performed in hospitalized patients with suspected influenza infection who have a negative rapid antigen or immunofluorescence antibody test. Influenza subtype testing with rRT-PCR or viral culture should also be prioritized for use in individuals who have died from suspected or confirmed influenza infection. The diagnostic tests for influenza are discussed in greater detail separately. 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