With laboratory-bred mosquitoes, scientists in Australia and China showed it may be possible to reduce the spread of dengue fever by using a parasite to shorten the life-span of the mosquito that carries the virus.
The study is the work of researchers from the School of Integrative Biology at the University of Queensland in Brisbane, and the College of Life Sciences at Central China Normal University in Wuhan, and is published on 2 January 2009 in the journal Science.
Efforts to use pesticides to wipe out the mosquito that carries the dengue fever virus have not been very successful, mostly because the insects develop immunity to the chemicals, so scientists have been looking for a new approach that circumvents this problem.
In their background information the authors explained how most pathogens transmitted by mosquito need to spend a long time in the insect before they can be passed onto the human host. Thus only older mosquito “vectors” pose a threat to humans, making the prospect of shortening mosquito lifespan, for instance with a parasite, an attractive alternative way of preventing disease spread.
In effect this is what the researchers were able to do, under laboratory conditions: they halved the lifespan of Aedes aegypti mosquitoes (the type that carry the dengue virus) by infecting them with a bacterial parasite called Wolbachia. They had to modify the parasite to make in infectious to Aedes aegypti mosquitoes, because they are not normally vulnerable to that particular bacterium.
They found that the association between the parasite and the host mosquito was “stable” and passed onto the next generation (“maternally inherited”) at a high frequency.
Not only did infected females pass on the bacterium, but infected males could only produce offspring with infected females, giving two powerful means by which a mosquito population can very quickly become infected with the parasite.
The researchers also found that the parasite is:
“Capable of inducing complete cytoplasmic incompatibility, which should facilitate its invasion into natural field populations and its persistence over time.”
“Our data suggest that targeting mosquito age with inherited Wolbachia infections may be a viable strategy to reduce the transmission of pathogens such as dengue viruses,” they concluded.
Commenting in the same issue of the journal, Drs Andrew Read and Matthew Thomas, who specialize in infectious disease dynamics at Pennsylvania State University in the US, said that while this study suggests there could be “substantial” reductions in disease transmission, many obstacles still remain.
For example, it remains to be seen how well the parasite spreads in the wild, and how many infectious mosquitoes it can remove. Also, the dengue virus could also adapt to survive, they said.
About Dengue Fever
People, many of them children under the age of 10, get dengue fever when they are bitten by a virus-carrying Aedes egypti mosquito. These mosquitoes are mostly found in countries of the southern hemisphere, either during or shortly after rainy seasons. They are particularly common in Africa, Southeast Asia, China, India, the Middle East, the Caribbean, Central and South America, Australia, and the South and Central Pacific.
Some eight days after being bitten by a virus carrying dengue fever, symptoms like high fever, headache, chills, swollen glands, red eyes, pain in the eyes, joint and muscle pains, back pain and fatigue suddenly emerge and last for two or three days. The person feels better for a little while but then the second stage begins, where a rash breaks out, the headache and fever returns, and this lasts four about 10 days.
The virus causes blood vessels to swell and leak blood into surrounding tissue where it produces small purple spots and bruises. Because bleeding also occurs in the digestive tract, the infected person gets stomach ache and their vomit is black. Damage to major organs can also occur and the more severe cases become dengue hemorrhagic fever (DHF) or dengue shock syndrome (DSS).
While the infected person’s immune system does respond with antibodies, and this offers protection for about 12 months, the most severe cases occur in people who have been infected before, where unfortunately the primed immune system over-responds and this results in the more severe symptoms. People with dengue fever are also more vulnerable to a range of other infections.
Like other mosquito-borne infections, dengue fever is treated by allowing it to run its course and giving medication to lower fever and reduce pain, with bed rest and fluids to replenish and prevent hydration. Almost all patients who get dengue fever recover completely, and if treated early, those with DSS also have a good chance of recovery.
“Stable Introduction of a Life-Shortening Wolbachia Infection into the Mosquito Aedes aegypti.”
Conor J. McMeniman, Roxanna V. Lane, Bodil N. Cass, Amy W.C. Fong, Manpreet Sidhu, Yu-Feng Wang, Scott L. O’Neill.
Science Vol. 323. no. 5910, pp. 141 – 144, 2 January 2009.
DOI: 10.1126/science.1165326
Sources: Journal abstract, Pennsylvania State University.
Written by: Catharine Paddock, PhD