The world is keeping a vigilant watch on the "Asian bird flu" situation in Southeast Asia and Eastern Europe. "Bird flu" is a severe form of avian influenza, a viral disease that normally only infects birds and pigs.
The avian influenza virus has many forms of pathogenicity ranging from mild to severe strains, the latter of which can produce devastating respiratory problems and rapid, widespread mortality in poultry and other birds.
The H5N1 avian influenza virus, more commonly known as the Asian bird flu, is a unique strain because it is not only highly pathogenic in poultry and other birds but has developed the rare ability to transmit directly from birds to humans in a limited number of cases.
Many experts feel that this virus may lead to a global epizootic in birds and, even worse, a pandemic in people. For a pandemic to occur, H5N1 would have to mutate to a different form that would be highly transmissible from human to human. There is no evidence of this change in the virus at this time.
In order to curtail the threat of further spread of the disease in poultry and, consequently, lessen the number of cases of infection and death in humans, world governments are exploring and implementing preparatory and response strategies.
Lessons from China
China, for example, has experienced widespread infection in its poultry. Moreover, a cultural practice of close living conditions between the Chinese people and their livestock has resulted in human infections and death. The Chinese government has, therefore, implemented mass vaccination of its poultry as a control effort.
Concerns have been raised about the mass vaccination of poultry in Asia. Is mass vaccination of poultry augmenting the spread of the virus between poultry, increasing infection of workers, and boosting the potential of a virus adapting to people, allowing it to transmit from human to human? What are the risks and benefits of mass vaccination?
One benefit of vaccination is the reduction of the amount of virus to which people are exposed. The vaccine will protect poultry against illness and death in addition to reducing the level of virus the birds are shedding and, therefore, the amount of virus in the environment. This is evidenced by the excellent results obtained in Hong Kong with vaccination and in research studies in poultry on avian influenza. Since the institution of vaccination in poultry as part of the control strategy, there have been no people reported ill with H5N1 in Hong Kong.
Another benefit is the active attempt to locate birds for vaccination and the potential identification of additional sick birds that should be culled. This again will reduce the amount of virus in the environment. The job of locating birds is difficult due to the large numbers, especially ducks, that are free to roam in remote areas.
On the risk side, the vaccinators can spread the virus between groups of birds. This is highly plausible because of the lack of supplies of coveralls, boots, masks, and disinfectants available.
An additional risk is the increased potential of vaccinators to become infected without the appropriate personal protection equipment. At this time, the relative risk of infection in people is low because H5N1 is not readily transmissible to people. It is important to improve the availability of personal protective equipment to mitigate this risk.
In addition, there is concern that the quality of the vaccines that are being used in China are not standardized or consistent. The vaccine efficacy may fall short of the level needed to provide protection for the birds, thus giving a false sense of security.
The Situation in the United States
By contrast, the policy in the United States for control of avian influenza in poultry is eradication or "stamping out" of the disease. This is based on a lesson learned in the 1983-84 outbreak of highly pathogenic avian influenza in poultry in the northeastern U.S. The lesson was to prevent viruses of low pathogenicity from circulating in a population, thus reducing their potential to change to highly pathogenic viruses.
The first case of avian influenza in poultry in April 1983 was caused by a virus of low pathogenicity, and by October of that year the virus had changed to a highly pathogenic form. That outbreak cost the federal government approximately 60 million dollars, and 17 million poultry were destroyed.
Avian influenza vaccine in poultry is effective in reducing the infection and shed of the virus but has not been used routinely as part of a U.S. control program because "stamping out," rather than reduction of the disease, is essential as a long-term strategy. Vaccination has been used as a tool for short-term control in certain countries, as part of their long-term strategy for eradication of the disease.
In general, vaccination for avian influenza is not used in the U.S., and approval for its use is strictly controlled by the USDA. Debate over its use in the U.S. revolves around the critical export market. Vaccine is available for use if needed, but it is highly unlikely H5N1 will enter through poultry in the U.S. and become widespread because of extensive surveillance, biosecurity, and response plans the poultry industry has in place.
Currently, the benefits of mass vaccination of poultry in Asia outweigh the risks of spread of the disease between poultry and infection of people. The amount of virus will be reduced, more sick birds will be culled, and more people will be educated concerning the risks of avian influenza.
Many tools must be implemented given the infrastructure and geographical dispersion of poultry in that part of the world, and appropriate biosecurity procedures must be enforced.
The task is daunting, but the U.S. and other countries are helping China to improve the infrastructure, equipment, surveillance, testing, and control efforts. The endpoint, for everyone's benefit, should be "stamping out" the virus rather than long-term vaccination.
Sherrill Davison, VMD, MS, ACPV, MBA, is Director of the Laboratory of Avian Medicine and Pathology, University of Pennsylvania School of Veterinary Medicine.
