How many mosquitoes does it take to eradicate disease?

Here is an intriguing new method for controlling, if not eliminating altogether, mosquito-borne diseases: RIDL (for Released Insect with a Dominant Lethal). Stanford University researchers suggest that releasing gentically modified male mosquitoes could eliminate the danger of dengue fever and other mosquito-borne diseases within a year in communities of up to a million people.

“The RIDL approach is an alternative that is also environmentally-benign,” say Lawrence Wein, the Paul E. Holden Professor of Management Science at the Stanford Graduate School of Business, one of the authors of a paper published by Proceedings of the National Academy of Sciences of the United States of America.

RIDL mosquitoes have proven more effective than insects which are irradiated to make them sterile because the genetically modified male insects are more physically fit to compete with the mosquitoes in the wild than those that have been subjected to radiation. Scientists, however, have had difficulty trying to measure the effectiveness of such campaigns to determine the right proportion of altered insects necessary to have an impact on infected mosquito populations. Wein and his coresearchers developed a mathematical model that predicts the effectiveness of RIDL eradication campaigns.

Mosquito-born diseases, such as dengue fever, are a major health problem in many countries, especially in the developing world. There is no licensed vaccine for dengue which affects up to 100 million people each year. Between 250,000 and 500,000 potentially fatal cases are reported annually. One of the known dengue-causing types of mosquitoes is endemic in the southeastern United States. The recent spread of the West Nile virus has raised concerns that the United States may be vulnerable to other serious outbreaks of mosquito-born diseases.

Health professionals have used different methods to battle dengue and other similar diseases, among them different approaches aimed at suppressing the infected insects’ ability to reproduce. One technique, called Sterilize Insect Technique, or SIT, introduces male mosquitoes that have been irradiated and rendered sterile in order to mate with infected female insects.

Wein said that based on their research into the RIDL method, “Eradication of the disease might be feasible within about one year for affected populations in the order of 100,000 to a million.”

Wein is highly regarded for his work exploring homeland security issues related to potential Anthrax attacks and the effectiveness of the two-finger fingerprinting system used on the U.S. border. He has also testified before a variety of government agencies considering policies affecting public health, epidemics, and bioterrorism.

The authors of the study write that “From a practical point of view, it is important to understand how many … mosquitoes are required for eradication and how long it takes to eradicate the virus…. Our model should suffice for an order-of-magnitude assessment of the effectiveness and practicality of the RIDL strategy.” Wein hastens to ass that the strategy also entails serious challenges. “The real drawback is the logistical consideration,” he said. “Once you release the mosquitoes, they only travel half a mile. So if you try to eradicate the disease in a rural area where there’s distances between neighbors, you pretty much have to hand deliver the insects.”