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February 5, 2002
SATELLITES VS. MOSQUITOES: TRACKING WEST NILE VIRUS IN THE U.S.
A NASA-funded study uses temperature and vegetation data from satellites to help track and predict where West Nile Virus is spreading in North America. Scientists and public health officials hope one day to use near real-time maps to focus resources and stave off the disease more efficiently.
The disease, first reported in the U.S. in 1999, causes flu-like symptoms that can lead to fatal encephalitis in people with compromised immune systems, like the elderly.
Though not yet proven, scientists believe the West Nile Virus may be spread across the country by infected birds traveling along their migration routes. Mosquitoes that act as a vector carry the virus, and pass it on when feeding on hosts like birds, livestock, other animals and people.
The satellite maps show nation-wide temperatures, distributions of vegetation, bird migration routes and areas pinpointing reported cases. The combined data helps scientists predict disease outbreaks by showing where conditions are right for the insects to thrive and where the disease appears to be spreading.
"The images are derived from satellite data that capture a number of variables that are crucial for detecting whether a habitat is suitable for a vector, like a mosquito that carries West Nile Virus," said David Rogers, the lead author of the study. Rogers is Professor of Ecology at Oxford University in the United Kingdom and a member of the International Research Partnership for Infectious Diseases (INTREPID) group, based at NASA's Goddard Space Flight Center. This paper is the cover story in the most recent issue of Photogrammatic Engineering and Remote Sensing.
"It's not a single variable that tends to determine whether a disease will occur, but rather a combination of variables," Rogers said.
Satellite sensors, like the National Oceanic and Atmospheric Administration's Advanced Very High Resolution Radiometer (AVHRR), provide information on vegetation and peak and average annual temperatures.
Mosquito populations are constrained by different factors in different places. In the North, colder temperatures hold mosquito populations back. In the south, where temperatures are higher, the insects are more dependent on the availability of moisture for survival. Lush vegetation provides a good indicator of areas with increased levels of moisture.
Mosquitoes tend to thrive in warmer, wetter areas. Satellites let researchers see which areas have these conditions, allowing for better prediction of disease transmission.
Similarly, satellites also help plot patterns like the timing of when temperatures peak during a year. Mosquito populations appear to increase during the first half of the year but only reach sufficient levels to transmit the disease during the second half of each year.
"The timing of a seasonal peak of temperatures determines the increase of vector populations in that place," said Rogers. "An early rise in spring temperature is likely to get the mosquito off to a flying start."
The idea is to let the satellite capture where the disease is spreading from year to year and make some predictions about where the disease is going. Computer models can determine which areas have the right combinations of temperatures and moisture levels most suitable for mosquitoes and transmission. Then, efforts and resources can target those high-risk areas.
The study suggests that a mild winter in 1998 to 1999 may have provided favorable conditions for mosquitoes to survive the winter, leading to a greater number of carriers in 1999. In that first year reported cases were found in three states. The number grew to 12 states in 2000, and more than 20 in 2001, despite the fact that the winters preceding these years were either average or slightly below average. "Once a disease like West Nile Virus establishes a foothold, and spreads, it will be very difficult to eradicate," said Rogers.
The methods used in this research may be modified and applied to study a variety of other diseases like malaria, dengue fever, Lyme disease, influenza and even asthma.
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Contact:
Cynthia M. O'Carroll
cocarrol@pop100.gsfc.nasa.gov
Goddard Space Flight Center
(Phone:301/614-5563)
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Monitoring the Spread of West Nile Virus in the U.S. - 52 KB JPEG
The image above represents a composite of Land Surface Temperatures between 1997 and 2000 to help monitor and predict the spread of West Nile Virus (WNV) in the United States.
In this figure, the mean land surface temperatures (LST) are in red; annual amplitude, or the difference between low and high annual temperatures are in blue; and annual phase, or the timing of annual temperature peaks appear in green. Brighter colors mean higher values. The major North-South temperature difference (dull red in the upper part of the image to bright red in the lower part) is considerably affected by the Rockies in the West and, to a much lesser extent, the Appalachians in the East. The brighter blue in the upper part of the image indicates the greater annual amplitude of the LST cycle at higher latitudes. Finally there is less variation in the timing of the annual peak of LST, which is earlier in the South than in the North.
Scientists working with the International Research Partnership for Infectious Diseases (INTREPID) program based at NASA are using such imagery to define and predict the conditions where mosquitoes transmit West Nile Virus in the U.S.
The conclusion about the importance of any single variable depends upon its absolute value and upon the context. A temperature of 30 degrees Celsius (86ø Fahrenheit) might be fatal for a mosquito at low humidity but survivable at higher humidities. The work done here on West Nile Virus and other diseases shows very clearly that it is a unique combination of temperature, humidity and vegetation variables that tends to determine mosquito and disease presence and abundance.
Black dots superimposed on this image are the locations (county geo-centers) where birds infected with WNV were reported between January and October 2001.
The image was produced by INTREPID from data taken by the National Oceanic and Atmospheric Administration?s (NOAA) Advanced Very High Resolution Radiometer (AVHRR) instrument.
Warmer Land Surface Temperatures May Provide Favorable Conditions For Mosquitos
These images were derived from data collected by the NOAA AVHRR since 1982.
The top image in this group is the 19-year mean land surface temperature (1982-2000) in false color; blue indicates low temperatures and red high. The three other images are the land surface temperatures for 1998, 1999, and 2000, showing the extent of inter-annual variability.
It is clear from these images that 1998 was warmer than average, 1999 was about average and 2000 was slightly cooler than average. The research suggests that the mild winter of 1998/99 provided conditions favorable for West Nile Virus transmission in 1999, perhaps by allowing greater over-winter survival of hibernating mosquitoes, leading to higher numbers of vectors in 1999.
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