The Human-Climate-Wildlife Nexus

Abstract

As more and more diseases are discovered to be of animal origin and the effects of climate change on disease become clearer, now is the time to examine how best to mobilize health professionals, policy makers, and security experts around these connections.

Data sometimes fail to paint an engaging picture in terms of human health. But consider these numbers: The U.S. Centers for Disease Control and Prevention (CDC) estimates that 75 percent of recently emerging human infectious diseases have an animal origin. 1 Another study reports that 80 percent of animal pathogens can circulate between humans and animals. 2 These studies certainly raise eyebrows in the public health community, but they demonstrate only one part of a much more complex situation.

That complexity comes with climate change, which is likely to have an impact on human health and disease spread. Under most climate change scenarios, it is anticipated that global temperatures and sea levels will continue to rise, and the severity and frequency of strong storms, flooding, and drought will increase significantly. Ecosystems will be altered, and with this, some environments will become suitable locations for the transmission and maintenance of infectious diseases that had never before been able to survive in these locales. At the same time, changing weather patterns have the potential to lead to the migration of large numbers of “climate refugees.” 3 Such displacements can favor the spread of diseases that are waterborne, food-borne, vector-borne (transmitted by insects or other arthropods), or zoonotic (transmitted by animals) in nature, since people may be forced to use compromised food, water, or shelter (potentially shared with wild or domesticated animal populations) simply to survive. These four types of disease are particularly sensitive to climate change because they have natural cycles that allow them to survive outside of a host for prolonged periods, are transmitted by vectors, or rely on non-human hosts as reservoirs of infection. 4 In fact, climate-related disasters are predicted to affect more than 375 million people by 2015, and large-scale human migration magnifies the risk of a pandemic. 5

Thus, there is a new urgency in the medical, veterinary, wildlife, and environmental health communities to understand the complex relationships between rapidly changing environments, human activities, and human-animal interactions in order to respond appropriately to an ever-changing health landscape. Recognizing the connections between these factors will allow the communities that must be involved (e.g., physicians, public health professionals, veterinarians, wildlife managers, environmental scientists, and policy makers) to take steps toward developing effective responses to emerging threats such as the spread of the H1N1 strain of influenza virus.

The human-climate-wildlife nexus illustrated.

Because climate, human health, and wildlife are connected in complex and varied ways, it is useful to present relevant examples that can illustrate these connections and allow us to explore larger relationships. Take, for instance, major climatic cycles such as El Niño/La Niña, which impact sea surface temperatures and that, in turn, influence rainfall patterns and temperatures on land. Precipitation can determine how much breeding habitat is available for arthropod vectors (e.g., mosquitoes, ticks, fleas, and similar organisms) early in their development, and temperature can influence the development rates and breeding of the same arthropod vectors. Increases in suitable breeding sites and longer periods in which to breed can lead to increased arthropod populations and more people being infected by arthropods carrying an infectious disease such as West Nile virus, which first appeared in North America in 1999 and spread to birds and humans throughout the contiguous 48 states by 2005. 6 A study of West Nile virus cases reported in the United States from 2001 to 2005 proposed that higher temperatures and humidity, as well as heavy rainfall, led to an increase in mosquito populations and a widespread outbreak of West Nile virus in humans. 7

A similar connection has been made with climate change, which is among the factors that may have affected the resurgence of dengue fever in the Americas (a disease transmitted by infected mosquitoes). 8 Warmer temperatures and changing precipitation patterns can, in some locations, increase mosquito populations by expanding habitat, speeding the rate of development of adult mosquitoes, and reducing winter temperatures. Mosquitoes, therefore, are able to survive in regions previously too cold. At the same time, the dengue virus replicates faster with increasing temperatures. A larger population of virus-carrying mosquitoes means a higher rate of prevalence of the disease because female mosquitoes have been shown to bite more frequently in warmer temperatures.

Disease emergence and spread are also affected by human actions. The unregulated importation and trade of exotic species, for example, allows for the transmission of what otherwise could be preventable disease spread. A billion-dollar industry, the commercial exploitation of wildlife has increased tremendously in the last 20 years. The illegal trade alone is worth an estimated $5 billion to $20 billion annually. As animal populations shift, so too do disease patterns. The rapid, widespread wildlife trade can move pathogens or infected animals rapidly into susceptible populations. 9 In the case of the U.S. monkeypox outbreak in 2003, a shipment of African rodents infected prairie dogs at a pet distribution facility. The prairie dogs were sold or distributed in several Midwestern states, and monkeypox was transmitted to humans after direct exposure to the infected animals. 10

Industrial food production methods hold animals in cramped quarters in exceedingly close contact. When this kind of intensive animal production occurs in areas of high natural biodiversity, it can provide a perfect incubator for the emergence of novel zoonotic pathogens.

