H5N2 Highly Pathogenic Avian Influenza Appears for the First Time at Taipei's Poultry Wholesale Market
After more than a year of quiet, highly pathogenic H5N2 avian influenza resurfaced: on May 2, the Council of Agriculture's Bureau of Animal and Plant Health Inspection and Quarantine announced that a case of a chicken infected with highly pathogenic H5N2 avian influenza had been detected at the Taipei poultry wholesale market. The infected bird was a red-feathered native chicken (Taiwan red-feathered free-range chicken) from Yunlin County. This is the first time a chicken with highly pathogenic H5N2 avian influenza has been found at a poultry wholesale market in the country, and the bureau has already traced back to the farm to carry out environmental disinfection and investigation.
Chang Shu-hsien, director-general of the Bureau of Animal and Plant Health Inspection and Quarantine, said that although all the chickens at that native chicken farm had already been sent to market and sold out, and no sick chickens were found at the breeding site, the H5N2 virus cannot withstand high temperatures, so as long as poultry meat is cooked thoroughly, consumers need not worry about contracting avian influenza from eating chicken. However, poultry farms, sorting facilities and transport operators must implement the required disinfection of poultry houses, transport vehicles, chicken cages, and egg boxes and egg trays.
Chang Shu-hsien said the reason a diseased chicken was found at the poultry market this time was that, to strengthen avian influenza surveillance and early warning, the bureau this year specifically stepped up sampling and testing of chickens at live poultry trading venues such as wholesale markets and sorting facilities. On April 15, the bureau collected and tested 100 chicken serum samples, 100 fecal swabs and samples from 6 chickens at the Taipei poultry wholesale market, and one of the samples tested positive for the H5N2 subtype of avian influenza virus.
Tsai Hsiang-jung, director-general of the Council of Agriculture's Animal Health Research Institute, said the H5N2 virus strain in this sample was cultured and multiplied by inoculation into embryonated chicken eggs, and then experimentally inoculated intravenously into chickens; all eight inoculated chickens died, a mortality rate of 100%. Analysis of the basic amino acid sequence at the HA protein cleavage site of the virus showed, for the first time, six basic amino acids, more than the four commonly seen in the past. According to the definition of the World Organisation for Animal Health (OIE), when the intravenous inoculation mortality rate in chickens reaches 75% or above, it can be judged as highly pathogenic avian influenza. Therefore, this case in which H5N2 avian influenza was detected was finally judged to be highly pathogenic.
Highly pathogenic, low mortality: the H5N2 virus in Taiwan, China is peculiar
Regarding this sample, Hsieh Kuai-le, professor emeritus in the Department of Veterinary Medicine at National Chung Hsing University, considers it very peculiar. He said that, according to the epidemiological investigation of the affected chicken farm, the Yunlin farm raised about 20,800 red-feathered native chickens, and clinically the daily mortality rate of the chickens was only 0.1%, far below the abnormal mortality rate of 1% commonly seen when highly pathogenic avian influenza generally occurs, and also below the 0.4% abnormal mortality standard set by the United States and the Netherlands. But why, when the virus shows six basic amino acids, was the clinical abnormal mortality rate of the chickens less than 0.1%? This phenomenon deserves further study.
The Bureau of Animal and Plant Health Inspection and Quarantine said that the highly pathogenic H5N2 virus strain found this time is the same as the virus strain from Taiwan, China's earliest series of low-pathogenic H5N2 avian influenza outbreaks between 2003 and 2004, and differs from the H5N2 virus strains found in neighboring mainland China. It is a "recombinant virus with surface antigens of the American virus group and internal structures of the Eurasian virus group."
Substandard avian influenza vaccines open the virus's Pandora's box
According to previous research by the Animal Health Research Institute, the H5N2 virus strain first discovered in Taiwan, China, in 2003-04, after analysis of viral evolution evidence and major viral antigens, is considered to be highly similar to and share the same characteristics as the Mexican virus strain, the main strain used worldwide to make avian influenza vaccines; it is not a virus strain produced under natural conditions, so it is inferred to have come from substandard vaccines, bringing the H5N2 virus strain into Taiwan, China.
In addition, what worries researchers at the Animal Health Research Institute most is that six internal gene segments of the H5N2 avian influenza virus strain found in Taiwan, China, are similar to the H6N1 virus strain that had already adapted to Taiwan, China's chicken flocks as early as 1972. If the two viruses circulate among chicken flocks, this may accelerate their adaptation to chickens and increase the virus's replication capacity.
This Taiwan, China-specific H5N2 avian influenza virus strain, born when the American H5N2 met Taiwan, China's H6N1, has now taken root and gradually evolved from low pathogenicity to high pathogenicity. Regarding this, Hsieh Kuai-le, professor emeritus in the Department of Veterinary Medicine at National Chung Hsing University, felt worried, and at a symposium held by National Taiwan University's College of Public Health on April 24 of the year before last, mentioned the possibility of vaccinating chickens against avian influenza.
However, at the bureau's press conference on May 2 this year, when Hsieh Kuai-le was asked whether he recommended vaccinating chickens against avian influenza, he only replied that the current government policy prohibits vaccination and did not answer the question directly. Chang Shu-hsien, the bureau's director-general, who was at his side, quickly stressed that, to give the poultry industry a chance to export in the future, the current policy still prohibits vaccinating chickens against avian influenza, requires poultry farms to implement self-directed disease prevention, and that once a highly pathogenic avian influenza outbreak is found, culling is still adopted, to avoid the virus multiplying in Taiwan, China's environment.