Dengue fever is caused by four related viruses that are spread by mosquitoes, affecting 100 million people each year and killing more than 12,000. The common fever is generally not fatal, but infection causes uncomfortable and unpleasant symptoms for a week or longer without any available treatment. Severe infections lead to hemorrhagic fever – dengue hemorrhagic fever has a high mortality rate due to blood loss and shock. With a lack of treatment options, prevention is the best way to combat the virus.
Combatting dengue virus
Because the mosquitoes that act as the primary vector for transmitting the virus, Aedes aegypti, are active during the day, the preventative measures used to curb other mosquito-borne diseases, like bed nets for malaria, are not helpful in preventing its spread. Scientists have had to get creative – infective agents and the introduction of anti-dengue genetic strains into the mosquito populations have been pursued as alternative prevention methods.
Wolbachia is an intracellular bacterium that tends to be endosymbiotic with the host mosquito and passed to its offspring. This bacterium has been found to inhibit viral replication within the mosquitoes that harbor it and prevent its transmission without the need for genetic engineering. In the journal PLoS Pathogens in 2010, researchers at Michigan State University showed that Wolbachia bacteria completely block the transmission of dengue from more than one-third of the infected Aedes aegypti mosquitoes they evaluated.
Several Wolbachia strains of bacteria have been identified, each with their own effects on the mosquitoes. The strain wMelPop-CLA was shown to reduce the lifespan of the Aedes aegypti mosquitoes, which in turn reduces the transmission of dengue. Another strain causes cytoplasmic incompatibility, which reduces the reproductive activity of the mosquitoes. Female mosquitoes infected with Wolbachia pipentis had half the reproductive lifespan as female mosquitoes that were not infected. Reducing the number of mosquitoes that a mosquito can have aids in controlling the dengue-spreading populations. This feature of Wolbachia infection has been known since at least the 1990s.
Preventing dengue spread
This strategy against dengue transmission has been shown to be possible based on the natural infection of two mosquito populations in Australia, as described in Nature in August 2011. Though the exact mechanism underlying the bacterium’s effect on the virus is uncertain, the current results are promising as the bacteria are naturally occurring endosymbiotes, spread quickly in the mosquito population, and appear to be effective at controlling both the mosquito populations and viral transmission.
Field studies began with laboratory raised populations of Wolbachia-infected Aedes aegypti mosquitoes in 2013. The small Vietnamese island of Tri Nguyen is the first testing ground. The strain being used blocks 100 percent of dengue but it is difficult to sustain in the mosquito populations.
Whether real world application actually leads to winning the battle with dengue is yet to be known. Additional field tests are planned and the scientists aren’t giving up on the promise of Wolbachia just yet.