Editor's Note: This article is an adaptation of a paper presented to The International Astronautical Federation 46th International Astronautical Congress, Oslo, Norway. The original paper contained numerous citations, only a few of which are shown here. The rest will be supplied by the author on request.

Remote satellite sensing (RS) of the oceans, land masses, ice cover, and atmosphere has been used for understanding biogeochemical cycles and biotic feedbacks. This paper addresses "side effects" of biotic processes that relate to understanding the habitat, ecological dynamics, and climatic forces affecting diseases, their carriers (vectors), and animal reservoirs.

Applications of RS for disease surveillance include: 1) monitoring coastal algal blooms (using the Coastal Zone Color Scanner (CZCS), Sea-viewing Wide Field-of-view Sensor (SeaWiFS), and Advanced Very High Resolution Radiometer (AVHRR)) and toxic phytoplankton for a global cholera and marine biotoxic Early Warning System (EWS); 2) observing terrestrial applications (using Landsat and Synthetic Aperture Radar (SAR)) for mosquito and rodent-borne disease EWS; 3) RS integrated into El Niño/Southern Oscillation (ENSO) models, using the teleconnections for predicting conditions conducive to disease outbreaks; 4) using RS as a component of models to project potential disease distribution as a function of climate change; and 5) using the Microwave Sounding Unit/High Resolution Infrared Radiation Sounder (MSU/HIRS) to detect tropospheric temperatures to help understand physical and biological changes occurring at high altitudes.

Coastal Algal Blooms

Since the 1960s, researchers in Bangladesh have associated the outbreaks of cholera with seasonal coastal algal blooms. Recently, a viable, non-culturable "dormant" form of Vibrio cholerae has been identified, which attaches to a wide range of marine life, and reemerges to an infectious state along with algal blooms.

Remote sensing from the CZCS and measurements of sea surface temperatures (SSTs) have been used to assess phytoplankton blooms and primary productivity (Aiken et al., 1992) and are currently used to follow potentially toxic phytoplankton blooms (confirmed with targeted sampling) associated with fish and shellfish poisonings.

Thus one can correlate phytoplankton blooms and their aftermath with the appearance of cholera in coastal populations. Consequently, a system to predict and monitor cholera outbreaks can be devised, and an analysis of existing data can help define uses and requirements for future satellite monitoring systems.

Terrestrial Ecosystems: Eastern Equine Encephalitis (EEE)

Knowing locations of temporary pools of standing water, when they will appear, and perhaps how long they will last, is necessary so that more environmentally appropriate actions can be taken to control the population of EEE-infected Aedes vexans mosquitoes in Massachusetts. Early use of Bti (Bacillus thuringiensis var.israelensis), a non-toxic, inexpensive larvicide, is the alternative to widespread spraying of the adulticide, malathion. Maturation of larvae to adults occurs in about seven days; therefore, accurate information on standing pools of water after a rain is necessary within two days to allow time for dip sampling and application of Bti (Epstein et al., 1993).

The best approach to creating a dependable set of maps would be the acquisition of remotely sensed images. These and other data layers could then be used together in an appropriate Geographic Information System (GIS) for analysis. "Real-time" information following summer rains, from oblique-angled SAR, can penetrate vegetative and cloud cover to distinguish smooth water surfaces, thus helping to focus dip sampling and application of larvicidal treatment in a timely fashion.

Landsat, with 30 m spatial resolution and coverage every 16 days (at best), or SPOT imagery (which has the advantage of relatively more on-demand coverage and both 20 m and 10 m spatial resolution) will be helpful for developing a series of base-line GIS-generated "risk" maps.

SAR may be most appropriate for providing real-time accurate estimates of the locations of standing water. Aircraft-collected SAR data could be acquired and processed at an appropriate scale and processed for use in a timely fashion.

Remote sensing has also been used to delineate the habitats of the vector-borne diseases (VBDs), including African sleeping sickness (Epstein et al., 1993).

ENSO Teleconnections

Dynamic atmospheric-oceanic coupled general circulation models (AOGCMs) involving remote sensing of sea surface temperatures have multiple applications for predicting conditions conducive to disease outbreaks. Teleconnections of ENSO events are based on the analysis of geographic patterns since 1887 (Glantz et al., 1991).

During an ENSO warm event, specific areas of the globe are consistently affected by drought, while others have excessive precipitation. While Southeast Brazil has rain, for example, the Northeast has intensified drought. The signal is stronger (more consistent) in some areas; Southern Africa repeatedly experiences drought during an El Niño--as is happening again this year. All tropical oceans warm in relation to the ENSO pattern, and evaporation from the Atlantic can flood a warmer Central Europe.

ENSO warm events are correlated with new appearances of harmful algal blooms (HABs) in Asia and along the U.S. Atlantic coast.

Tracking ENSO events and epidemics is a key to unfolding the impacts of climate variability and weather on disease patterns. Associations in themselves are not "proof" of causation; but a preponderance of evidence, globally distributed, and a plausible mechanism (extremes of precipitation and temperature) lend credence to a strong role for climate in disease distribution.

El Niño warm events are associated with upsurges of cholera in Bangladesh; typhoid, shigellosis, and hepatitis in South America (after flooding); viral encephalitides (Murray Valley and epidemic polyarthritis, from Ross River virus) in Australia; and Eastern Equine Encephalitis in Massachusetts.

