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New Swine Flu Viruses Identified in 2 Children in U.S.

Last month 2 children in Indiana and Pennsylvania were diagnosed with novel strains of swine-origin influenza A (H3N2), according to a September 2, 2011, early online edition of the CDC's Morbidity and Mortality Weekly Report. Both children with the virus -- which is distinct from the H1N1 pandemic influenza A identified in 2009 -- had fevers and other typical flu symptoms, but recovered without complications.

Influenza A viruses are endemic in many species, including humans, swine, and wild birds, and sporadic cases of transmission of between humans and animals can occur, the report authors noted as background. Genetic analysis can distinguish animal-origin flu viruses from typical seasonal human influenza viruses that cause annual epidemics.

The report describes 2 cases of febrile respiratory illness caused by swine-origin influenza A (H3N2) identified on August 19 and August 26. The first case involved a boy younger than 5 years in Indiana. He experienced fever, cough, shortness of breath, diarrhea, and sore throat on July 23 and was taken to a local emergency department. The child was discharged without antiviral treatment but returned to the ED the next day and was hospitalized for pre-existing chronic health problems; he was again sent home on July 27 and has since recovered from this illness. After a respiratory specimen tested positive for influenza A (H3), the Indiana State Department of Health identified a suspect swine-origin influenza A (H3N2) virus on August 17, which was confirmed by the CDC on August 19.

While the boy had no known direct exposure to pigs, a caretaker who provided care 2 days before the boy became ill did report direct contact with asymptomatic swine in the preceding weeks. Neither the caretaker nor any member of the boy's family nor any close contacts have since developed respiratory illness.

The second case involved a girl, also younger than 5, in Pennsylvania. This child experienced acute onset of fever, nonproductive cough, and lethargy on August 20. She too was taken to a local hospital ED, tested positive for influenza A, and was discharged the same day with no antiviral therapy. The girl has since completely recovered from this illness. On August 23 the Pennsylvania State Department of Health identified a suspected swine-origin influenza A (H3N2) virus, which was confirmed by the CDC on August 26.

The girl had visited an agricultural fair on August 16, where she had direct exposure to pigs and other animals. As with the previous case, none of her family members or close contacts has since come down with a similar illness, though the health department is investigating whether other fair-goers might have been infected.

Public health officials have not identified any epidemiologic link between the Indiana boy and the Pennsylvania girl. Genetic sequencing revealed that the viruses are similar to each other, but not identical. Out of 8 flu genes, 7 are similar to those of swine H3N2 influenza A viruses circulating among pigs in the U.S. since 1998, which have been responsible for 8 other swine flu infections in humans since 2009.

The 1 difference is a matrix gene acquired from the 2009 influenza A (H1N1) virus, which is thought to have been transmitted from humans to pigs. This particular genetic combination is unique and has not been reported previously in either swine or humans, according to the report authors. Genotypic testing indicates that the newly identified H3N2 viruses are resistant to amantadine (Symmetrel) and rimantadine (Flumadine), but susceptible to oseltamivir (Tamiflu) and zanamivir (Relenza).

According to an editorial note, the CDC has received 3-5 reports of human swine flu infection per year since 2007. Between December 2005 and November 2010 there were 21 reported cases of human infection with swine-origin flu viruses: 12 with influenza A (H1N1), 8 with influenza A (H3N2), and 1 with influenza A (H1N2). Only 6 of these 21 cases occurred in people known to have direct exposure to pigs, 12 reported being near pigs, and 2 cases had contact with another ill person who had exposure with pigs, suggesting probable human-to-human transmission.

"Although the vast majority of human infections with animal influenza viruses do not result in human-to-human transmission, each case should be investigated fully to ascertain whether these viruses are transmitted among humans and to limit further exposure of humans to infected animals, if infected animals are identified," the authors wrote.

"Clinicians should consider swine-origin influenza A virus infection as well as seasonal influenza virus infections in the differential diagnosis of patients with febrile respiratory illness who have been near pigs," the editorial advised.

"Non-human influenza virus infections rarely result in human-to-human transmission, but the implications of sustained ongoing transmission between humans is potentially severe; therefore, prompt and thorough identification and investigation of these sporadic human infections with non-human influenza viruses are needed to reduce the risk for sustained transmission," the report concludes.

9/6/11

Reference

K Nalluswami, A Nambiar, P Lurie, et al. Swine-Origin Influenza A (H3N2) Virus Infection in Two Children -- Indiana and Pennsylvania, July-August 2011. Morbidity and Mortality Weekly Report 60;1-4 (full text). September 2, 2011 Early Release.