CDC reports swine influenza A infection in two California children

Two children that live in neighboring counties in southern California have a type of swine influenza A (H1N1) made up of genetic components that have not been reported among swine or human influenza viruses in the United States or elsewhere, researchers from the CDC reported yesterday.

The viruses from the two cases are closely related genetically and are resistant to amantadine and rimantadine.

Neither child — a 10-year-old boy who lives in San Diego County and a 9-year-old girl who lives in Imperial County — had contact with pigs, making human-to-human transmission a likely source, CDC researchers said. The children apparently had not been in contact with each other.

The boy presented to an outpatient clinic with cough, fever and vomiting on March 30, 2009. He received symptomatic treatment, all his symptoms resolved and he traveled to Dallas by plane with his family on April 3. Officials at the clinic took a nasopharyngeal swab, but initial testing was negative for human influenza subtypes H1N1, H3N2 and H5N1. The CDC determined the virus was swine influenza A (H1N1) on April 14.

The girl presented to an outpatient facility on March 28, 2009, with cough and fever on March 28. She was treated with amoxicillin/clavulanate potassium and an antihistamine; she has since recovered. Neither child had received influenza vaccine.

On April 17, CDC officials identified the second specimen from the girl as a similar type. Researchers said the hemagglutinin genes in these two types are similar to those of swine influenza viruses that have circulated among U.S. pigs since 1999; however two genes encoding for the neuraminidase and matrix proteins are similar to swine flu viruses of the Eurasian lineage.

CDC researchers noted that although this is not a new subtype of influenza A in humans, “concern exists that this new strain of swine influenza A (H1N1) is substantially different from human influenza A (H1N1) viruses, that a large proportion of the population might be susceptible to infection, and that the seasonal influenza vaccine H1N1 strain might not provide protection.”

Since 2005, the CDC has received 12 reports of human infection with swine influenza, most of which occurred in patients who either had direct exposure to pigs or reported being near pigs. Health officials said that increased surveillance and improved testing methods in recent years may be increasing the rate of reported cases.

CDC officials are contacting several contacts of the children, starting with close family members. Each child had two family members who were sick within the week of their illness, but they were not tested for influenza at the time.

Physicians are being asked to “consider animal as well as seasonal influenza virus infections in their differential diagnosis of patients who have febrile respiratory illness and who 1) live in San Diego and Imperial counties or 2) traveled to these counties or were in contact with ill people from these counties in the seven days preceding their illness onset, or 3) had recent exposure to pigs.” They added that physicians who suspect this virus should get a specimen and contact state health officials. – Colleen Zacharyczuk

MMWR. April 21, 2009. 58 (Dispatch);1-3

The concern stems from the fact that these children had no known exposure to pigs. This suggests that there may be human-to-human transmission, which would imply that further spread is possible.

There have been about 10 cases of swine influenza in humans since 2007 when it was first looked for in a systematic way. All but one of these cases had contact with swine. The main focus at the present time would be in the San Diego area where both of these cases occurred. They do not seem to be particularly virulent but increased human passage with the possibility of recombination could result in a virus with different characteristics.

–Philip Brunell, MD

Infectious Diseases in Children Chief Medical Editor