Opinion

Towards an Interdisciplinary Ecological Approach to Infectious Disease

07/25/2010

From a 30,000-foot view, it is clear that disease and the environment are intertwined. The flu virus pays a visit to the northern hemisphere each winter, toxic algal blooms in coastal waters precipitate massive fish kills and illness, even death, among consumers of shellfish from the Gulf of Mexico each summer, and cholera rears its head in the Indian subcontinent each spring, following a centuries-old pattern. The reason for this pattern is straightforward: environmental conditions dictate the occurrence and survival of pathogens.

Much of my research over the past two decades has focused on understanding the environmental controls of endemic cholera in Bangladesh, where Vibrio cholerae serves as an important model pathogen for the links between disease and the environment. Seasonal fluctuations in sea temperature and estuarine salinity, weather events, and nutrient runoff control the population of V. cholerae, which grows in association with chitinaceous zooplankton and shellfish. Human exposure occurs via contact with surface water or with these organisms. In historical cholera outbreaks, initial cases frequently occur among fishermen or boatmen. The threat is particularly acute in developing countries like Bangladesh, where poor hygiene, lack of sanitation, contaminated drinking water, and overcrowding allow the cycle of transmission to persist.  

This pattern is neither unique to Bangladesh nor to cholera because the oceans are incubators of great biodiversity and thereby serve as incubators for emerging disease. Worldwide, human activity encourages favorable conditions for the growth of deadly microorganisms, whether because of increased pollutant loads or because of rising sea temperatures due to climate change, thus adding to the potential and actual pathogen load. An ecological infectious disease perspective applies to pathogens like cryptosporidium, enteric viruses, Vibrio vulnificus (a.k.a. flesh-eating bacteria), and many more.

Shrimp
Vibrio sp. associate with chitin, like that found in the shells of these shrimp. photo by Rafael Ortega Díaz, Creative Commons Attribution-Share Alike 1.0, 2.0, 2.5

Despite the clear links between disease and environment, modern medicine has not taken a 30,000-foot view, but has taken a near-sighted view instead and focused on the pathogen and the patient, that is, on treatment but not prevention. Prevention of disease should be a major focus for public health globally, and an interdisciplinary ecological approach to infectious disease is the cornerstone of prevention and prospective medicine.  

The major challenges of infectious disease prevention for the next decade are twofold. The first challenge is to link human activity, conditions in the environment, and outbreaks of known and emerging diseases and the second challenge is to develop a system to anticipate, detect, and rapidly respond to these threats. 

"Environmental conditions dictate the occurrence and survival of pathogens."

Already, scientists have developed a variety of tools that will prove useful in meeting these challenges. Remote sensing of water bodies from satellites is used to monitor sea surface temperature, turbidity, chlorophyll, and sea surface height, which has allowed us to understand parameters that most strongly correlate with epidemics. A second tool, ever-advancing chemical sensing technologies, continues to show new ways in which human disturbances upset delicately balanced ecosystems, leading to harmful algal blooms. Modern genomics, a third tool, reveals that harmful algal blooms introduce species that outcompete native species, posing ever-new threats to human health. And a fourth tool, the Demographic Surveillance System (DSS), allows us to detect outbreaks well before they are underway and determine the patterns that lead to outbreaks. A DSS involves continuous, extensive monitoring of population health and behavior and various environmental conditions, so it allows us to translate social and environmental conditions into human health risks.

Child with water bucket
How clean is this water? DSS can help answer this question and determine whether water interventions improve health.

Wielding these tools, we can meet the first challenge of infectious disease prevention and link human activity, environmental conditions, and disease. In a recent paper published in Nature, my co-authors and I describe just how informatics and computer models can pave the way forward.  We present a framework in which data on ecosystems, organisms, and genes are collected and synthesized on an informatics platform, leading to accurate assessments of microbial activity. We envision miniaturized ecogenomic sensors spread throughout the seas, collecting continuous genetic information to measure microbial activity and sharing this information wirelessly through satellite networks, where it can be linked to other data and give a holistic view of environmental processes. The technology embedded in this system is close to reality, and will provide a deep understanding of ecosystem health at the microbial level. Such surveillance systems need not be limited to the ocean, but can include rivers, lakes, and streams. Adding to this vision of integrated environmental data collection, Demographic Surveillance Systems can close the gap between infected patients and the environment, painting a full picture of the cycle of transmission.T hen, statistical and mechanistic models will allow us to predict current and future human health risks.  

Satellite image
Remote sensing can help predict disease outbreaks. Shown here is a NASA satellite image of the Ganges River delta.

Such integrated models will result in actionable information that is useful both for immediate interventions and for setting long-term health and environmental policy to protect man and the environment, which, of course, meets the second challenge of infectious disease and is our ultimate goal. The way to realize this goal in the next decade is to develop one or more Interdisciplinary Center of Ecological Health, test beds wherein all of the complex and interacting factors are monitored, modeled, and managed. These centers will involve collaborations of researchers in environmental science, hydrology, meteorology, metagenomics, microbiology, computer science, public health, medicine, and public policy. In many ways, this embodies the mission of our very own Johns Hopkins University Global Water Program. To meet the challenges of disease prevention, these centers would also involve participation of groups that implement strategies for disease treatment and control, and environmental management. This includes international groups like the Gates Foundation or UNICEF, as well as local governments and NGOs. Ideally, such centers would be strategically located to be the most generalizable to the rest of the world, in diverse places like the Chesapeake Bay, the Bay of Bengal, the East China Sea, or the dozens of inland DSS sites scattered throughout Africa and Asia.

"The way to realize this goal is to develop an Interdisciplinary Center of Ecological Health"

Today, in the wake of the oil spill in the Gulf of Mexico, we are left without answer to a simple, but critical question: what is the impact of the oil spill on human and ecosystem health, in the near and long term? Will pathogenic organisms multiply on the rich organic matter now inundating the water column? An established Interdisciplinary Center of Ecological Health in the Gulf of Mexico would be able to immediately shed light on these issues.  But regardless of where we chose to first pursue them, these centers would take the 30,000-foot view and develop an interdisciplinary ecological framework for monitoring, modeling, and managing environmental and human health risks that will become as relevant for the Chesapeake Bay as for the Bay of Bengal.

References

Bowler, C., D. M. Karl, et al. (2009). "Microbial oceanography in a sea of opportunity." Nature 459(7244): 180--184.

Colwell, R. R. (1996). "Global Climate and Infectious Disease: The Cholera Paradigm." Science 274(5295): 2025--2031.

Harvell, C., K. Kim, et al. (1999). "Emerging marine diseases--climate links and anthropogenic factors." Science 285(5433): 1505.

Sack, R. B., A. K. Siddique, et al. (2003). "A 4-Year Study of the Epidemiology of Vibrio cholerae in Four Rural Areas of Bangladesh." The Journal of infectious diseases 187: 96--101.

Keywords:

infectious disease, disease prevention, interdisciplinary ecological health, cholera, emerging disease, health surveillance

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