STUDY ON THE IMPACTS OF DEFORESTATION ON MALARIA TRANSMISSION
Source: http://www.nhm.ac.uk
Author: Leah Gainey September 2014
Study on the Impacts of Deforestation on Malaria Transmission: September 2014
Contents Introduction .......................................................................................................................................... 2 1. Malaria .............................................................................................................................................. 3 1.1 Overview ..................................................................................................................................... 3 1.2 Anopheles darlingi ...................................................................................................................... 4 2. Deforestation and Malaria ................................................................................................................ 5 2.1 Removal of Competition ............................................................................................................. 6 2.2 Removal of ‘Dead-End’ Host ....................................................................................................... 6 2.3 Creation of New Suitable Habitats .............................................................................................. 6 3. Malaria Mitigation Strategies............................................................................................................ 7 4. Conclusion......................................................................................................................................... 7 5. References ........................................................................................................................................ 9
Trocano Araretama Conservation Project - Study on the Impact of Deforestation on Malaria Transmission 2014
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Study on the Impacts of Deforestation on Malaria Transmission: September 2014
Introduction Vector-borne diseases refer to illnesses caused by pathogens and parasites in human populations, and account for over 17% of all infectious diseases (World Health Organization, 2014). The total number of global cases of vector-borne diseases diagnosed every year is approximately 1 billion, with about 1 million of these resulting in death. One of the most prevalent vector-borne diseases is malaria, which is accountable for the highest global mortality rate of any tropical disease (Cruz et al., 2013). Historically widespread, malaria has now been eradicated in many parts of the world; however it remains a prevalent threat in many regions surrounding the equator, such as in many parts of South America, including Brazil. The Pan American Health Organisation classifies Brazil as ‘medium risk’ for malaria, implying that the annual parasite incidence (API) is between 0.1-1% of the inhabitants (Pan American Health Organization, 2008), however the distribution of malaria cases are not uniform throughout the country; in 2007 there were an approximate 460,000 cases of malaria reported in Brazil, but 99.9% of these originated in the Amazon (Hahn et al., 2014). In 2010, ~580,000 cases of malaria were reported in the Amazon Basin, and 281,586 of these were in Brazil (Cruz et al., 2013). As the Trocano Araretama Conservation Project comprises 1.3 million hectares of the Brazilian Amazon, this high incidence of malaria is of key relevance to the residents of the project area. The main vector of malaria transmission in the Brazilian Amazon is the mosquito Anopheles darlingi. The survival, density, and distribution of the mosquito is significantly influenced by minor changes in their environmental conditions, making them one of the most sensitive forest species to environmental changes caused by deforestation. Changes in variables such as temperature and availability of breeding sites can have major consequences on the mosquito population demographic (Yasuoka & Levins, 2007). Deforestation in the Amazon has proceeded at a rate of between 600028,000 km2/year in the last decade (Hahn et al., 2014), and as this deforestation has impacted the distribution and abundance of a multiplicity of forest plants and animals, recent studies have begun to explore the possibility that arthropod disease vectors such as A. darlingi may also be affected (Vittor et al., 2006). This would in turn have a potential consequence on disease patterns in affected areas. This study aims to address the subject of malaria and its primary vector in Brazil, and presents a variety of literature surrounding the topic of malaria transmission and the potential implications of deforestation on its prevalence. Continued development in the Amazon in the form of agriculture, construction of roads, and logging (among other factors) have resulted in an increasing level of deforestation, and if a positive correlation between deforestation and malaria transmission is confirmed, then the negative implications for the residents of the Trocano Araretama Project could be significant.
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Study on the Impacts of Deforestation on Malaria Transmission: September 2014
1. Malaria 1.1 Overview Malaria is an infectious parasitic disease caused by four Plasmodium species in humans: -
Plasmodium falciparum; Plasmodium vivax; Plasmodium malariae; Plasmodium ovale;
The parasites are transferred to humans through a vector species, namely Anopheles mosquitos, which bite their human hosts between dusk and dawn. There are more than 60 species of Anopheles worldwide that are recognised as vectors, but the most effective species in the Amazon Basin is Anopheles darlingi, which become infected with either P. falciparum or P. vixax, and transmit the parasite to a human host (Figure 1). According to the latest estimates by the World Health Organization, there were approximately 207 million cases of malaria in 2012 (with an uncertainty range of 135 million to 287 million) and an estimated 627,000 deaths (with an uncertainty range of 473,000 to 789,000) (WHO, 2013) and malaria mortality rates have fallen by 42% globally since 2000.
