The rapid emergence of the H1N1 strain of influenza in North America and its subsequent global spread have reminded the world that viruses and other microbes are often not limited to specific species and have little regard for international boundaries. While the world was watching to see if the highly pathogenic H5N1 influenza, the so-called bird flu, in Asia and Africa would mutate to make transmission easy among humans, the new H1N1 influenza -- first called swine flu because it shares several gene sequences with influenza viruses found in pigs -- seems to have already made that transition.

Changes to influenza viruses can happen as a result of quick mutations or as the end result of a slower drift in genetic material. At the same time, different influenza viruses exchange entire genetic segments with one another when they simultaneously infect the same person or animal. One of the virus's genetic segments controls the hemagglutinin antigen, which sits on the surface of the virus and allows it to attach itself to the cells of the host. Another segment controls neuraminidase, a protein that enables the virus's release from the host cell after multiplying. The combination of H's and N's provides the name used for strains of influenza viruses such as H5N1 and H1N2.

The host, whether it is a person, bird, or pig, provides a place for influenzas to make contact, exchange genetic material, and form new viruses. Such genetic intermixing is not uncommon: the new H1N1 influenza virus, for example, appears to have formed from components of previously circulating viruses with a mix of influenza genes for three different host groups: birds, pigs, and humans.

This specific H1N1 virus rapidly circulating around the world, then, is not a new phenomenon but simply a new strain of influenza. This does not, however, mean that there is no need for concern. Each new strain of influenza has the potential to cause mild to very serious disease, and when new strains first emerge, it is extremely difficult to predict a trajectory for infection or its consequences.

As I described in a 2005 article in Foreign Affairs, the health sciences are segregated into many specialty areas, such as human medicine, livestock disease, and wildlife health. The compartmentalization of these fields has hampered efforts to effectively control or prevent the emergence and spread of diseases that move among species, which together represent approximately 60 percent of all infectious organisms.

Even as national and global health agencies are scrambling to react to the continuing spread of the H1NI virus, a major influenza virus that emerged in the last decade has not been adequately addressed. The highly pathogenic avian influenza, subtype H5N1, is still killing millions of domestic chickens and ducks in Asia and Africa. This has caused economic hardship for people and has hit many poor countries with trade restrictions, especially Bangladesh, Egypt, Indonesia, and Vietnam. Avian flu still presents the ongoing threat of a potential human pandemic, with more than 400 cases reported as of April 2009, resulting in at least 250 human deaths.

Although collaboration among health organizations and local governments has vastly improved since H5N1 first emerged, much remains to be done to implement better control mechanisms in the poultry industry, such as improved sanitation and good vaccination programs. In East Asia, where the highly pathogenic H5N1 influenza first developed in domestic ducks, chickens, and geese, the virus has spread not only to other farms and flocks but also to wetlands, where it can live for years and infect wild bird populations.

In many countries where contact between domestic and wild animals is commonplace, early disease-detection systems are still not as good as they could or should be. In the case of H5N1, new vaccines developed in the last few years for both humans and birds may help to reduce risks, but local capabilities to vaccinate people are often lacking. Even the United States has yet to approve new vaccine production technologies that would cut the wait time for producing massive quantities of human-influenza vaccines from the current minimum of eight months or so to just half of that or less.

A broader understanding of the connection between human and animal health demands a more unified approach such as that offered by the One World-One Health program, an ongoing series of multidisciplinary dialogues led by the Wildlife Conservation Society. The resulting programs bring together experts ranging from biologists and sociologists to economists and natural-resource managers. The rapid partnering of animal- and human-influenza specialists on the current H1N1 outbreak is an example of how a more comprehensive approach can speed the response to a potential pandemic.   

Over the last few years, the One World-One Health concept has gained wide acceptance in the scientific community as well as the attention of policymakers and the development community. International bodies such as the World Health Organization and the World Bank have adopted the One World-One Health approach in their collaborative efforts to control avian and pandemic influenza and other diseases of global concern.

