The damage caused by mercury contamination of fish – 04/27/2023 – Ciência Fundamental

In the early 1950s, residents of the paradisiacal Minamata Bay, a fishing village in southern Japan, observed that some cats were acting strangely, with falls, jumps and tremors accompanied by terrifying meows. The “dancing cat fever”, as it became known, caused many animals to die.

Shortly thereafter, the city began registering cases of children and adults with walking difficulties, falls, speech and cognition problems, seizures and vision and hearing loss. Nobody knew the causes of the symptoms of “Minamata disease”, and to try to solve this mystery, teams from the University of Kumamoto and the Ministry of Health went to the place.

Epidemiological studies in 1956 found 55 cases of the disease and 17 deaths, and indicated that the symptoms were associated with the ingestion of fish and shellfish that were probably contaminated with some heavy metal. In 1959, the hypothesis was presented that the origin of the contamination would be in the oceanic discharges of the Chisso Corporation, an industry in the region that produced acetaldehyde, raw material for the manufacture of plastics, and used mercury as a catalyst.

Mercury was released into the sea in an inorganic form, but due to the action of bacteria in the environment, it was transformed into the most harmful form to health – methylmercury, absorbed by plants and animals. Once in the aquatic environment, the phenomenon of bioaccumulation begins: the plants are ingested by fish, which in turn are ingested by larger fish, and each of them accumulates more methylmercury. When other animals or humans feed on these fish, they become contaminated and may show different symptoms, depending on the ingested and accumulated dose.

In our body, methylmercury binds to a specific amino acid, and thus manages to cross the brain barrier and the placenta. In addition, when entering cells, it impairs several reactions essential to metabolism.

Cells of the nervous system are among the main victims of methylmercury, and developing neurons are the most sensitive to its toxic effects. Pregnant women exposed to mercury, even asymptomatic or with few symptoms, can give birth to babies with severe neurological disorders. Thus, even if occasional, exposure to mercury can compromise several generations.

In the case of Minamata, in 2005 the number of patients with the disease reached 2,955. For many years, the number of newborns with cerebral palsy in the region was extremely high, as were the rates of miscarriages and birth defects. Although there are no visible changes or symptoms in the first months of life, excessive exposure to methylmercury can have consequences years later, with a reduction in attention parameters, verbal memory, motor development and performance in IQ tests.

Despite what happened in Minamata, cases of major contamination by methylmercury continued to occur in several countries, including Brazil. Studies indicate that the Munduruku people, in Pará, and Yanomami, in Roraima, have high levels of mercury. In general, contamination is higher precisely in the populations most impacted by illegal mining. For some years, researchers have been alerting to this reality, including the many cases of indigenous people with neurological and psychological alterations, as well as children with developmental delay. The situation is so similar to that of the first cases of mercury contamination recorded in the world that this Amazon region is called by some “the new Minamata”.

Analyzes indicate that fish, the main source of protein in these communities, are also contaminated, but stopping eating them is not a plausible solution, also because food is an important source of essential fatty acids for the development of the fetal nervous system. Researchers and health professionals guide pregnant women to eat two to three servings of fish per week, avoiding carnivorous species at the top of the food chain.

Changing the landscape of methylmercury contamination from industries and mining is essential for a healthy present and future. Far beyond astrological predictions, mercury in fish can represent a tragedy announced and that must be strongly fought in all countries.

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Rossana Soletti has a PhD in morphological sciences and is a professor at the Federal University of Rio Grande do Sul.

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