Mercury is a widespread element that acts as a potent neurotoxin after conversion into methylmercury (MeHg). Mercury in aquatic environments is methylated through the action of anaerobic bacteria, and enters the food chain in this form. As a consequence, the main path to contamination of the general population is through fish consumption.
Contamination of populations in the Amazon basin has been recognized since the 1990s and was attributed to gold-mining activity (Branches, Erickson, Aks, & Hryhorczuk, 1993). Mercury is used to extract gold found in river beds and is evaporated from the amalgam by heating. Less than 30% of the mercury is recovered, while more than 60% is released into the atmosphere and returns to the soil with the rains or is directly rejected into the river (Maurice-Bourgoin & Quiroga, 2002).
Various studies reported that riverside Amazonian populations, especially fish-eating groups, were exposed to mercury contamination (Santos et al., 2000). Moreover, a neurological impact was discernible in adults as well as in children (Grandjean, White, Nielsen, Cleary, & de Oliveira Santos, 1999; Lebel et al., 1998).
The situation in the Andean tributaries of the Amazon has not been extensively elucidated despite the fact that huge quantities of mercury have been used for centuries (Maurice-Bourgoin & Quiroga, 2002). Gold-mining activity is not the only source of mercury: erosion of contaminated soils and volcanic activity must also be considered (Maurice-Bourgoin, Quiroga, Chincheros, & Courau, 2000; Roulet et al., 1999). Substantial gold mining took place in the upper Beni, however, and involved about 200 cooperatives. It was estimated that each year, 250 to 500 kg of mercury were used and that 50%-70% of the mercury was directly released into the environment (Maurice-Bourgoin & Quiroga, 2002). Contamination of populations living downstream could therefore be suspected. A survey performed in 1995-2000 reported fairly high concentrations of mercury in the hair of members of an Amerindian community living on the bank of the Beni (Maurice-Bourgoin et al., 2000). The expansion of the Beni just at the exit point of the Andes, its contribution to forming the Rio Madeira (one of the main tributaries of the Amazon), and the existence of sites favorable to mercury methylation justified a more in-depth survey of the process of mercury contamination of riverside populations.
Hence, the purpose of our study was two-fold: first, to document mercury contamination in a group of communities living in the flood plain of the Beni River and second, to examine risk factors associated with their lifestyle.
Subjects and Methods
Within the framework of a multidisciplinary study on mercury contamination in the flood plains of the Beni River, it was decided that a study be performed of human populations living in the area, from the Andean piedmont to 120 km downstream. The survey was performed from March to August 2004.
Subjects and Sampling
According to a national census in 2001, 14,000 people lived in the project area, but only 4,000 actually lived along the river bank. Since the present study considered the risk in the general population, we decided to include only the women and their children. The rationale for this choice was that prenatal mercury contamination could severely impair cerebral development of infants due to the capacity of methylmercury to cross the placenta barrier (Gilbert & Grant-Webster, 1995). The population was scattered throughout small communities and hamlets with frequent displacement for their subsistence. It was not possible to draw a random sample, and we preferred to examine each mother and her children present at the site during the visit. In all, 173 mothers, 354 children 0-10 years of age, and 104 children 10-15 years of age were surveyed.
Ecological and Cultural Setting
The area under study is part of the northern Amazon region of lowland Bolivia. It encompasses 10 millions hectares with a population of around 170,000 inhabitants. This region has a tropical climate with an annual rainfall ranging from 1,800 to 2,200 mm. Two ethnic groups live in this area. The largest ethnic group is the Tacanas, and the smaller ethnic group is the Tacanas, and the smaller ethnic group is a closely related tribe, the Ese Ejjas. The Tacana population is made up of around 3,400 people, on the basis of self-designation (Diagnostico Socioeconomico, 2005).
Tacanas are mainly farmers who grow maize, rice, tubers (cassava roots), plantains, and bananas. They practice a system of slash and burn to cultivate pieces of land that can reach one hectare. They obtain a substantial part of animal foods from hunting and fishing. They are well integrated into the Bolivian society. The majority of Tacanas speak Spanish and are able to read and write. The Ese Ejjas form the second group. They are believed to currently number 600-700 members. The clan living in the study area had approximately 200 people. Few adults have gone to school or speak Spanish, and they continue to use their vernacular language. Their productive activities are oriented towards use of forest and river resources. They cultivate the same products as the Tacanas (rice, maize, tubers, plantains) but in smaller parcels. They dedicate a large part of their time to fishing either in groups or individually. They have preserved a seminomadic way of life, following migration of fish and returning to their village during the flooding period.
Information collected during the visits indicated a mediocre health status in this population. Results have been partly published elsewhere (Benefice, Monroy, Jimenez, & Lopez, 2006). Fifty percent of the mothers surveyed had lost at least one child. Mothers usually breastfed their children for more than one year. The prevalence of anemia among women was high: 42% (95% CI [confidence interval] = 34.6%-49.8%) presented moderate to severe forms of anemia. This figure was 1.5 times higher than the national level. Chronic malnutrition represented by growth retardation was very frequent in children: 41% (95% CI = 33.9%-49.0%) of preschoolers were affected. Intestinal parasitism was very frequent in children.
This study was approved by the "Comite Nacional de Bioetica de Bolivia" and by the "Comite Consultatif de Deontologie et Bioethique" (IRD, France). Subjects or their parents were individually informed concerning the...