Mosquito Control

June 1st, 2011

Mosquito Parasite Fights Infectious Disease

Deadly malaria, painful filariasis and other mosquito-borne diseases can likely be controlled using a parasite known as Wolbachia.

The research could create a new way to manage mosquito populations and eventually save millions of lives each year.

“We took a strain of mosquitoes, transinfected them with the inherited bacteria Wolbachia, and looked at gene expression compared with mosquitoes that didn’t have Wolbachia,” said Steven Sinkins, a scientist at the University of Oxford who co-authored a paper appearing today in the journal Science.

“We found a surprising number of immune genes were turned way up.”

For years scientists have known that mosquitoes, fruit flies and other insects infected with Wolbachia live about half as long as their uninfected counterparts.

Scientists have also known that infected animals have dramatically lower transmission rates for a number of diseases.

What scientists didn’t know is why the Wolbachia-infected insects lived half as long, or why they were less likely to pass on diseases to humans. Sinkins and his colleagues have now answered a large part of both questions.

For their study, the researchers infected the mosquito eggs with Wolbachia. When the eggs hatched and the mosquitoes grew up, the scientists then infected the mosquitoes with the nematode that causes filiariasis.

The mosquitoes’ immune system, already in overdrive because of the Wolbachia infection, destroyed as much as 100 percent of all the nematodes.

Presumably malaria, yellow fever, dengue fever and other diseases will be similarly destroyed, although more testing is required to definitively prove the effectiveness of this method, said Sinkins.

The Wolbachia infection is especially taxing on mosquitoes’ immune system and takes a lot of energy. Wolbachia-infected mosquitoes live half as long as uninfected mosquitoes.

Not only does Wolbachia decrease the total number of disease-carrying, bloodthirsty, flying syringes, it also decreases their numbers. Old mosquitoes are more likely to carry disease causing infections than young mosquitoes.

For public health officials, the next trick is getting the Wolbachia into the mosquitoes more easily. The scientists injected the mosquitoes with a special strain of fast-growing Wolbachia originally found in fruit flies. This strain has been casually dubbed “popcorn” because of the popcorn-shaped structures formed inside the gut of fruit flies.

To infect Aedes aegypti mosquitoes, which carry yellow and dengue fever, and malaria-carrying Anopheles mosquitoes, scientists injected Wolbachia into tiny mosquito eggs.

Only 10 percent of A. aegypti mosquitoes survived, and only 5 percent of Anopheles mosquitoes survived.

Once a Wolbachia-infected mosquito, always a Wolbachia-infected mosquito, even after the original infected mosquito dies. All the offspring from a Wolbachia-infected female mosquito are also Wolbachia-infected, since the parasite is passed from mother to daughter.

Given adequate funding and local cooperation, Wolbachia could spread though a local mosquito population in a year, says Sinkins.

French Polynesia, Burkina Faso and Kenya are likely to be the first countries to develop field trials for Wolbachia-infected mosquitoes, if they pass all the appropriate tests for biological control agents.

Using one bacteria to stop another is like using probiotics to aid in digestion, said George Dimopoulos, a researcher at Johns Hopkins School of Public Health.

“I think we will see a number of interesting applications in the near future based on the introduction of certain bacterial populations,” said Dimopoulos. “In fact, this may be happening all the time naturally.”

In the field, there is often a huge variation between local mosquito population in their ability to transmit diseases said Dimopoulos, a variation that is likely explained by the various bacteria found in the mosquitoes guts.

If Wolbachia can influence the transmission of filariasis-causing nematode, other bacteria should be able to influence other diseases as well.

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