Color of rivers in the Amazon may be linked to the incidence of malaria

Fiocruz Amazônia, in partnership with the Federal University of Amazonas (Ufam) and the French Research Institute for Development, has carried out a study with an unprecedented approach to assess the relationship between the color of rivers and the incidence of malaria. According to the research, a relationship was observed between the two factors, which is why, in extreme situations, localities near black colored rivers have a higher incidence of malaria when compared to the so-called whitewater rivers, such as the Solimões, Amazonas, Madeira and Juruá. The results of the study, published in a scientific article in the Malaria Journal, indicate that the waters of dark colored rivers, such as the Rio Negro (Black River), because they carry less sediment and are more acidic, can favor the proliferation of the Anopheles darlingi mosquito, which transmits the disease.

Led by researcher Fernanda Fonseca, deputy head of the Laboratory of Modeling in Statistics, Geoprocessing and Epidemiology (Legepi/Fiocruz Amazônia), the work is also authored by Fiocruz Amazônia's Public Health researchers Jesem Orellana and Antonio Balieiro; Naziano Filizola (Inpa); Jean-Michel Martinez (French Research Institute for Development); and James Dean Santos (Ufam). The results, according to the authors, help to identify the places most at risk of malaria transmission, which can contribute to more precise planning of actions aimed at controlling the disease in the region. According to Fernanda Fonseca, the research was carried out in a region with hydrographic characteristics heterogeneous enough to allow an analysis that contrasted the different colors of the rivers and covering almost the entire state of Amazonas. The work was the subject of a recent article in Forbes magazine.

The analyses include data from 50 of the 62 municipalities in the state of Amazonas and were based on satellite images from the Google Earth app, data from the National Water Resources Information System (SNIRH), the database of the Observatory of the Environment, Research in Hydrology and Geodynamics of the Amazon Basin and the Malaria Epidemiological Surveillance System (Sivep-Malária) of the Ministry of Health.

"In Brazil, malaria is concentrated in the Amazon region and rivers play a major role in the life cycle of the disease, as the vector reproduces in aquatic environments," points out the research summary. The waters of Amazon rivers have different chemical characteristics, mainly due to the geology of the region, the type of vegetation, the presence of decomposing organisms and the climate. The research analyzed malaria incidence data, between 2003 and 2019, from 50 municipalities in the state of Amazonas located on the banks of rivers with black, white and mixed (black and white) water, with a 99% probability; "In this context, the study proposed an approach based on hypotheses that correlated the colors of the water and the incidence of the disease, in order to determine whether the characteristics of the type of water color influence the distribution of the disease," explains Fernanda.

The researcher points out that, despite the progress made in recent years in eliminating the disease, in 2021, countries like Venezuela, Brazil and Colombia accounted for almost 80% of cases in the region of the Americas. "The epidemiological situation remains worrying and challenges such as having reliable data on the incidence and distribution of the disease, as well as locally adapted strategies to improve early diagnosis or timely access to treatment, remain important challenges, especially in relation to vulnerable groups and regions," says the researcher, mentioning as examples indigenous peoples who live in remote and degraded areas, often difficult to access or insufficiently reached by health services, such as in areas of illegal logging and mineral extraction in the Amazon.

The study reveals that, when comparing the lowest values of the malaria incidence indicator (IPA) between the three colors of water assessed, the mean levels of the blackwater rivers were substantially higher than the other two, since the probability of malaria incidence is higher in blackwater rivers compared to those of white water and mixed water, close to 99% in both comparisons.

Amazonas

Fernanda notes that, as Amazonas has historically been one of the states with the highest incidence of the disease in Brazil, since 2002, the Ministry of Health's National Malaria Control Program has strongly encouraged the state to tackle the disease. Actions were taken to strengthen local management, with technical support, greater access to long-lasting insecticidal nets, conventional and rapid diagnostic tests, improvements to diagnostic laboratory networks, health education and training for health workers.

The study showed that there was a favorable downward trend in the incidence of malaria in Amazonas between 2003 and 2019. However, epidemiologist Jesem Orellana, head of Legepi/Fiocruz Amazônia, points that more recently the scenario has become unfavorable, as in 2021, Amazonas was among the five states in the Amazon region that failed to reach the maximum target for autochthonous cases, with 17.4% (60.380) more cases than expected (51.416). Another piece of information that also shows the worsening epidemiological situation of malaria in Amazonas is that of the Amazonian municipalities considered to be at high risk, since of the 29 municipalities classified as high risk by the Ministry of Health, 14 or near half were in Amazonas in 2021.

Jesem Orellana also highlights the pioneering nature of the study, as it not only explores a little understood topic, but also combines the expertise of scientists from the fields of hydrology, geoprocessing and remote sensing, statistics and mathematics applied to health and epidemiology. "Interdisciplinary work is crucial to the consolidation of research cooperation networks and to a broader understanding of challenging health problems, such as the elimination of malaria," he said.

Researcher Antonio Balieiro, who took part in the construction of the statistical model developed especially for analyzing the data collected, explains that the following methods were used in bayesian inference, which consists of assessing hypotheses by maximum likelihood; dynamic models and by mixing likelihood distributions. "In fact, the person responsible for building a specific model to work with the data that we had on malaria was Professor James Dean, from Ufam, one of the collaborators on the article. We developed a first model, but then we realized that we would need more refined statistics to analyze thse data and Professor James' expertise was key for this," explained Balieiro.

It is expected that the results will help to improve the control or elimination of the disease by expanding knowledge on the identification of susceptible areas or those at greater risk of transmission. "Our results can be extrapolated to regions with health and hydrological characteristics similar to those observed in the Brazilian Amazon," said Fernanda.

Sediments

The presence of suspended sediments in rivers, usually sand, clay particles and silt, changes the characteristics of the water, such as its physical and chemical properties and, above all, the color of the water. These can harm or favor the breeding sites of malaria vectors and, consequently, have an impact on the transmission of the disease. On Amazon, the colors of rivers are classified as white, black or clear. Whitewater rivers carry a large amount of suspended sediment and have a pH close to neutral. These include the Madeira, Purus, Juruá and Solimões/Amazonas rivers.

Blackwater rivers, on the other hand, get their name from their characteristic color, which results from the substances dissolved therein. These rivers have an acidic pH, carry a lot of organic matter and have a low concentration of suspended sediments in their waters. The bigger of these is the Rio Negro. Finally, clear water rivers (for example, the Tapajós river) are greenish or transparent. These rivers carry a small amount of dissolved sediments, have a certain level of dissolved organic matter and a pH close to neutral. In this context, the study proposed an approach based on hypotheses that correlate the colors of Amazon river waters and the incidence of malaria, in order to assess whether the characteristics of the type of water color influence the distribution of the disease.