Parasites that cause schistosomiasis are a major health hazard in many tropical/subtropical regions of the globe. Disease control programs need basic environmental risk assessment capabilities for this human parasitic disease. The application of advanced satellite remote sensing and geographic information systems (GIS) technology is opening new technically and cost-effective approaches to these long-term public health problems.
In this study, sponsored initially by the National Institutes of Health/National Institute of Allergy and Infective Diseases grant R15, then by the Binational Fulbright Commission (Cairo, Egypt), and the Egyptian Ministry of Health/USAID, we experimented with AVHRR thermal imagery and a multi-year epidemiological data base.
The basic hypothesis was that diurnal surface temperature difference maps (dT) calculated from the early morning and mid afternoon NOAA/AVHRR 1.1 km resolution overpasses, would allow us to discriminate soil moisture domains in the vicinity of the Nile Delta. A further hypothesis was that the wettest of these domains would harbor the parasites and drier zones would have lower infection rates or be clear of the parasite.
The mechanism: a portion of the life cycle of these parasites requires snails as host‹fewer snails, fewer parasites. In the drier zones of the region snail populations are suppressed or die back, collapsing the infection rate. Infective stages invade the host during water contact after skin penetration of cercaria released by snail vectors that live in irrigation canals and other water bodies.
In the preliminary studies, surface temperature patterns in the Nile Delta region were characterized in a classification of Tmax, Tmix, and dT images for August 16, 1990, that reflected the classic S. mansoni prevalence maps described by Scott in 1937. The broad thermal domains seen in the August 16, 1990 image were seasonally stable in the October 18, 1990 and February 14, 1991 dT images, suggesting that permanent features, not vegetation or climate, were responsible for patterns seen. Normalized difference vegetation index images (NDVI) for each day revealed no apparent relationship to thermal patterns.
Using the TeraScan(TM) software, an analysis was done on the diurnal dT AVHRR images of August 1990 and February 1991 to study the relationship of regional thermal domains to historic S. mansoni and S. haematobium distribution in the Nile delta. In both dT images, a series of temperature/distance profiles revealed a decrease in dT values of approximately 2 degree C at points approximating Scott's transition from low to high prevalence of S. mansoni. Analysis continued with an epidemiological data base at 41 rural sites involved in surface surveys conducted in 1935, 1983, and 1991. An analysis using the Spearman rank correlation coefficient between median dT values and S. mansoni prevalence indicated that lower dT values reflect wetter hydrological regimes that are more suitable for the parasite propagation (Malone, J.B. et al., 1994). These results were confirmed by similar analyses of individual images, seasonal composites, and annual composites of a 1990-91 monthly dT and NDVI time series. In preliminary work, snail vector population density was greatest in wetter moisture domains of low dT.
Dramatic recent success in schistosomiasis control has been realized in Egypt by a campaign of mass chemotherapy supplemented by molluscicide use and a television-based public education program (El Khoby, personal communication). Identification of environmental indicators of high risk of infection may facilitate provision of more-frequent chemotherapy and focus molluscicide application for areas of high prevalence that may serve as reservoirs for reintroduction into well-controlled areas.
Satellite environmental remote sensing capabilities combined with GIS technology are providing new ways to address classic concepts of landscape epidemiology, i.e., diseases having natural habitats in well-defined ecosystems where pathogens, vectors, and natural hosts form associations within which the pathogens circulate.
References:
Huh, Oscar K., 1991: Limitations and capabilities of the NOAA satellite advanced very high resolution radiometer (AVHRR) for remote sensing of the Earth's surface. Preventive Veterinary Medicine, 11, 167-183.
Malone, J. B., O. K. Huh, D. P. Fehler, P. A. Wilson, D. E. Wilensky, R. A. Holmes, and A. I. Elmagdoub, 1994: Temperature Data from Satellite Imagery and the Distribution of Schistosomiasis in Egypt. Am. J. Trop. Med. Hyg., 50, (6), 714-722.