Altering behavior: Another viral trick?

Mosquito (Aedes aegypti)
Photo by USDA

Dengue virus (DENV) is one of the world’s fastest growing health problems.  Each year, about 500,000 dengue infected people are admitted to the hospital with life-threatening symptoms, including gastrointestinal bleeding, seizures, and slow heart rate.  More alarmingly, no vaccine or treatment is available for those infected.  DENV is spread by mosquitoes, much like malaria and West Nile virus.  Therefore, controlling the mosquito vector remains the primary defense against dengue.  While DENV causes illness in humans, researchers have uncovered different effects in the mosquito that may impact viral transmission.

In order to be transmitted to a human host, DENV must first infect a female mosquito’s salivary glands.  These glands contain compounds that help the insect feed.  It would be in the virus’ best interest to encourage the presence of these compounds to increase transmission rates.  Therefore, scientists wanted to investigate any changes in the salivary glands during DENV infection. 

To detect any changes, researchers screened the entire genome looking for differences in expression of individual genes in DENV infected mosquitoes compared to healthy mosquitoes.  They found a total of 147 genes being regulated by DENV.  Most of these genes were involved in the immune response, food digestion, and metabolism.  However, there were a few surprising genes being regulated by DENV.  These genes are actually involved making proteins that bind odorant, allowing the insect to detect odors.  Scientists engineered mosquitoes that could never express these specific odor binding protein genes.  These insects took significantly longer to find and bite their host. 

This suggests that by increasing the expression of these odor binding proteins, the insects will be able to detect their hosts more quickly, resulting in better transmission of the virus.  This is the first hint that human pathogens may be able to increase their transmission by affecting the behavior of their vector animal.