Characterization of larval habitats of Anopheles (Nyssorhynchus) darlingi and associated species in malaria areas in western Brazilian Amazon

Abstract

Background: Anopheles darlingi is the most efficient vector of malaria parasites in the Neotropics. Nevertheless, the specificities of its larval habitats are still poorly known.

Objectives: Characterize permanent larval habitats, and population dynamics of An. darlingi and other potential vectors in relation to climate, physicochemical variables, insect fauna and malaria cases.

Methods: A 14-month longitudinal study was conducted in Porto Velho, Rondônia, western Brazilian Amazon. Monthly, 21 permanent water bodies were sampled. Immature anophelines and associated fauna were collected, physicochemical characteristics, and climate variables were recorded and analyzed.

Findings: Five types of habitats were identified: lagoon, stream, stream combined with lagoon, stream combined with dam, and fishpond. A total of 60,927 anophelines were collected. The most abundant species in all habitats were Anopheles braziliensis and An. darlingi. The highest density was found in the lagoon, while streams had the highest species richness. Abundance was higher during the transition period wet-dry season. There was a lag of respectively four and five months between the peak of rainfall and the Madeira River level and the highest abundance of An. darlingi larvae, which were positively correlated with habitats partially shaded, pH close to neutrality, increase dissolved oxygen and sulphates.

Main conclusions: The present study provides data on key factors defining permanent larval habitats for the surveillance of An. darlingi and other potential vectors as well as a log-linear Negative Binomial model based on immature mosquito abundance and climate variables to predict the increase in the number of malaria cases.

Fig. 1:. monthly rainfall (mm) and Madeira River level (m) between April 1998 and May 1999. Porto Velho, Rondônia, Brazil.

Fig. 2:. relative location of permanent larval habitats of anophelines in the Municipality of Porto Velho, Rondônia, Brazil.

Fig. 3:. total number of Anopheles spp. immature stages identified collected monthly in all type of habitats in relation to rainfall (mm) in Porto Velho, Rondônia, Brazil. April 1998 - May 1999.

Fig. 4:. cross-correlation analysis between the variable rainfall and number of the four most abundant Anopheles species collected. Porto Velho, Rondônia, Brazil. April 1998 - May 1999.

Fig. 5:. (A) Correlation biplots based on distance-based redundancy analysis (dbRDA) ordination of environmental variables, months, and Anopheles species in streams. Porto Velho, Rondônia, Brazil. April 1998 - May 1999. Vectors (arrows) point in the direction of increasing values for the respective variables; longer vectors indicate stronger correlations between variables scores and axes. (B) Correlation biplots based on dbRDA ordination of aquatic insects and Anopheles species in streams. Porto Velho, Rondônia, Brazil. April 1998 - May 1999. Vectors (arrows) point in the direction of increasing values for the respective variables; longer vectors indicate stronger correlations between variables scores and axes.