CIDRAP Update: Novel H1N1 Influenza (Swine Flu)

Last updated August 14, 2009
Excerpts:

Information about the 2009 Novel H1N1 Virus

The 2009 novel A H1N1 virus appears to be of swine origin and contains a unique combination of gene segments that has not been identified in the past (see References: Garten 2009, Zimmer 2009).

* The NA and M gene segments are in the Eurasian swine genetic lineage; they were originally derived from a wholly avian influenza virus and likely entered the Eurasian swine population in 1979. Until emergence of the current novel H1N1 strain, these gene segments had not been identified outside Eurasia.
* The HA, NP, and NS gene segments are in the classical swine lineage; they likely entered the swine population around 1918 and are common in North America.
* The PB2 and PA gene segments are in the swine triple reassortant lineage; viruses of this lineage entered pigs in North America around 1998. Viruses that seeded this lineage were originally of avian origin.
* The BP1 gene segment is in the swine triple reassortant lineage and was seeded in pigs from humans also around 1998; this virus was also originally from an avian source.

A recent molecular analysis of the novel H1N1 virus demonstrates that the virus possesses a distinctive evolutionary trait (genetic distinctness) that may be characteristic in pig-human interspecies transmission of influenza A (reported cases of similar reassortant viruses of swine origin occurred in Iowa, Maryland, and Wisconsin between 1991 and 2006) (see References: Nava 2009).

Clinical findings

From April 15 through May 5, 642 confirmed cases of novel H1N1 infection were identified in the United States (see References: Novel swine-Origin Influenza A [H1N1] Virus Investigation Team. In patients for whom clinical information was available, the most common presenting symptoms were fever (371 of 394 [94%]), cough (365 of 397 [92%]), and sore throat (242 of 367 [66%]). Diarrhea was present in 82 of 323 patients (25%) and vomiting in 74 of 295 patients (25%). Of the 399 patients for whom hospitalization status was known, 36 (9%) required hospitalization. Two patients died; one was a 23-month-old child and one was a pregnant 33-year-old woman.

In a report from the WHO dated May 22, 2009, key clinical features of laboratory-confirmed cases for which data are available include the following (see References: WHO 2009: Human infection with new influenza A [H1N1] virus: clinical observations from Mexico and other affected countries, May 2009):

• Most patients appear to have uncomplicated, typical influenza-like illness and recover spontaneously. The most commonly reported symptoms include cough, fever, sore throat, malaise, and headache. Fever has been absent in some patients.
• Almost one half of cases in the United States requiring hospitalization as well as 21 (46%) of 45 fatal cases in Mexico for which data are available involved underlying conditions, including pregnancy, asthma, other lung diseases, diabetes, morbid obesity, autoimmune disorders and associated immunosuppressive therapies, neurologic disorders, and cardiovascular disease.
• Among 20 pregnant women with H1N1 in the United States, three required hospitalization and one of these died.
• Among 45 fatal cases in Mexico, 54% of patients were previously healthy and most were 20 to 59 years of age. The median time from symptom onset to death was 10 days (range, 2 to 33 days). The clinical course for fatal cases in Mexico has been characterized by:
• Severe pneumonia with multifocal infiltrates (including nodular alveolar and, less frequently, basilar opacities) on chest x-ray (bacterial coinfections were documented in three fatal cases)
• Rapid progression to acute respiratory distress syndrome (ARDS) and renal or multiorgan failure (24% of fatal cases)

Another study from Mexico found a significant increase in the rate of severe pneumonia between March 24 and April 29, 2009 (during the time of peak novel H1N1 influenza activity), with a shift in the age distribution to a younger segment of the population. The age-group 5 to 59 represented 87% of deaths from severe pneumonia compared to approximately 17% on average during other influenza epidemic periods (see References: Chowell 2009). A case series of 18 patients with pneumonia caused by H1N1 influenza who were hospitalized in the Mexico City area during March and April 2009 reported that 12 patients required mechanical ventilation and seven died (see References: Perez-Padilla 2009); most patients were previously healthy adults.

These findings clearly demonstrate that novel H1N1 can cause severe disease and death in otherwise healthy persons.

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