Figure 1: Distribution of malaria attributed to P. falciparum and P. vivax in Brazil. Source: PAHO, 2008
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Study on the Impacts of Deforestation on Malaria Transmission: September 2014
1.2 Anopheles darlingi
Figure 2: A. darlingi. Source: grogdocarlosreis.blogspot.com
Anopheles darlingi is classified as a riverine mosquito, and is one of the most effective vectors of New World malaria as it can become infected with both P. vivax and P. falciparum. They are typically found in rural, lowland forest locations, with a wide geographic location ranging from northern Argentina to southern Mexico (Brochero et al., 2005) (Figure 3). They are unable to survive in dry climates; the preferable larval habitat of A. darlingi comprises natural bodies of water such as lakes or slow flowing steams with shaded, clear water. Larvae are commonly encountered in patches of floating debris along river margins, however it is postulated that they may possess a level of adaptability to areas associated with human intervention. This is supported by the fact that larvae have been identified in slightly brackish water; in turbid, polluted water; and in abandoned gold mine dugouts, all uncharacteristic locations for the species (Sinka et al., 2010). A. darlingi feeds on warm blood, and are described as endophilic; this means that they are ecologically associated with humans and their domestic environment. They tend to rest outside, regardless of whether they have taken their blood meal indoors or outdoors, and generally bite throughout the night between dusk and dawn. Their host preference varies from one place to another, and studies suggest that their biting patterns may represent an adaptation to human behaviour; a study by Moreno et al. (2007) concluded that the all night activity of the species in the southern Venezuelan goldmines may be an adaptation to the all night activity of the miners there. The atmospheric variable affecting the species most significantly is precipitation, which increases river flow and height and can flush away the A. darlingi larvae (Sinka et al., 2010). While land surface temperature does not have as significant an effect on the species distribution, it may be indicative of hotter, dryer climates outside the forest habitat that are unsuitable for an A. darlingi population.
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Study on the Impacts of Deforestation on Malaria Transmission: September 2014
Figure 3: Predicted distribution of A. darlingi based on 318 occurrence records (black dots). Source: Sinka et al. (2010).
2. Deforestation and Malaria Deforestation in Brazil has been progressing at a steady rate, now perceived to account for almost 24% of the overall global greenhouse gas emissions from land cover change (Aragão & Shimabukuro, 2010). While Brazil’s National Institute for Space Research (INPE) have been monitoring deforestation since 1988, these estimates do not account for forest alterations that selectively thin and kill the canopy; it is therefore postulated that logging could account for an additional 60-123% of forest damage than has been reported (Hahn et al., 2014). Recent studies have examined deforestation and forest disturbance, exploring the potential correlation between them and the transition of malaria. Olson et al. (2010) conducted a study analysing health district reports collected in 2006 by the Programa Nacional de Controle da Malária and concluded that a 4.3% increase in the percentage of deforestation from August 1997-August 2000 was associated with a 48% increase in the incidence of Trocano Araretama Conservation Project - Study on the Impact of Deforestation on Malaria Transmission 2014
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Study on the Impacts of Deforestation on Malaria Transmission: September 2014
malaria in the areas. Another study in 2006 examined the effect of deforestation on the humanbiting rate of A. darlingi and found that the biting rate was more than 278 times higher in deforested areas than in that of intact forest (Vittor et al., 2006). Three main consequences of deforestation have been found to affect the biting prevalence of A. darlingi, and thus increase the chances of malaria transmission: -
Removal of competition Removal of ‘dead-end’ hosts Creation of new suitable habitats
2.1 Removal of Competition Anopheles darlingi is a vector species of mosquito, but an intact forest supports the biodiversity of non-vector mosquito species’ as well. These non-vector species act as direct competition for A. darlingi in terms of obtaining a blood meal, thus acting as a regulator of population (Yasuoka and Levins, 2007). Disturbing the forest compromises the ability of the non-vector species to thrive, thus removing a main competitor of A. darlingi from the ecosystem and allowing for their population to expand.
