New Strain of Swine Flu Investigated
Wednesday, April 22, 2009
Public health authorities are investigating two highly unusual cases of a previously unknown strain of swine flu that was found in the San Diego area late last month.
The cases occurred almost simultaneously in children who had no contact with pigs or each other, a scenario that raised the possibility that the illnesses may be the sign of an emerging pandemic strain of influenza.
More than 50 scientists and epidemiologists at the Centers for Disease Control and Prevention in Atlanta are studying the strain, and dozens of public health investigators in Southern California are looking for more cases among the those who had contact with the children.
"While we have a low index of suspicion that this is a pandemic, we're being very careful in our investigation to rule out every possibility," said Lyn Finelli, an epidemiologist in the CDC's influenza division.
Neither of the children -- a 10-year-old boy in San Diego County and a 9-year-old girl in Imperial County, just to the east -- was seriously ill. The cases were detected because both children were treated at clinics that took nose or throat swabs looking for influenza and passed the samples on to health department labs when they could not identify the strains.
"It was a very fortunate lightning strike," Finelli said yesterday.
Both children have recovered. The boy, however, took an airplane trip to Texas with his younger brother while at the tail end of his illness before it was known that he had an unusual strain of flu. Health officials in the Dallas area are looking for cases there, as well as among airline employees who assisted the two children, who traveled as "unaccompanied minors."
Public health officials in the two California counties -- both of which border Mexico -- are urging physicians and hospitals to look carefully for cases of flu and report any to local health departments.
Flu season, which officially ended this month, was mild this year. However, Stephen Munday, the health officer for Imperial County, said his jurisdiction saw cases late in March, although not enough to qualify as an outbreak. Whether any other cases involved swine flu is unknown at this point.
The Imperial County girl fell ill on March 28 with cough and a high fever, and the San Diego boy came down with similar symptoms, as well as vomiting, two days later. People in both households became sick before and after the children did, although health officials have not determined whether they also had swine flu.
Munday said his department has drawn blood from more than 20 people to be tested for antibodies to the swine flu strain. The antibodies would be a sign that those people were infected even if they never had symptoms. He said some had traveled into Mexico recently but would not describe them further.
"As of yet, we have not been able to come up with any explanation of why anyone would have swine flu," he said.
Molecular analysis of the virus suggests that it is the product of a rare event called a "gene reassortment." In a reassortment, two distinct strains of virus infect the same cell. The viruses take over the cell's genetic machinery to make copies of themselves, mingling the genes of the two strains to create a new, essentially hybrid, strain. Six of the eight genes in the new strain are from the North American lineage of swine flu, but two are from the Eurasian lineage.
The reassortment probably occurred in a pig sometime in the past decade. Both sets of genes are slightly different from those of their original lineage -- a sign that time has passed. However, it is unlikely that they have been in humans very long.
"If these viruses had been circulating at low levels in humans for several years, we probably would have detected them," said Nancy J. Cox, head of the CDC's influenza division.
The ability to find and identify rare strains of influenza virus has improved greatly in the past decade, spurred in part by the "bird flu" outbreak in Asia and the anthrax attacks of 2001.
In the past three years, the CDC has investigated 12 cases of human illness caused by swine flu strains. In 11 of the cases, however, the infected people had direct or indirect exposure to pigs.
In 1976, a strain of swine flu caused illness in 13 soldiers at Fort Dix in New Jersey, killing one.
Fearing a pandemic might be in the offing, the federal government ordered emergency production of a vaccine and made plans to administer it to millions of elderly and vulnerable Americans. Mass immunization was halted, however, when the virus did not spread and some vaccine recipients developed a rare neurological disorder called Guillain-Barre syndrome.