Wolbachia Bacteria and Biocontrol of Arboviral Diseases
Introduction
Arboviruses (arthropod-borne viruses) are transmitted by arthropod vectors such as mosquitoes and ticks and cause significantly human morbidity and mortality. There are more than 80 different arboviruses that are known human pathogens with dengue virus (DENV) having the greatest impact on humans. DENV is within the genus Flavivirus that also includes medically important viruses such as West Nile virus (WNV), Yellow fever virus (YFV) and Japanese encephalitis virus (JEV) that are transmitted by mosquitoes. Estimates of annual global DENV infections range from 100–390 million, with 100 million symptomatic infections leading to 12,500 deaths per year (Bhatt et al. 2013). The resulting disease, dengue fever, is classified as ‘re-emerging’ predominantly because the range of the principal mosquito vector, Stegomyia (Aedes) aegypti, is increasing alongside the secondary vector Stegomyia (Aedes) albopictus. The range of these two invasive mosquito species is expanding due to several factors including glob lization and climate change (Kilpatrick & Randolph 2012). Dengue outbreaks typically occur in tropical areas of Southeast Asia and South America in countries where disease has a significant impact on struggling economies (Guzman et al. 2010). Early recognition and supportive treatment can help lower the risk of severe morbidity or mortality although diagnosis is often difficult in developing countries. Mosquito vector control has been the only way to prevent transmission, as there are currently no vaccines or drugs available for dengue.
Wolbachia-induced cytoplasmic incompatibility (CI) in mosquitoes. Wolbachia-infected mosquitoes are shown in blue. Image courtesy of Claire Jeffries, london School of Hygiene and Tropical Medicine.
Successful mosquito control for dengue has been problematic for several reasons. Stegomyia aegypti are anthropophilic mosquitoes (preferring to bite humans) and are extremely well adapted to living in urban environments in close proximity to dense human populations. This species of mosquito has a preference for laying eggs in artificial containers (eg. water tanks and unused tires) so widespread larval source reduction is difficult to implement. Another widely used method has been outdoor spraying of insecticides such as DDT and malathion. Singapore is a good example of how dengue continues to persist even with a strictly implemented vector control program. Dengue was first reported in 1960 and government interventions through integrated mosquito control programs reduced the incidence of transmission by the 1970s. However, Singapore has recently seen resurgence in dengue cases despite a significant control program targeting mosquitoes. If a country like Singapore is unable to prevent dengue transmission, current vector control methods are clearly not working and novel methods need to be developed.
This is an extract of the full article published in: Outlooks on Pest Management – August 2015 issue.
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Author: Thomas Walker – outlines the rationale of using Wolbachia as a bio-solution to insect vectored arboviruses.
Category: Public health
