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Swiminization of H5N1 in Asymptomatic Indonesian Pigs
May 17, 2005
>> C.A. Nidom, the university researcher who discovered the infection in pigs, said he had found the virus in the blood of 10 pigs out of 20 he checked. A laboratory in Tokyo conducted the tests, he said.
"I found it in the snout to begin with, but I was doubtful whether this meant they were truly infected," he said in a telephone interview.
"So I then proved it by finding a matching strain in the blood."
Nidom said the samples were taken from pigs living 100 yards away from a chicken farm on Java island that was struck by bird flu last year. None of the pigs showed any signs of illnesses, he said. <<
The finding of H5N1 in asymptomatic pigs on a farm in Indonesia is cause for many concerns. Last fall WHO warned about H5N1 in asymptomatic ducks. The H5N1 in that case was isolated from fatal human infections in Vietnam. Since then there has been mounting evidence of H5N1 in asymptomatic ducks and more recently in asymptomatic chickens. By definition these virus are difficult to monitor because the host does not display symptoms. Since the host appears healthy, more virus can be shed for a longer time period and the host offers a provocative target for additional infections. Dual infections lead to the exchange of genetic information via reassortment and recombination, leading to increased genetic instability of more rapid evolution.
The H5N1 bird flu detected throughout eastern Asia has frequently been the Z genotype. Although these viruses have the same constellation of genes, there are regional polymorphisms that distinguish the isolates. There are some that are specific for Vietnam and others specific for Thailand, but a number that are found just in H5N1 isolates from Vietnam and Thailand. However, these polymorphism are present in other serotypes typically found in mammalian isolates, such as swine or humans. This acquisition of mammalian polymorphisms via recombination has been called swimanization and these polymorphisms are associated with the H5N1 that cause fatal infections in humans. Finding Z genotype H5N1 in swine raises the possibility is swimanization in Indonesia, leading to an increased ability of H5N1 to infect humans.
H5N1 infections of swine in Vietnam and Thailand have not been reported. However, testing of asymptomatic swine has not been reported in Vietnam. Similarly, proclamations by Thailand concerning the lack of H5N1 causing illness in pigs do not address H5N1 infections in asymptomatic pigs. Difficulties in detecting H5N1 or other viruses have been described previously. False negatives in PCR tests in both northern and southern Vietnam have been reported.
In addition, South Korea has had difficulty detecting human WSN/33 in swine. Some of these swine are reported infected with three distinct flu viruses, H1N1, WSN/33, H1N2 reassortant similar to the reassortants found in pigs in the United States, and H9N2 containing Korean avian genes.
The failure of government to monitor and report these infections is cause for additional concerns.
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