National governments are also beginning to acknowledge the links among wildlife, domestic animal, ecosystem, and human health and the resulting need to address the threat of disease across sectors. In October 2007, the Brazilian government held its first One World-One Health congress, which brought together the ministries of agriculture, environment, and health along with major livestock producers. One outcome of the Brazil meeting was the increased recognition that forest degradation can not only lead to an increase in greenhouse gases but also act as a driver of both wildlife and human disease. For example, clearing tropical forests can create breeding grounds for several types of Anopheles mosquitoes that carry malaria.

In the United States, the Centers for Disease Control created the National Center for Zoonotic, Vector-Borne, and Enteric Diseases in 2007. Staffed with more than 600 epidemiologists, physicians, and veterinarians, this new center brings together some of the oldest components of the agency -- those dealing with viral, bacterial, parasitic, and other communicable diseases -- to examine diseases that can affect both wildlife and humans within a larger ecologic context.

Improvements in global information-sharing have allowed scientists from around the world to more easily communicate findings and ideas, an essential step toward meeting the challenge of protecting people and animals from infectious diseases. New networks for sharing viral gene sequences are beginning to release the world from the old system of having to wait for a few individuals to identify a new pathogen or for them to publish their findings a year or two later in a scientific journal. With the H1N1 outbreak, as soon as the new strain was identified, its genetic code was transmitted to influenza experts around the world for further evaluation.    

When avian influenza emerged in 2005, scientists and researchers had little information on how common it was and where it was occurring in wild and domestic birds. Those studying the disease needed to know where wild birds were migrating and whether they carried the virus from its incubation points in large duck and chicken farms. The creation of the Global Avian Influenza Network for Surveillance in 2006 was designed to answer some of these questions. Participants from 30 GAINS partner organizations around the world have since recorded the movements and locations of more than 100 million wild birds and collected over 40,000 samples for influenza testing. These findings are available in a public database and mapping system. For the first time in history, thousands of people from government agencies, universities, and NGOs have been trained to safely and effectively participate in a global wildlife-health-monitoring system.

Expanding this approach to a wide range of diseases would enhance the early detection of new diseases, as well as the occurrence of known diseases in new places, and help to rapidly identify the genetic origins of pathogens. The U.S. Agency for International Development is now using GAINS as a model for a new program designed to predict and respond to diseases emerging from animals around the world. It will focus on building local capacity in places such as the Amazon basin, Asia, and the Congo basin, where diseases such as avian influenza, Ebola hemorrhagic fever, SARS, and HIV/AIDS first emerged.    

New challenges will continue to arise. Climate change, for example, is a concern not only because of the environmental fallout but also because warmer temperatures and altered rainfall levels may foster the spread and emergence of infectious diseases. Wildlife may be the canary in the coal mine for the dangers of climate change: even minor disturbances in the environment can influence its susceptibility to disease. Microbes are even more responsive; temperature and rainfall patterns significantly affect the viability of viruses, bacteria, and parasites. Little data currently exists on how diseases will spread in response to climate change. To uncover such trends, disease surveillance efforts should be examined together with meteorological data. 

The drivers of global change -- namely, population growth, expanding agriculture and livestock production, wildlife trade, biodiversity loss, and climate change -- all disturb the balance of disease pathogens in the world's ecosystems. Scientists are beginning to gain a better understanding of the vulnerabilities of certain species, disease transmission routes, and ways to prevent disease problems from occurring.

But there is a long way to go. Organisms dangerous to people and animals continue to mutate, adapt, and spread, and opportunities for new diseases to emerge and travel globally are increasing. In response, it is necessary to quickly build new partnerships and strengthen capabilities for global health surveillance that are not limited to disease in humans. It is equally important to find innovative ways of using that information to more effectively protect the health of wildlife, domestic animals, and people everywhere.

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  • WILLIAM B. KARESH is Vice President and Director of the Global Health Program and President of the Wildlife Conservation Society’s World Animal Health Organization (OIE) Working Group on Wildlife Diseases.
  • More By William B. Karesh