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Control of avian influenza A(H5N1): public health concerns
10 February 2004
The current outbreaks of highly pathogenic H5N1 avian influenza in poultry in parts of Asia have had immediate and severe consequences for the agricultural sector.1 Human cases, with a high fatality, have been reported in two countries, Viet Nam and Thailand, with very widespread outbreaks in poultry.
It can be anticipated that human cases will also be detected in other countries where outbreaks in poultry are rapidly spreading.
The number of human cases presently detected is very small compared with the large number of infected birds distributed over a wide geographical area. This suggests that the H5N1 virus strain may not easily infect humans.
To date, no human-to-human transmission is known to have occurred. However, the continuing presence of infection in poultry may also create opportunities for the emergence of a new influenza virus subtype with a capacity to spread easily among humans, thus marking the start of an influenza pandemic. Should this rare event occur (three pandemics occurred during the previous century), it would immediately have serious consequences for human health throughout the world.
For this reason, public health concerns about the present H5N1 situation must be given the highest priority when weighing the immediate and measurable economic losses in animals against possible yet unpredictable consequences for humans.
Several other diseases in animals can be transmitted to humans. Experience with such diseases, known as “zoonoses”, has shown that strict measures on animal health, imposed by the need to protect human health, helped rebuild consumer confidence.2
Recent experience has also shown that measures for the control of zoonotic diseases, aimed at halting further spread in animals and minimizing economic losses, need to be closely coordinated with measures that minimize the longer-term risks to human health. In the present situation, measures aimed at eliminating the disease in poultry will also reduce the presence of the virus in the environment and thus reduce opportunities for human exposures and infections. These measures must be carried out urgently, giving highest priority to the protection of human health. Previous outbreaks of highly pathogenic avian influenza associated with human infections occurred in areas, such as Hong Kong and the Netherlands, with industrial poultry production and well developed health and agricultural infrastructures. Even so, elimination of infection in poultry was a complex, difficult, and costly undertaking. Both outbreaks were eventually controlled through immediate culling of infected flocks, quarantine and disinfection of farms, strict biosecurity, restrictions on the movement of animals, and compensation for farmers.
The present situation is different. Control of outbreaks of highly pathogenic avian influenza is known to be especially difficult in areas where poultry range freely. In several affected countries, up to 80% of the total poultry population is raised in small backyard farms. Most rural families keep a small free-range flock.
Given these features of the present situation there is potential that the H5N1 virus could become established in bird populations in this geographical region and possibly spread to other parts of the world. This was one of several conclusions reached during a joint FAO/OIE/WHO technical consultation on the control of avian influenza, held in Rome from 3–4 February.
No single blueprint for control in animals, and thus reduction of risks for humans, is available. Over the past four decades, only 18 outbreaks of highly pathogenic avian influenza, most caused by strains other than H5N1, have occurred throughout the world. Existing evidence will not suffice to provide universally applicable recommendations for a rapid and effective response in affected countries.
Control measures must be tailored to each country’s unique epidemiological situation and unique capacity, with health and agricultural sectors working hand-in-hand. Agricultural authorities face the immediate challenge of rapidly eliminating the H5N1 reservoir in poultry. Authorities in all affected countries need to work together in a coordinated way
Transparency in reporting of human and animal disease is absolutely essential.
Despite the uncertainties, experts fully agree that immediate culling of infected and exposed birds is the first line of defence for both the protection of human health and the reduction of further losses in the agricultural sector. Other measures, such as the vaccination of healthy flocks, may play a supportive role in some cases when undertaken in conjunction with measures for preventing further spread of infection. WHO has repeatedly stressed the need to ensure that culling is carried out in a way that does not fuel more human cases. and that vaccination of poultry should not lead to the dropping of vigilance or compromise other necessary control measures.
In responding to the situation, WHO emphasises three strategic goals: to avert an influenza pandemic, to control the present human outbreaks and prevent further spread, and to conduct the research needed for better preparedness and response, including the immediate development of a new vaccine for humans against H5N1. WHO has issued a series of technical guidelines aimed at minimizing the risk of further human cases and facilitating a coordinated international response.
1 Highly pathogenic avian influenza is categorized by OIE as a “list A” disease. List A includes transmissible diseases “which have the potential for very serious and rapid spread, irrespective of national borders, which are of serious socio–economic or public health consequence and which are of major importance in the international trade of animals and animal products.”
2 One example is the spread of bovine spongiform encephalopathy, or “mad cow disease”, in cattle, which led to the emergence of a rare yet invariably fatal new disease in humans.
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