An ecosystem approach for the study of neurobehavioral effects of methylmercury in the Brazilian Amazon

Donna Mergler PhD
CINBIOSE
WHO-PAHO Collaborating Centre for the Prevention of
Université du Québec à Montréal
CP 8888, Montréal, Québec , Canada

During the past 10 years, an interdisciplinary team of Canadian and Brazilian researchers from the natural, health and social sciences have used an ecosystem approach, with a strong participatory research component, to examine the pathways of mercury contamination, human exposure and health effects, mitigation measures and their efficiency, in collaboration with communities living along the Tapajós River, a major tributary of the Amazon in Brazil. Previous reports of high levels of mercury in fish and in humans attributed the source to gold mining.

The study revealed that large-scale deforestation, mainly from “slash-and-burn” agricultural practices, was the major culprit, through soil erosion and lixiviation (washing the soluble matter), releasing mercury into the river. The deforested areas are increasing in size due to large in-migration and the need clear the forest to grow food to feed the growing population.. Climatic conditions and aquatic vegetation are optimal for mercury methylation, accelerating the incorporation of mercury into the trophic chain and contaminating the fish, a dietary mainstay of this population. Measurements of hundreds of fish samples showed that mercury concentrations varied greatly, depending upon feeding habits, growth rate, age, and location. Mercury in humans, measured in hair samples, cut in centimeters, provided a chronological portrait of exposure. Exposure increased with fish consumption, was higher among those who ate more piscivorous (fish-eating) fish and varied seasonally. Evaluation of nervous system functions showed significant declines in motor and visual functions in relation to increasing exposure.

The integrated results were returned to the communities and village workshops examined short- medium-, and long-term solutions with respect to diet, fishing, and farming practices. Since fish is a highly nutritious food and the major source of animal protein, a positive slogan, inviting people to “Eat more fish that don’t eat other fish” was adopted. In addition, a chart with drawings of 42 fish species in red (high mercury), yellow (medium levels) and green (low mercury) was posted in every house.

Re-assessment of fish consumption, exposure and neuro-outcomes, 5 years later, showed that the villagers ate the same amount of fish, but had reversed the proportion of herbivorous to piscivorous fish. Exposure had decreased by 40 percent! There was improvement in motor functions, but visual functions continued to decline in correlation with previous exposure levels.

To further foster maximizing nutritional input from fish and minimizing toxic risk, an extensive dietary study was undertaken with 26 village women, coordinated by the village midwife; for 13 months, they kept daily food-frequency diaries. Comparison of monthly hair mercury levels with fruit intake, after controlling for fish consumption, showed that those who ate more fruit had lower mercury. This positive influence of fruit consumption was confirmed in an epidemiological study of more than 400 persons.

Ongoing health studies are examining the progression of visual decline. The environmental team is working with fishers to identify “hot spots” for methylation and with farmers to modify agro-forestry practices to reduce soil erosion. Analysis of communication networks within the villages is helping to identify how to implement programs for improving the environment and health. The success of this project is attributed to the synergy of scientific inputs, coupled with community participation and an ecosystem approach to human health.