HONG KONG - An apparently new strain of influenza type-A virus known only to infect aquatic birds has been isolated here in a 3-year-old boy with Reye syndrome who died of acute respiratory distress.
World Health Organization (WHO) officials said this is the only case so far in humans. Although no more instances of type A (H5N1) virus have been isolated in humans, efforts are being made to determine whether other Chinese may have been infected with this strain. A WHO-sponsored team from the Centers for Disease Control and Prevention (CDC) are in Hong Kong conducting extensive investigations to determine its public health impact.
"There is no indication that this strain has spread from person-to-person yet, so there is no need for special measures to be taken immediately," said Daniel Lavanchy, MD, medical investigator for WHO's Division of Emerging and other Communicable Diseases Surveillance and Control. "But this should be considered a wake-up call for epidemiologists."
Viruses carrying the H1N1, H2N2, and H3N2 combinations were responsible for the Spanish flu of 1918-1919 that killed an estimated 20 million people, the Asian Flu of 1957 and Hong Kong flu in 1968. The recent discovery of an influenza type A virus of the H5N1 variety has sparked discussion on the potential for a new human influenza pandemic.
"Trying to predict the next pandemic is like trying to accurately predict the weather," said Dominick Iacuzio, MD, program officer at the Infectious Disease Branch of the National Institutes of Health (NIH). "So, the question is what is preventing another pandemic from coming - well, truthfully nothing. We've seen new strains in the past and there's no reason why we should not see them now or in the future."
The contribution of the influenza A H5N1 infection to the child's disease is not yet clear, but the virus identification is important as it is the first documented isolation of this subtype from humans. H5N1 influenza viruses can cause lethal disease among fowl which can decimate entire poultry flocks. Health officials say that so far these virus strains have not been identified in pigs, which is where the lethal 1918 strains originated. But the key to understanding how virulent the strain is lies in its genetic coding.
For example, the chicken influenza virus of the 1980s had three amino acids at a location where the hosts enzymes had to break a protein for the virus to infect a cell. At that particular spot there should have been only one amino acid present. Investigators are still looking at the possibility that the boy could have died from a more virulent version of this, but have made no definitive links between the genetic codings.
CDC investigators in Hong Kong are looking into several possibilities as how the boy may have contracted so rare an influenza strain. The boy belonged to a pre-school where the teachers had set up a small petting zoo that contained five chickens and eight ducks. According to investigators, one of the chickens died shortly before the boy's symptoms appeared, which could explain how it was so easily transmitted.
On the fifth day of his hospitalization, the 3-year-old died suddenly in the intensive care unit of Victoria Hospital in Hong Kong with a diagnosis of Reye syndrome complicated by acute influenza pneumonia and respiratory distress syndrome. A series of x-rays and liver biopsies revealed progressive viral pneumonia. But doctors said they could not detect any underlying immunodeficiency or cardiopulmonary disease during the boy's autopsy.
"Typically, new influenza viruses pass through and are genetically modified in other mammals, like pigs, before reaching humans. A unique feature of this new virus of the H5N1 subtype is that it managed to cross the avian-human species barrier without prior adaptation in other mammalian species," said Robert G. Webster, PhD, chairman of the St. Jude Children's Research Hospital Department of Virology and Molecular Biology in Memphis. "Fortunately, there are no indications that more infections with this subtype have taken place in humans or that the virus has spread among humans. The subtype does not seem to be a pandemic, or the start of a world epidemic, at present."
So far, more than 460 blood samples have been taken from relatives and neighbors exposed to the victim and another 2,000-3,000 samples were taken from people with flu-like symptoms. But so far none have the strain that killed the Hong Kong boy. The CDC is developing a new test for detecting the virus and the Advisory Committee on Immunization Practices is holding talks on the new strain.
As the investigators try and solve their influenza mystery, in the tundras of Norway a team of Canadian-led international researchers are trying to unearth the Spanish flu virus from the graves of seven Norwegians who died in 1918. Because of the tremendously cold temperatures there, the scientists are hoping to find preserved specimens of the virus from the seven victims, who died while traveling on an Arctic passenger ship.
Peter Lewin, MD, of the Hospital for Sick Children in Toronto and one of the few experts in the field of medical archeology, told New York Magazine that a thawed Spanish flu virus could be just as deadly now as in 1918. It would be comparable to thawing out a smallpox virus found in permafrost, he said.
"It would cause tremendous danger if exposed because no one's immunized against smallpox anymore," he said.
For more information:
- Osterhaus, A. Webster, RG. Lim, WL. A Pandemic Warning. Nature. 1997; VOLUME:389; 554.
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