Stepping into a dangerous quagmire: Macroecological determinants of Bothrops envenomings, Brazilian Amazon

Abstract

Despite significant and successful efforts in Brazil regarding snakebites in the areas of research, antivenom manufacture and quality control, training of health professionals in the diagnosis and clinical management of bites, little is known about determinants of snakebites incidence in order to further plan interventions to reduce the impact of this medical condition. Understanding the complexity of ecological interactions in a geographical region is important for prediction, prevention and control measures of snakebites. This investigation aims to describe spatial distribution and identify environmental determinants of human envenoming by lancehead pit vipers (Bothrops genus), in the Brazilian Amazon. Aggregated data by the municipality was used to analyze the spatial distribution of Bothrops bites cases and its relationship with geographic and environmental factors. Eight geo-environmental factors were included in the analysis as independent variables: (1) tree canopy loss increase; (2) area with vegetation cover; (3) area covered by water bodies; (4) altitude; (5) precipitation; (6) air relative humidity; (7) soil moisture; and (8) air temperature. Human envenoming by lancehead pit vipers (Bothrops genus) in the Amazon region is more incident in lowlands [Adjusted regression coefficient [ARC] -0.0007 (IC95%: -0.001; -0.0006), p<0.0001], with high preserved original vegetation cover [ARC 0.0065 (IC95%: 0.0071; 0.0060), p<0.0001], with heaviest rainfall [ARC 0.0001 (IC95%: 0.00009; 0.0001), p<0.0001] and higher air relative humidity [ARC 0.0082 (IC95%: 0.0108; 0.0056), p<0.0001]. This association is interpreted as the result of the higher prey availability and further abundance of pit vipers in such landscapes.

Fig 1. Some species responsible for Bothrops envenomings in the Brazilian Amazon and surrounding cerrado areas.

a) Bothrocophias hyoprora; b) Bothrops atrox; c) Bothrops bilineatus bilineatus; d) Bothrops brazili; e) Bothrops moojeni; f) Bothrops mattogrossensis.

Fig 2. Incidence rates of Bothrops envenomings presented by state.

The highest incidence rates were reported in the states of Pará (57.0 per 100,000 inhabitants/year), Roraima (53.3 per 100,000 inhabitants/year) and Tocantins (49.7 per 100,000 inhabitants/year). The lowest incidence rates were reported in the states of Maranhão (15.9 per 100,000 inhabitants/year), Rondônia (22.6 per 100,000 inhabitants/year) and Amazonas (29.1 per 100,000 inhabitants/year).

Fig 3. Spatial distribution of Bothrops snakebites in the Brazilian Amazon from 2010 to 2015.

Map were created using incidence per 100,000 inhabitants. Snakebites are largely distributed in the Amazonian states, with several counties presenting incidences higher than 100 cases per 100,000 inhabitants. The Northwest of the State of Amazonas, North of Roraima, North of Pará on the border with Amapá and central part of the state of Tocantins, show spots of higher incidence rates.

Fig 4. Descriptive analysis of geo-environmental variables.

A) Tree canopy loss. Mean tree canopy loss in the study area was 35.6% (±33.4%), being lower in Western Brazilian Amazon, namely in the North and West of the state of Amazonas, West of the state of Acre and Amapá. A higher tree canopy loss is observed in the state of Maranhão, in the extreme East of the states of Pará and Acre, and in some municipalities of the state of Mato Grosso and Rondônia. B) Area with vegetation cover. The mean area with vegetation cover is 25.31% (±27.4%), being lower in the states of Tocantins, Maranhão and in the South of the state of Mato Grosso. C) Area covered by water bodies. The mean area covered by water bodies is 2.9% (±6.9%) and is larger in the municipalities located in the banks of the Solimões, Negro and Amazonas rivers and in the Pantanal region of the state of Mato Grosso. D) Altitude. The whole study area had a mean of 153.7 meters above sea level (±148.6 meters). Altitude is lower in municipalities located in the banks of the Amazonas river and higher in the states of Mato Grosso, Tocantins, south of the Maranhão, peaking in the north of the state of Roraima. E) Precipitation. The average value of accumulated rainfall in the study area is 9,356.2 millimeters (±2,335.1 millimeter), ranging from <5,000 millimeters in some municipalities in the periphery of the region to >13,000 millimeters in the extreme Western Amazon. F) Air relative humidity. Mean air relative humidity in the study area is 82.9% (±6.7%), ranging from ~70% in municipalities Southern Amazon border to >95% in the Western Amazon. G) Soil Moisture. Mean soil moisture in the study area is 0.3270047 cm³ water/cm³ soil (±0.0280811 cm³/cm³), being lower in the states of Maranhão, Tocantins and Mato Grosso and higher in the state of Amazonas. H) Air temperature. Mean air temperature in the region is 25.2°C (±0.81°C), ranging from ~23°C in the state of Roraima to >27°C in the extreme Eastern Amazon.

Fig 5. Temporal and spatial distribution of Bothrops envenomings and precipitation.

Map was created using the absolute number of Bothrops snakebites and the accumulation of precipitation per month during the years studied. Seasonality of Bothrops bites over the year, with a pronounced increase of cases in the rainiest trimester across the study area. In the bar graph is the absolute number of snakebites and in the line graph is the cumulative value of precipitation.