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H5N1 Bird Flu Silently Speading in Northern Vietnam
June 26, 2005
>> Laboratory tests in Vietnam and abroad have indicated that the antigen structure of bird flu virus strain H5N1 in the country is changing, local newspaper Labor Monday quoted a Vietnamese Health Ministry report as saying.
The change, which might increase the possibility of bird flu infections on human, explains why people in Vietnam have still been infected with H5N1 in recent months, although it has detected no major outbreaks.
The mutation also makes it more difficult for health agencies to identify high-risk areas, since poultry infected with H5N1 with a change in the antigen structure do not exhibit the disease's symptoms, said the report.<<
The above comments suggest H5N1 is silently spreading in northern Vietnam in bird and human populations. Last fall WHO had issued a warning on asymptomatic infections of water fowl. H5N1 from fatal cases in 2004 was used ti infect ducks. The laboratory infected ducks did not display symptoms, yet harbored large amounts of H5N1 in their intestines and excreted high concentrations of virus that were unusually stable.
This season the H5N1 in southern Vietnam and Cambodia was almost universally fatal. In contrast, the H5N1 in the north produced a milder disease. Some data suggested that changes in the sequences recognized by primers might generate false negatives in southern and northern Vietnam.
Comments over the weekend appeared to support those data. The statement above suggests that the difficulties in detecting H5N1 in the lab due to genetic changes were compounded by the announcement that the H5N1 in the north also was asymptomatic in birds.
Thus, H5N1 in the north produces no symptoms in water fowl and mild symptoms in people. Since the detection tests are not as accurate this season, H5N1 can silently spread.
This type of spread would explain why 1000 serum samples were collected. The assay for the virus was generating false negatives, and many of the suspect cases recovered quickly. The infections could still be determined by measuring H5 antibodies. Media reports indicated that the samples were sent to the CDC in Atlanta in April and results were expected by the end of that month.
Instead of announcing results, an urgent meeting was called for May 6-7 in Manila. Although the results were not publicly disclosed, statements on the evolution of H5N1 and possible changes in the receptor binding domain suggested that human-to-human spread was becoming more efficient and many of the collected serum samples from patients with mild disease would be expected to be positive.
Media reports suggest that western blot testing of these samples were positive. However, WHO declared these test results to be experimental requiring confirmation by a HI assay, which should have been done in Atlanta in April.
Although the positive western blot data has not been confirmed, positive data would be consistent with more clusters in the north which are large and extend over a longer time period. The alarming increase in admission is further cause for concern because the admitted cases have mild disease, suggesting an even larger number of unreported cases with slightly milder disease.
This H5N1 is silently spreading mild disease in human and asymptomatic infections in poultry, which would move the pandemic to phase 6. The seeding of the human population with H5N1 sets the stage for further recombination in the fall when migratory birds bring in new sequences, which will cause new problems.