2.2 Removal of ‘Dead-End’ Host Intact forests also serve as habitat to many warm-blooded hosts of malaria. These species are known as ‘dead-end’ hosts as the mosquitos feed on them but we do not, so the infection is restricted to the ‘dead end’, non-competent host (Laporta et al., 2013). Disturbance of the forest can result in the migration of such species away from human interference, thus removing a source of prey for the mosquitos and leaving them no alternative other than to feed on the remaining warm blooded species: the human population.
2.3 Creation of New Suitable Habitats A. darlingi seek out partially sunlit, clear water in which to lay their larvae. In undisturbed forests they are generally never found in standing bodies of water due to the fact that the forest canopy provides too much shaded area. Reducing the forest canopy through deforestation increases the sunlit bodies of water available for habitation by the vector-carrying mosquitos (Hahn et al., 2014). In a study of deforestation and malaria in Mâncio Lima County, Brazil, Olsen et al. (2010) identified a positive correlation between the habitat preference of A. darlingi and an existing malaria surveillance programme. The study found that shrub land cover, which typically develops ~5 years after initial deforestation (and becomes classified as secondary growth after ~15 years) had a significantly greater population of A. darlingi than forested land, and thus suggests that human malaria risk is specifically associated with deforestation that occurred 5-10 years previous. Outbreaks of malaria in areas that are not typically associated with high disease prevalence have occurred previously with the onset of human disturbance. For example, in 2006 a malaria epidemic occurred in the southern periphery of the city of São Paulo, where residents of the Marsilac district began to construct houses in a previously undisturbed area of the Serra do Mar Natural Forest Reserve (Laporta et al., 2013). Trocano Araretama Conservation Project - Study on the Impact of Deforestation on Malaria Transmission 2014
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Study on the Impacts of Deforestation on Malaria Transmission: September 2014
3. Malaria Mitigation Strategies Despite the growing body of literature that supports the positive correlation between deforestation and malaria incidence, there are also contradictory theories which postulate that intact forest may in fact result in a higher incidence of malaria. In one study of the Brazilian Amazon, the authors concluded: ‘We find that cities close to protected areas (PA’s) tend to have higher malaria incidence than cities far from PA’s. Using future LULC* scenarios, we show that avoiding 10% of deforestation through better governance might result in an average 2-fold increase in malaria incidence by 2050 in urban health posts’ (Valle & Clark, 2013). *LULC= land use/land change While this theory is not as widely supported as the former, it still warrants consideration as the research surrounding the topic of linking deforestation and malaria is still relatively new. Valle & Clark (2013) do not, however, promote deforestation as a means to reducing malaria. On the contrary, they instead recommend implementing ‘Malaria Mitigation Strategies’ as part of rainforest conservation efforts. As part of incorporating a Malaria Mitigation Strategy into the Trocano Araretama Project, the following objectives could be considered: -
Supplying long-lasting, insecticidal bed nets to residents of the project area Carrying out residual indoor spraying of DDT in an effort to kill present mosquitos Building of additional malaria detection and treatment outposts Exploring better drainage systems for agricultural land Running an awareness campaign concerning prevention and treatment
Further research into addressing the specific needs of the residents of the project area in terms of combating malaria, in combination with reducing deforestation, will help the team at Celestial Green Ventures to develop an efficient Malaria Mitigation Strategy that can be incorporated into all areas of the project. One obstacle in the mitigation against malaria is encouraging the residents to comply with preventative measures, as they have learned to live with the threat of the disease and therefore may not recognise the importance of taking the preventative measures in an effort to eradicate the parasite and its disease altogether. For this reason an awareness campaign would be particularly efficient.