Other ENSO-related outbreaks of disease include: worldwide distribution of malaria outbreaks (Bouma et al., 1994); malaria in Pakistan; malaria and dengue ('breakbone') fever upsurges in Costa Rica; dengue fever in northeast Brazil; epidemic malaria in the Indian subcontinent, India and Sri Lanka (1874-1945); agricultural rodent infestations in Zimbabwe (1973-1983, 1984); and monsoons (biennially related to ENSOs) directly related to the spread of brown plant hopper (Nilaparvata lugansCoccidiomycosis immitis) in the 1980s, 1200 cases occurred in 1991, and over 4000 in 1992 and 1993. An earthquake and a prolonged drought followed by torrential rains are considered to have been contributory factors.

Additionally, disease events across taxa appear to cluster during ENSO years. Disease events along the U.S. Atlantic coast during 1987, an ENSO year, heralding the warmest year yet this century, include: a) extensive Caribbean coral bleaching; b) a large Florida sea grass die off; c) agent of neurological shellfish poisoning (Gymnodynium breve) transferred from the Gulf of Mexico to North Carolina; d) large die-off of sea mammals in New England (and the North Sea); e) emergence of amnesic shellfish poisoning in Prince Edward Island (new diatom toxin--domoic acid--later appearing worldwide); and f) an outbreak of spruce budworms in NE Canadian balsam forests.

Climate Change

Animals and plants have clear thresholds for viability and temperature and humidity ranges in which they mature, replicate, and thrive (Gill, 1920). Shifts in temperature isotherms in latitude and altitude with climate change could thus have profound impacts on ecotones, on biota and--in particular--on the distribution of pests and pathogens.

Several models using RS/GIS and GCMs have been used to project areas of the world where conditions conducive to vector-borne diseases may change with global warming scenarios. These include malaria, schistosomiasis, and dengue fever. The figure 1 shows the output of one such model for malaria.

Figure 1. Projected Changes in malaria risk associated with potential pattern of global warming

High Altitudes

Recent reports indicate that malaria and dengue fever are apppearing at higher latitudes and altitudes than at any time during this century. In addition, plants have been observed to be moving to new altitudes on 26 Alpine peaks, in the U.S. Sierra Nevada and Alaska, and in New Zealand. Moreover, summit glaciers are retreating on many continents, and ice caps show evidence of accelerated warming this century. (Additionally, Antarctica shows signs of instability, with significant breakages in 1995.)

An initial examination of Microwave Sounding Unit (MSU) and HIRS/2 data provided by Joel Susskind of the NASA/Goddard Space Flight Center suggests that, in El Niño years, warming in the upper atmosphere may exceed warming occurring on Earth's surface.

[There are several possible contributing factors to explain these observations. One is the increased relative heat absorption by carbon in the upper troposphere, because it is cooler at higher altitudes. Second, increased tropospheric water evaporation due to deep oceanic warming (SW Pacific, Atlantic, and Indian) (Parilla et al., 1994), and exaggerated during El Niño years (Graham, 1995), may augment greenhouse warming and increase high, heat-trapping, clouds. Third, sulfur-enriched lower clouds may also increase with increased atmospheric water vapor, and may reflect and absorb solar energy and thus cool Earth's surface.]

Conclusions

The costs of not understanding present climate instability and likely changes in climate due to human activities may be enormous. Disease outbreaks cause disability and mortality, and the impacts can ripple through societies and economies. To date, the dengue outbreak has cost Central American nations $7.5 million in control efforts alone; Peruvian fisheries lost $750 million in seafood exports during the 1991 cholera epidemic; and airline and hotel industries lost an estimated $2 billion from the Indian plague last fall. The global resurgence of malaria, dengue fever, and cholera--and emergence of relatively new diseases like Ebola--can impact development, trade and tourism, agriculture, and livestock.

Remote sensing alone or integrated into GISs and GCMs has multiple applications for understanding biological processes and, in particular, disease phenomena. A cholera EWS using RS to detect coastal algal blooms and target surveillance has immediate relevance to protecting populations. Prediction of conditions conducive to VBD outbreaks--associated with climate variability and teleconnections to ENSO events--can also be of use to public health authorities.

Additionally, RS can help project future potential disease distribution due to climate change. Finally, RS can play a central role in a multidisciplinary exploration of current physical and biological changes occurring at high altitudes, thus providing important information on climate trends and their impacts to better inform policymakers.

References

Aiken, J., G.F. Moore, and P.M. Holligan, 1992: Remote sensing of oceanic biology in relation to global climate change. J. Phycol. 28, 579-590.

Bouma, M.J., H.E. Sondorp, and J.H. van der Kaay, 1994: Health and climate change. Lancet, 343, 302.

Epstein, P.R., D.J. Rogers, and R. Sloof, 1993: Satellite imaging and vector-borne disease. Lancet, 341, 1404-06.

Gill, C.A., 1920: The relationship between malaria and rainfall. Indian Journal of Medical Research, 37, 618-632.

Glantz, M.H., R.W. Katz, and N. Nicholls, 1991: Teleconnections Linking Worldwide Climate Anomalies. Scientific basis and societal impact. Cambridge University Press, NY.

Graham, N.E., 1995: Simulation of recent global temperature trends. Science, 267, 666-71.

Parrilla, G., A. Lavin, H. Bryden, M. Garcia, and R. Millard, 1994: Rising temperatures in the subtropical North Atlantic Ocean over the past 35 years. Nature, 369, 48-51.

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