The Institute of Medicine has stated, “The significance of zoonoses in the emergence of human infections cannot be overstated,” particularly when it comes to human action. 11 Industrial food production methods hold animals in cramped quarters in exceedingly close contact. When this kind of intensive animal production occurs in areas of high natural biodiversity, it can provide a perfect incubator for the emergence of novel zoonotic pathogens. Such was the case with Nipah virus, which emerged in Malaysia and Singapore in the late 1990s among slaughterhouse pigs and farm workers. Epidemiological studies following the outbreak showed that the virus was originally transmitted to pigs by flying foxes (which are, in fact, large bats). 12 Flying foxes are a natural reservoir for Nipah virus, and these particular bats were driven into cultivated orchards and pig farms that abutted the forests' edge after a drought had reduced fruit availability in the forest. The virus was transmitted to pigs by bat excretions and was subsequently passed to pig farmers in the area. The outbreak killed 105 people and threatened to collapse the billion-dollar pig industry in Malaysia. The likelihood of sudden outbreaks on this scale will only increase with the added pressures of climate-influenced migration.

The current state of the debate.

Climate change can exacerbate the potential for these diseases to be transmitted to humans and hamper people's ability to protect themselves against new diseases. There are more than 200 zoonoses currently causing a variety of human illness. In addition, as yet undefined zoonotic diseases probably exist that pose infectious risks for people.

It is worth noting that there is a growing understanding internationally about the human-climate-wildlife nexus. A number of “hot spots” of emerging infectious diseases have been identified, especially in zones of high natural biodiversity and high human population density. 13 These are the zones with the greatest frequency and complexity of human-animal-ecosystem contact and in some cases, zoonotic transmission–for example, in southern and eastern Asia, central Africa, Western Europe, Central America, and localized zones in North and South America. 14

Furthermore, the World Health Organization (WHO), the Food and Agriculture Organization, and the World Organisation for Animal Health have jointly established the Global Early Warning System for Major Animal Diseases, Including Zoonoses (often shortened under the acronym GLEWS) with the purpose of sharing information on the occurrence of animal diseases to facilitate the identification, tracking, and early response to disease outbreaks. The GLEWS disease priority list includes 25 diseases and disease categories, 19 of which are zoonoses, including anthrax, Mad Cow disease, the arthropod-borne disease Crimean Congo hemorrhagic fever, and West Nile virus. 15 Although GLEWS is primarily a web-based electronic platform for information sharing among the three organizations, it has the ability to convene an ad hoc scientific panel that can assist the GLEWS task force on emerging scientific questions if necessary. 16

There also is growing agreement among public health professionals on the importance of surveillance and an active, trained workforce to control disease. 17 In short, an effective surveillance system is vital to community disease control. Early warning mechanisms give local authorities time to put control measures into effect to reduce the community's health care costs and allow trained staff to quickly enact control measures. Of course, examining current surveillance techniques reveals gaps in local, national, and international systems. At present there are no long-term studies designed to quickly identify new and emerging pathogens; establish baseline data on the distribution and spread of disease agents, their hosts, and their vectors; or monitor the occurrence of disease outbreaks among humans and other animals. Nor are there enough laboratory studies in enough locations to complement field studies or develop predictive models that can serve as tools for anticipating potential disease risks in relation to climate change. 18

In the field of veterinary medicine, animal surveillance for infectious diseases (e.g., bubonic plague, equine viral encephalomyelitis, hantavirus, and rabies) is already done routinely by some animal health departments. 19 This is particularly important since animal outbreaks of these diseases typically precede human outbreaks. Consider West Nile virus. When health and veterinary professionals actively are monitoring for the disease, it usually occurs first in avian populations. Animals can reveal health hazards associated with environmental pollution and change. The classic example was the use of canaries in coal mines. Monitoring how changing air and water temperatures, rainfall patterns, and other environmental conditions affect the location and abundance of vector populations (and whether they harbor disease pathogens) while simultaneously monitoring changes in human and animal illness cases will be especially important to improve preparedness and the ability to respond to future outbreaks. The U.S. Geological Survey is working to address some of these important issues in its National Wildlife Health Center. One of its major research programs focuses on invasive wildlife pathogens and emerging diseases in the United States. The objective is to “understand associations between host, agent, and environment that lead to disease emergence, expansion, or escalation.” 20 Currently, however, the program is only conducting research on West Nile virus.

In the aftermath of 9/11, anthrax (a zoonosis) was used as a bioweapon. The attempt was partly successful, and it led to increased efforts in zoonotic preparedness. These activities also subsequently resulted in enhanced support in the United States to deal with other “agroterrorism threats.”

What needs to be done?

Although actions to curb climate change are being designed with a long-term view, disease control can be carried out immediately and with visible results through concrete, specific actions geared toward protecting the community as a whole. In the aftermath of 9/11, anthrax (a zoonosis) was used as a bioweapon. The attempt was partly successful, and it led to increased efforts in zoonotic preparedness. These activities also subsequently resulted in enhanced support in the United States to deal with other “agroterrorism threats” (i.e., the threat or deliberate introduction of an animal or plant disease that could impact U.S. food systems). 21 However, there are still numerous steps that can, and should, be taken to more effectively respond to future disease outbreaks or pathogen misuse.

