Two swine-origin influenza A (H3N2) virus strains that triggered febrile respiratory illness in two children this summer contain genetic material from the 2009 influenza A (H1N1) virus – a genetic reassortment that hasn’t been seen before, according to the Centers for Disease Control and Prevention.
The report coincides with the Food and Drug Administration’s approval of a new in vitro diagnostic kit for seasonal influenza and novel influenza A viruses with pandemic potential. The kit will be distributed at no cost to qualified international public health laboratories.
Both of the H3N2 cases were reported in August. There were no epidemiologic links between the cases, and no other human infections with the virus have been detected, although investigations are ongoing, according to the CDC’s Sept. 2 report (MMWR 2011;60:1-4).
The first case involved a boy younger than 5 years, who has since recovered. The Indiana State Department of Health Laboratories reported the case Aug. 17.
The child, who had received flu vaccine in September 2010, presented in July 2011 with fever, cough, shortness of breath, diarrhea, and sore throat. A respiratory specimen taken at a local emergency department tested positive for influenza A (H3). Further testing of the specimen, including testing by the CDC, confirmed swine-origin influenza A (H3N2).
The child had no direct exposure to swine, but a caretaker of the child reported direct contact with asymptomatic swine in the weeks before onset of the boy’s illness.
On Aug. 24, the Pennsylvania Department of Health reported the second case, which involved a girl younger than 5 years who also has completely recovered. That child also had received influenza vaccine in September 2010, and presented in August 2011 with acute onset of fever, nonproductive cough, and lethargy.
A nasopharyngeal swab at a local emergency department was positive for influenza A. Additional testing, including genome sequencing by the CDC, confirmed the virus as swine-origin influenza A (H3N2).
The girl had recently visited an agricultural fair, where she had direct exposure to swine and other animals.
The two viruses are similar but not identical, the CDC reported.
"Seven of the eight gene segments, including the hemagglutinin (HA) and neuraminidase (NA) genes, are similar to those of swine H3N2 influenza viruses circulating among U.S. pigs since 1998 and previously identified in the eight other sporadic cases of human infections with swine-origin influenza A (H3N2) viruses in the United States since 2009," the report’s authors noted.
There’s a notable difference from those eight earlier cases, however: The two new viruses have a matrix gene from the 2009 influenza A (H1N1) virus, which has replaced the matrix gene found in the previous eight swine-origin infections in humans.
Although reassortment between swine influenza and 2009 influenza A (H1N1) has been reported in pigs, that particular genetic combination of swine influenza virus segments is unique, and has not been previously reported in swine or humans.
Researchers have since identified two additional influenza A (H3N2) isolates from swine containing the M gene from the 2009 influenza A (H1N1) virus. Those isolates are undergoing genome sequencing to characterize their genetic composition.
Although nothing is known about the new viral strains’ ability to transmit efficiently in humans or swine, or between swine and humans, they have been found to be resistant to amantadine and rimantadine. They are susceptible, however, to the neuraminidase inhibitor drugs oseltamivir and zanamivir.
The CDC offered clinicians the following diagnostic and treatment recommendations:
– If influenza virus infection is suspected in an individual with recent exposure to swine, a nasopharyngeal swab should be obtained for timely diagnosis at a state public health laboratory.
– Empiric neuraminidase inhibitor antiviral treatment should be considered to quickly limit potential human transmission.
New Test Could Speed Diagnosis
The newly approved diagnostic test – the CDC Human Influenza Virus Real-Time RT-PCR Diagnostic Panel – utilizes a three-module design that incorporates and streamlines previous versions of the CDC’s two separate FDA-cleared diagnostic test kits, according to a CDC statement.
The first module identifies and distinguishes influenza A and B viruses. The second module classifies influenza A viruses by subtype, such as H1N1, H3N2, or 2009 H1N1. The third module detects highly pathogenic avian influenza A (H5N1) viral infection in human respiratory tract specimens.
CDC officials are hopeful that the test, which uses specimens from a patient’s upper or lower respiratory tract, will improve the ability to make quick diagnoses.
"As the spread of the H1N1 pandemic slowed last year, we conducted an end-to-end review of our nation’s medical countermeasure enterprise, which showed a clear need for better diagnostic tests," Dr. Nicole Lurie, assistant secretary for preparedness and response in the Department of Health and Human Services, said in a statement.