4. Conclusion Malaria transmission is a widespread problem in the Brazilian Amazon, including the area comprising the Trocano Araretama Project, and is a public health issue that needs to be addressed. Deforestation has long been recognised as a leading driver of climate change, and recent studies suggest that it may exacerbate the prevalence of malaria in some areas. The transformation of ecosystems in the interim between high-impact selective logging and ecosystem recovery may replicate the ecological transitions that support mosquito breeding that are generally associated with malaria, such as partial shade at forest fringes. Another hypothesis Trocano Araretama Conservation Project - Study on the Impact of Deforestation on Malaria Transmission 2014
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Study on the Impacts of Deforestation on Malaria Transmission: September 2014
suggests that forest disturbances from roads, fire, and selective logging increase malaria risk in a municipality, and this hypothesis is one that is being given increasing attention (Hahn et al., 2014). High levels of biodiversity associated with intact forests are seen to aid in the control of malaria transmission, by facilitating non-vector hosts which act as competition to the main malaria vector in the area, Anopheles darlingi, through contention for blood meals. Additionally, the facilitation of warm-blooded, incompetent hosts for the mosquitos to feed on reduces the chances that the parasite (P. vivax or P. falciparum) will be transmitted to humans. Deforestation threatens to disrupt this natural control of A. darlingi, as well as opening up areas of the canopy that create new suitable breeding grounds for the mosquito’s larvae, and such disturbance could ultimately favour a rapid increase in the abundance of infected A. darlingi and consequently, human malaria incidence. The incorporation of Malaria Mitigation Strategies in concomitance with efficient deforestation management should be implemented in the Trocano Araretama Project in order to reduce and control the malaria prevalence in the area. Further research surrounding effective treatment and prevention measures, as well as specific statistics surrounding malaria levels within the project area, should be carried out to facilitate a future study.
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Study on the Impacts of Deforestation on Malaria Transmission: September 2014
5. References Brochero HL, Rey G, Buitrago LS, Olano VA (2005) Biting activity and breeding sites of Anopheles species in the municipality Villavicencio, Meta, Colombia. J Am Mosq Control Assoc, (21): 182-186 Cruz, L. R., Spangenberg, T., Lacerda, M. V. G., & Wells, T. N. C. (2013). Malaria in South America: a drug discovery perspective. Malaria Journal, 12(1), 168. doi:10.1186/1475-2875-12-168 Hahn, M. B., Gangnon, R. E., Barcellos, C., Asner, G. P., & Patz, J. a. (2014). Influence of deforestation, logging, and fire on malaria in the Brazilian Amazon. PloS One, 9(1), e85725. doi:10.1371/journal.pone.0085725 Laporta, G. Z., Lopez de Prado, P. I. K., Kraenkel, R. A., Coutinho, R. M., & Sallum, M. A. M. (2013). Biodiversity can help prevent malaria outbreaks in tropical forests. PLoS Neglected Tropical Diseases, 7(3), e2139. doi:10.1371/journal.pntd.0002139 Moreno JE, Rubio-Palis Y, Paez E, Perez E, Sanchez V (2007) Abundance, biting behaviour and parous rate of anopheline mosquito species in relation to malaria incidence in gold-mining areas of southern Venezuela. Med Vet Entomol, (21): 339-349 Olson, S. H., Gangnon, R., Silveira, G. A., & Patz, J. a. (2010). Deforestation and malaria in Mâncio Lima County, Brazil. Emerging Infectious Diseases, 16(7), 1108–15. doi:10.3201/eid1607.091785 Pan American Health Organization: Report on the Situation of Malaria in the Americas, 2008. Washington, D.C: PAHO; 2010. http://www2.paho.org/hq/dmdocuments/2011/PAHO_ENG_Malaria_LR.pdf. Sinka, M. E., Rubio-Palis, Y., Manguin, S., Patil, A. P., Temperley, W. H., Gething, P. W., … Hay, S. I. (2010). The dominant Anopheles vectors of human malaria in the Americas: occurrence data, distribution maps and bionomic précis. Parasites & Vectors, 3(1), 72. doi:10.1186/1756-3305-3-72 Valle, D., & Clark, J. (2013). Conservation efforts may increase malaria burden in the Brazilian Amazon. PloS One, 8(3), e57519. doi:10.1371/journal.pone.0057519 Vittor, A. Y., Gilman, R. H., Tielsch, J., Glass, G., Shields, T., Lozano, W. S., … Patz, J. a. (2006). The effect of deforestation on the human-biting rate of Anopheles darlingi, the primary vector of Falciparum malaria in the Peruvian Amazon. The American Journal of Tropical Medicine and Hygiene, 74(1), 3–11. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/16407338 World Health Organization (2014) http://www.who.int/mediacentre/factsheets/fs387/en/ Yasuoka, J., & Levins, R. (2007). Impact of deforestation and agricultural development on anopheline ecology and malaria epidemiology. The American Journal of Tropical Medicine and Hygiene, 76(3), 450–60. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/17360867
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