Thu, Dec 30, 2010 - Page 8 News List

Revival of bird flu likely in Asia

By King Chwan-Chuen 金傳春

On Nov. 18, Hong Kong health authorities reported that a woman was suffering from the highly pathogenic (A)H5N1 subtype avian influenza virus, commonly known as bird flu. Then, on Dec. 9, the Indonesian Ministry of Health announced that a 21-year-old female from Bandung City in West Java was a new human case of H5N1. These human H5N1 cases have prompted public health systems in Asia to step up their pandemic influenza preparedness. Now that winter has arrived in Taiwan, we need to strengthen surveillance of both humans and animals to guard against highly pathogenic bird flu viruses and prevent them from spreading to humans.

Conventional infectious disease surveillance is generally passive in that it depends on doctors notifying health authorities when cases occur. This is a low-cost system, but it depends on doctors being keen enough to notify the authorities. Another problem arises if there is a high proportion of asymptomatic cases for a particular illness — meaning that many people who are infected do not fall ill — and this is true of the influenza virus.

Besides, the notification rate for infectious diseases of the respiratory and digestive tracts tends to be rather low. If we want to be sure about whether one of the many subtypes of influenza virus that can spread quickly among animals can also cross over to human hosts, a passive surveillance system might be too slow and that could lead to unfortunate consequences. This is especially true of emerging novel influenza viruses, which can spread quickly because most people are susceptible, having no antibodies. Novel virus infections can break out all over the place, with high morbidity rates, leading to a heavy death toll.

The best approach is to routinely conduct serological surveillance studies on people belonging to high-risk groups — ie, those exposed to potential carrier animals — to detect early on whether they have antibodies to animal influenza viruses. This contrasts with the past practice of flu-affected Asian countries, which had been to take body temperature tests in areas where there have been incidents of bird flu.

Enhanced virological surveillance is needed for people who have close contact with animals, such as those working in live poultry markets and livestock farming, when they exhibit respiratory tract symptoms and also for travelers and traders who fall ill, and in places where clusters of influenza infections occur, so as to ascertain whether there have been interspecies transmissions from animals to humans.

When interspecies infections do happen, the eight genomic segments of the viruses must be analyzed to investigate their origin and find out how they are related to animal-borne influenza viruses. It is also important to determine what key risk factors there are for humans to acquire such infections in the locality.

For example, surveillance has been implemented in Indonesia, a country badly affected by bird flu. In West Java Province, it was found that 47 percent of chickens in live poultry markets were infected with bird flu viruses, 44 percent of which had the H5N1 subtype. It was also found that unhygienic waste disposal in such markets and poorly managed poultry transport was an important risk factor.

Such conditions have shown again and again how important it is to conduct routine virological and serological surveillance at live poultry markets where people come into close contact with domestic fowl.

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