The use of a vest equipped with a global positioning system to assess water-contact patterns associated with schistosomiasis.

The real exposure to many of the tropical diseases is difficult to assess at the individual-level due to problems of recall, self-reported diaries, personnel requirements, and altered behaviour related to observation. We present a study in an area endemic for Schistosoma japonicum in which global positioning system (GPS) receivers were used for personal time-activity monitoring to assess water-contact associated with schistosomiasis transmission. The study subjects were equipped with a vest with an embedded GPS receiver for 8-hour periods. The resulting data were used to create hourly time-activity maps, which were subsequently used in interviews to ascertain the timing and location of the water-contacts. Based on a sample of twenty-four 8-hour person-days we found that individuals averaged 1.4 +/- 1.2 water-contacts per day, and were surprisingly mobile, with 39% of the participants having spent time out of the village (0.8 +/- 1.4 hours outside of village). Such mobility suggests the need for further research into social patterns that may facilitate the spread of parasites, and contribute to sustained transmission. We present an assessment of the accuracy of cheaper commercially- available GPS units that have shown promise in such applications. We feel that a speed-filtering method is effective in managing measurement errors commonly encountered during personal activity monitoring with GPS. We conclude that personal GPS units can help reduce recall problems associated with other methods of assessing water-contact, and that they offer valuable insights into time-activity patterns that influence schistosomiasis transmission.