|Wolbachia infected insect cell, by Scott O'Neill|
Dengue fever is caused by four viruses that are spread by mosquitoes. Some infections are severe, leading to a hemorrhagic fever. The common fever is generally not fatal, but infection causes uncomfortable and unpleasant symptoms for a week or longer without any available treatment. Dengue hemorrhagic fever, on the other hand, has a higher mortality rate due to blood loss and shock. Dengue fever affects 100 million people each year and kills more than 12,000. Because the mosquitoes that transmit the virus 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. Infective agents and the introduction of anti-dengue genetic strains into the mosquito populations have been pursued as alternatives.
Aedes aegypti mosquitoes are the primary vector of dengue virus. 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.
As noted in the journal Nature in August 2011 by Walker et al., several Wolbachia strains 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 (as noted in PNAS in 2004).
This strategy against dengue transmission has been shown to be possible based on the natural infection of two mosquito populations in Australia, as described by Hoffmann et al. in Nature in August 2011. Though the exact mechanism underlying the bacterium’s effect on the virus is not certain, researchers find the current results promising as the bacteria are naturally occurring endosymbiotes, spread quickly in the mosquito population, and appear effective at controlling both the mosquito populations and viral transmission.