Create and fund one centralized, public repository for surveillance data from both humans and animals that can harbor zoonotic pathogens. It would require environmental monitoring and data collection at a number of widely dispersed geographical and temporal data points that would aid in the understanding of both climate and non-climate factors influencing zoonoses. Although the CDC, U.S. Department of Agriculture, and other agencies provide domestic and international leadership and support on research, disease tracking, laboratory services, and epidemiology response activities, today no single agency monitors these issues for potential threats or dangers. To some extent this is not unexpected, since each agency has its own priorities and responsibilities. But, in some cases, the lack of cooperation simply is due to the need for better communication and support for interagency coordination, with clearly defined responsibilities for surveillance, outbreak response, or control associated with zoonotic diseases. The U.S. Agency for International Development has taken steps to improve interagency coordination with the recently launched Emerging Pandemic Threats Program, which will provide funding and direction to combat emerging zoonotic diseases at their source. 22 Roughly $350 million will be distributed among five projects with specific goals: predict, respond, identify, prevent, and prepare. Among the partners: the Wildlife Trust, the University of California-Davis's School of Veterinary Medicine, Global Viral Forecasting, Inc., the WHO, and the Food and Agriculture Organization.

Establish clear jurisdictional guidelines regarding who will enforce rules and regulations associated with human-animal interaction threats. At the same time, there is a need for interdisciplinary teams that specialize in constructing models that reflect changes in weather events and human responses to zoonotic outbreaks so that early warning systems become more widespread. Of course, especially in the current economy, the lack of personnel and other resources for disease monitoring, prevention, and control activities can be a significant problem. Nevertheless, this is not a luxury but a necessity.

Establish better communication channels between veterinarians, physicians, nurses, and public health officers. One key success story in the public health field was the 1999 diagnosis of West Nile virus in New York City, which involved critical outreach and communication between zoo managers, local wildlife experts, the City Department of Health and Mental Hygiene, and local clinicians when they began seeing a new infection in their patients. More broadly, ongoing funding is needed at the state and local levels to help foster more of these cross-disciplinary, long-term partnerships. The challenge of West Nile virus, which has affected more than a million humans and had devastating effects on some avian populations, actually presented opportunities for some of those partnerships to take root. Now they must be built upon and used as models in other locations.

Diversify and unite disparate communities. There is a persistent human-animal divide in thinking about health. To overcome this, a sustained, global, multidisciplinary effort to track new infectious pathogens and illnesses in humans, wildlife, domesticated animals, and insect vectors is necessary. One way to look beyond this is by working within the One Health paradigm, which focuses on bringing the human and animal health communities together. 23 Climate change affects all species, so it is important to put in place monitoring programs that cut across the previous frame of the human-nonhuman divide.

It has been suggested that climate-health preparedness is “a puzzle that is slowly coming together” and that monitoring systems may soon synthesize climate and weather data; local environmental data; disease case data from people, pets, and wildlife; and information on the movement of goods and populations across borders.

In addition to the public health, wildlife management, and veterinary communities, security and defense experts have been leaders on the issue of climate change impacts. The insurance industry and wildlife conservationists have been as well. Now it is time for these seemingly disparate research and service communities to unite around securing a healthy future for all species. It is particularly important considering the attention that has been given to the connection between climate change and national security, since climate impacts are likely to destabilize societies in ways that will demand diplomatic or military interventions to restore order. 24 Humanitarian aid groups are already expressing concerns that their budgets cannot cope with the scale of climate-related disasters that have been observed thus far, and more challenges are projected in coming years. 25

Expand public education about the threats and risks that the consequences of climate change pose to human health. To reduce risks for emerging zoonoses, information about the risks associated with wildlife, bushmeat, and the exotic pet trade should be shared with the public. Citizens also should be engaged–whether in Boston or Botswana–in implementing proper surveillance systems to help minimize local risk. Once informed about signs and symptoms of climate-sensitive illnesses, people should be educated on how to report potential cases to their physicians, veterinarians, or wildlife management agencies.

It has been suggested that climate-health preparedness is “a puzzle that is slowly coming together” and that monitoring systems may soon synthesize climate and weather data; local environmental data; disease case data from people, pets, and wildlife; and information on the movement of goods and populations across borders. 26 As the connections are drawn between climate change, zoonotic diseases, and public health, it is easy to see why emerging and reemerging infectious diseases have received increased attention in the medical and veterinary communities since the end of the twentieth century. Infectious pathogens of wildlife affect not only human health and agricultural production but also wildlife-based economies and wildlife conservation. And we know that as climate change continues, zoonotic pathogens that infect domestic animals and wildlife hosts are more likely to emerge and continue to spread. It is time for this issue to be taken more seriously. If we invest in multilevel zoonotic climate-health preparedness, the future will be healthier and more secure.