Avian influenza – situation in Viet Nam |
18 August 2004
Avian influenza: H5N1 detected in pigs in China
20 August 2004
Avian influenza - update: Implications of H5N1 infections in pigs in China
25 August 2004
******** Avian influenza – situation in Viet Nam *********
18 August 2004
In the present outbreak in Viet Nam, first reported last week, three fatal human cases of avian influenza have now been laboratory confirmed, two in the north and one in the south. For two of these cases, further testing has identified the H5N1 strain as the causative agent. The most recent case died on 6 August and no new cases have been identified since then.
With support from the Ministry of Health in Viet Nam, arrangements are under way to send specimens from these cases to a laboratory in the WHO Global Influenza Surveillance Network. The laboratory will perform gene sequencing and other analyses of the virus in order to yield information immediately relevant to assessment of the public health risk.
Studies will determine whether the virus responsible for these cases has mutated. It is particularly important to learn whether the H5N1 virus strain remains entirely of avian origin.
Following a meeting held earlier this week, health officials in Viet Nam have called for close cooperation between WHO and the country’s two institutes equipped to test specimens for avian influenza. These are the National Institute of Hygiene and Epidemiology in Hanoi and the Pasteur Institute in Ho Chi Minh City. This cooperation is expected to expedite the sharing of test results on any additional cases that might require investigation.
A small team of WHO staff will be traveling to Viet Nam over the weekend. The team will be working closely with the Ministry of Health to assess the present risk to public health and to work out plans to strengthen surveillance capacity for the detection of human cases.
******** Avian influenza: H5N1 detected in pigs in China ********
20 August 2004
A researcher from China’s Harbin Veterinary Research Institute has today presented initial evidence that pigs from farms in parts of China have been infected with the H5N1 strain of avian influenza. The findings, set out in a table and without further supporting data, were presented today at an international symposium on SARS and avian influenza held in Beijing.
WHO has requested confirmation and further details about this study.
Pigs are known to be susceptible to infection with avian influenza viruses. However, natural infection of pigs with the H5N1 strain has not been previously reported.
In order to assess the implications for human health, it is important to know whether the reported infections in pigs are rare events, possibly caused by contact between pigs and wild birds. Wild aquatic birds, which are the natural reservoir of all influenza A viruses, can carry the H5N1 strain without developing symptoms, and are known to excrete large quantities of the virus in their faeces.
A comparison of the H5N1 strain isolated in pigs with strains recently circulating in poultry populations in parts of Asia is needed to determine whether the virus is being passed directly from poultry to pigs. Evidence of direct transmission of H5N1 from poultry to large numbers of pigs would be of particular concern, as this would increase opportunities for a new influenza virus with pandemic potential to emerge.
Pigs have been implicated in the emergence of new influenza viruses responsible for two of the previous century’s influenza pandemics. Pigs have receptors in their respiratory tract that make them susceptible to infection with human and avian influenza viruses. If a pig is simultaneously infected with both a human and an avian influenza virus, it can serve as a “mixing vessel”, facilitating the exchange of genetic material between the two viruses in a process known as “reassortment”. The resulting new virus, which will not be recognized by the human immune system, will have pandemic potential if it retains sufficient human genes to allow efficient human-to-human transmission, and if it causes severe disease in humans.
Confirmation of H5N1 infection in pigs would add complexity to the epidemiology of this disease, but needs to be viewed in perspective. During the peak of the poultry outbreak of H5N1 in Viet Nam earlier this year, extensive testing of pigs on farms where poultry were heavily infected failed to find evidence of infection in pigs. In addition, Hong Kong authorities regularly perform random testing for the H5 avian influenza virus subtype in pigs imported from mainland China. No infection in pigs has been detected to date.
WHO, in collaboration with FAO and OIE, will be assessing the implications of reported H5N1 infection in pigs as further details become available.
******** Avian influenza - update: Implications of H5N1 infections in pigs in China *********
25 August 2004
Considering the widespread nature of the current H5N1 outbreak in Asia and the capability of influenza viruses to jump the species barriers, it is inevitable that H5N1 virus will be detected in some pigs. Pigs can be infected with both avian and human influenza A viruses-for instance, human influenza H3N2 viruses have been detected in pigs in Asia, Europe and Africa.
Some of these human and avian influenza viruses might become adapted to pigs and then begin circulating in pig populations. The co-circulation of avian, human, and pig viruses in pigs is of significant concern because of the potential for a genetic exchange, or "reassortment," of material between these viruses. Such an occurrence has the potential to produce a new, pandemic influenza strain.
Last week, a researcher at China's Harbin Veterinary Research Institute announced that pigs from farms in parts of China had been infected with the H5N1 strain of avian influenza (see previous report). China's Ministry of Agriculture has since confirmed the researcher's findings. What is unclear from the few studies which have been conducted is whether the H5N1 virus has already become established in pig populations in China.
Because the findings remain preliminary and are not necessarily indicative for widespread infection among pigs, assessing the consequences of this information for public health is difficult. Providing a detailed risk assessment of the current situation requires an understanding of the main factors influencing the potential for the emergence of a pandemic influenza strain: the prevalence of H5N1 and human H3N2 virus in pigs in Asia, and the likelihood of a reassorted virus and its possible pathogenicity.
The role of pigs in genetic reassortment is not fully understood. While there has been no known natural occurrence of reassortment of influenza viruses in pigs that resulted in a new pandemic strain, the probability of this occurrence is not negligible.
The chances for genetic reassortment depends upon both the duration of H5N1 circulation in pigs as well as the simultaneous presence of human and pig influenza A viruses (such as H3N2 or H1N1). As long as human and avian influenza viruses are co-circulating--whether in humans or in pigs-- the possibility of an exchange of genetic material--exists.
To better understand the implications of the findings in China, WHO is encouraging that additional studies be conducted on H5N1 and other influenza A viruses in pigs in China, as well as in other countries which have experienced H5N1 avian influenza outbreaks. In addition, laboratory experiments would be required to shed some light on the probability for virus reassortment, the possible pathogenicity of a reassorted virus, and the chance that pigs will act as a pathway for the emergence of a potential pandemic strain. These results will help national and international public health authorities not only