The spread of disease is influenced by a multitude of factors. Chagas disease is an inflammatory disease that is spread by insects of the genus Rhodnius, which feed on the blood of vertebrates. These insects of the Rhodnius genus frequently interact with a variety of vegetation, debris, and vertebrate and invertebrate species within the palm crown of the Attalea butyracea tree. The Attalea butyracea palm tree is a tropical plant that grows in the dry forests across the Amazon, and the crown (branches and leaves) of this tree provides an ideal experimental environment to study how microclimates (the climate of a small area) and food web relations affect Chagas disease in certain species.
CEID’s Nicole Gottdenker and a team of experts performed an observational study of these palm crowns to evaluate the factors associated with the abundance of the Rhodnius insects and the Trypanosoma cruzi parasite, which causes Chagas disease. A total of 1098 Rhodnius pallascens insects were collected from 105 palm crowns in three locations in rural Panama: Santa Fe, La Chorerra and Capira.
The team recorded the microclimate and the habitat conditions for 49 of these palm crowns. The number of vertebrate and invertebrate species present in each palm was recorded to examine the food webs within these crowns. Associations between these food webs and Chagas disease were then examined to assess two important components of these food webs: vulnerability (the average number of predators per prey) and generality (average number of prey per predator).
Linear models were developed using the collected field data, and the models showed that microhabitat conditions (average temperature, palm tree height, canopy cover, etc.), geographic location, and different land types all impacted the abundance of these insect vectors (organisms which transmit the parasite to other organisms) and T. cruzi parasite infection.
Out of the 1098 insects captured, 476 were tested for the Trypanosoma cruzi parasite, and 181 of these insects tested positive for the Chagas parasite. Across the different locations, the percentage of insects that tested positive for the T. cruzi parasite varied significantly. In Santa Fe, 57% of sampled insects tested positive, while only 28% tested positive in La Chorerra. Further, there was variation in percentage of infected insects between the type of area where the palms were located. 61% of the R. pallascens insects from palms in forest remnants (areas of forest surrounded by deforested areas) tested positive, 28% were positive in pastures, and 43% were positive in peridomestic landscapes (within or 100 meters from human settlement). Insect vectors in palms with mammals were about 25 times more likely to be positive for T. cruzi infection, compared to vectors in palm trees without mammals.
Overall, the abundance of these insect vectors was negatively associated with species diversity within the palm crown. These findings indicate that a comprehensive study examining local environmental factors and food webs can inform disease management strategies across different habitats.
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By: Brenna Daly