The biological control of mosquito-borne disease
Dengue became a widely established disease throughout the tropics with the development of commercial shipping during the 18th century. In 1905 during a large dengue outbreak in Brisbane (Queensland, Australia), local medical practitioner Thomas Bancroft was the first to demonstrate that the mosquito, Aedes aegypti, was a vector of dengue. The epidemiology of dengue was transformed in Southeast Asia during World War II. Troop deployments facilitated the movement of different dengue viruses across the region. This resulted in hyperendemicity (co-circulation of multiple dengue serotypes), a major factor related to the emergence of epidemic DHF in the Philippines and Thailand during the 1950s. By the 1970s regular DF/DHF epidemics were common throughout Southeast Asia.
Dengue fever is a mosquito-borne viral disease that occurs in over 100 countries worldwide and puts up to 40% of the world’s population at risk of infection. With no known vaccine or cure this can result in 50 – 100 million cases and upwards of 40,000 deaths a year.
There are four dengue virus serotypes all of which can cause dengue fever (DF) and dengue haemorrhagic fever (DHF), a more severe and potentially fatal form of the disease. Dengue imposes a significant global disease burden with some 2.5 billion people – 40% of the world’s population – at risk of infection. The World Health Organization (WHO) reports 50 to 100 million cases of dengue annually.
An innovative biological approach to manipulate mosquito populations to make them incapable of transmitting dengue viruses between people. If successful it will greatly reduce our reliance on conventional insecticide-based mosquito control.
Wolbachia pipientis is the only species of the genus Wolbachia, family Anaplasmataceae, order Rickettsiales, class a-proteobacteria. The genus Wolbachia is currently defined as monophyletic. W. pipientis are obligate intracellular bacteria that live in the vacuoles of eukaryotic cells. Wolbachia are coccoid or bacilliform in structure and range in size from 0.8 to 1.5 micrometers in length. Wolbachia are gram negative bacteria. They also have two cell membranes and are enclosed within a vacuole.
The approach is centered on a naturally occurring bacterial agent known as Wolbachia pipientis. This bacterium is estimated to occur naturally in up to 70% of all insect species. Considering that scientists estimate there may be around 5 million different insect species on the planet, Wolbachia is extremely common in the environment. We know that in addition to well-known insects like Birdwing butterflies, Wolbachia also occurs naturally in many mosquito species that bite humans. Interestingly it does not occur naturally in the mosquito species that are known to be of major importance in transmitting pathogens like malaria and dengue between people.
Program is investigating whether we can use Wolbachia strains that occur naturally in fruit flies to influence the ability of the mosquito Aedes aegypti to transmit dengue viruses between people.
Results & Conclusions
Researchers have shown that cells infected with Wolbachia display inhibition of dengue virus replication, that the extent of inhibition is related to bacterial density and that Wolbachia infection, although costly, will provide a fitness benefit in some circumstances. Our results parallel findings in mosquitoes and flies, indicating that cell line models will provide useful and experimentally tractable models to study the mechanisms underlying Wolbachia-mediated protection from viruses.