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Schistosomiasis affects 200 million people in 74 countries worldwide; 120 million of these are symptomatic and 20 million have severe disease. (1) Unlike the other three schistosome species affecting humans, Schistosoma japonicum is a true zoonosis, infecting 46 species of mammals, including humans, all of which are definitive hosts necessary for transmitting the infection. (2-4)
S. japonicum is endemic in China, parts of Indonesia and the Philippines. (5) These parasites are found in the 22 provinces in the Philippines that do not have a definite dry season; this leads to continual transmission. (6,7) Approximately 6.7 million people live in endemic areas in the Philippines. (8) Transmission to humans requires that they come in contact with fresh water colonized by amphibian snail hosts (Oncomelania hupensis quadrasi) that have become infected with S. japonicum through contamination by faeces from infected mammals. (9) Pigs, dogs, cats, rats, cows and water buffalo may become infected and may potentially play a role in the transmission of S. japanicum to humans, (6,10,11) but the association between infection levels in animals and humans has not yet been estimated.
An ecological study conducted in China used simple correlation methods to estimate the association between the prevalence of S. japonicum in water buffalo and that in humans over 10 years. The researchers assumed that all observations were independent, which is a questionable assumption that may have resulted in biased estimates. (12)
The objective of our study was to estimate the strength of the association between the intensity of S. japonicum infection in animals and humans across 50 villages in the province of Western Samar, the Philippines.
Study design and location
A cross-sectional study was conducted between August 2003 and November 2004 in the province of Western Samar (on the island of Samar) in the Eastern Visayas region of the Philippines. In 2002, there were estimated to be 57 033 farms with an area of 101 954 hectares in Western Samar. (13) In Western Samar, S. japanicum is considered to be endemic in 13 municipalities and 133 villages by the National Schistosomiasis Control programme (R Martinez, personal communication, May 2002).
Selection of study villages
Of the 133 villages, 25 with predominantly rain-fed farms and 25 with manmade irrigation systems were chosen for the primary objective of determining the effect of irrigation on transmission of S. japonicum. The following steps were taken to select these 50 villages. A total of 58 villages were excluded because: 3 were inaccessible; 5 had fewer than 50 households; and 15 were near the coast or urban areas where there was little rice farming, leaving 75 villages. Based on the information from farmers or village leaders as to which type of irrigation systems were used, we selected the 25 rain-fed villages and 25 irrigated villages that formed the study villages.
Selection of participants
Thirty-five eligible households were randomly selected within each village. Households had to have at least five members to be eligible for participation. Households that declined to participate were replaced by the next available household on the list. A maximum of six participants were selected from each household. Five individuals in the household were selected at random in addition to at least one randomly selected full-time farmer. If the household contained fewer than six individuals, all members were recruited. A sociodemographic interview was held with each participant and it provided data on age, sex, occupation and health history regarding schistosomiasis.
Selection of animals
In the first 10 villages, all animals from each species belonging to different owners were selected. In the next 40 villages, an animal census determined the number of animals of each species in each household, and a random sample of households was generated for each species. …