Deforestation: effects on vector-borne disease

Abstract

SUMMARYThis review addresses' changes in the ecology of vectors and epidemiology of vector-borne diseases which result from deforestation. Selected examples are considered from viral and parasitic infections (arboviruses, malaria, the leishmaniases, nlariases, Chagas Disease and schistosomiasis) where disease patterns have been directly or indirectly influenced by loss of natural tropical forests. A wide range of activities have resulted in deforestation. These include colonisation and settlement, transmigrant programmes, logging, agricultural activities to provide for cash crops, mining, hydropower development and fuelwood collection. Each activity influences the prevalence, incidence and distribution of vector-borne disease. Three main regions are considered – South America, West & Central Africa and South-East Asia. In each, documented changes in vector ecology and behaviour and disease pattern have occurred. Such changes result from human activity at the forest interface and within the forest. They include both deforestation and reafforestation programmes. Deforestation, or activities associated with it, have produced new habitats for Anopheles darlingi mosquitoes and have caused malaria epidemics in South America. The different species complexes in South-East Asia (A. dirus, A. minimus, A. balabacensis) have been affected in different ways by forest clearance with different impacts on malaria incidence. The ability of zoophilic vectors to adapt to human blood as an alternative source of food and to become associated with human dwellings (peridomestic behaviour) have influenced the distribution of the leishmaniases in South America. Certain species of sandflies (Lutzomyia intermedia, Lu. longipalpis, Lu. whitmani), which were originally zoophilic and sylvatic, have adapted to feeding on humans in peridomestic and even periurban situations. The changes in behaviour of reservoir hosts and the ability of pathogens to adapt to new reservoir hosts in the newly-created habitats also influence the patterns of disease. In anthroponotic infections, such as Plasmodium, Onchocerca and Wuchereria, changes in disease patterns and vector ecology may be more difficult to detect. Detailed knowledge of vector species and species complexes is needed in relation to changing climate associated with deforestation. The distributions of the Anopheles gambiae and Simulium damnosum species complexes in West Africa are examples. There have been detailed longitudinal studies of Anopheles gambiae populations in different ecological zones of West Africa. Studies on Simulium damnosum cytoforms (using chromosome identification methods) in the Onchocerciasis Control Programme were necessary to detect changes in distribution of species in relation to changed habitats. These examples underline the need for studies on the taxonomy of medically-important insects in parallel with long-term observations on changing habitats. In some circumstances, destruction of the forest has reduced or even removed disease transmission (e.g. S. neavei-transmitted Onchocerca in Kenya). Whilst the process of deforestation can be expected to continue, hopefully at a decreased rate, it is expected that unpredictable and sometimes rapid changes in disease patterns will pose problems for the public health services.

Bayesian spatial modelling and the significance of agricultural land use to scrub typhus infection in Taiwan

“…The legacy of frontier cultivation in Taiwan may, therefore, be increased risk of scrub typhus infection. The association of frontier colonization with outbreaks of scrub typhus has also been documented in several other countries (Traub and Wisseman, 1974), although our study is the first one to quantitatively investigate such an association, suggesting that, similar to other vector-borne diseases, land use policy that encourages agricultural practices in the frontiers should also contemplate the potential for elevated burdens of scrub typhus infection and other vector-borne diseases (Walsh et al, 1993;Singer and de Castro 2001).…”

Ecology of the Leishmania Species in the Guianan Ecoregion Complex

“…12,13 Overall, leishmaniases have a wider geographic distribution than previously believed and are now being reported in areas that were previously non-endemic in the GEC, as in the rest of South America. [14][15][16][17][18] The different Leishmania species isolated from human cases in the GEC will be referred to according to the clinical forms of disease they cause in each country.…”

“…Any modification of the environment may influence to various degrees the prevalence of leishmaniases in a given region. Several general 15,16,[205][206][207] or more specialized 14,17,18,173,[208][209][210][211] reviews report and exemplify different risk factors linking leishmaniases to environmental modifications.…”

The association of Triatoma maculata (Ericsson 1848) with the gecko Thecadactylus rapicauda (Houttuyn 1782) (Reptilia: Squamata: Gekkonidae): a strategy of domiciliation of the Chagas disease peridomestic vector in Venezuela?

“…This peridomiciliation is of particular interest in Chagas disease transmission. The adaptation of vector species to human dwellings in deforested areas is thought to be linked to recent findings of T. cruzi in human populations in the Amazon [30,31] where the low frequency of human autochthonous cases of Chagas disease contrasts with the high infection index found in reservoirs and vectors [32] .…”