Why we're fighting mosquito-borne diseases - with more mosquitoes

Mosquito-borne diseases are a global plague. Malaria is the most famous - and most deadly. But there is also a family of terrible viral diseases spread by mosquitoes that includes dengue fever, chikungunya, yellow fever, and now, of course, Zika.

Now two new, related approaches that use clever biology to attack this problem are being tested. Strangely, both involve releasing even more mosquitos into the environment. And both are backed by big money.

The Debug Fresno program is from a company called Verily, owned by Google's parent company, Alphabet. They are releasing 20 million mosquitoes, loaded with a secret weapon, in Fresno, California.   

Dr. Stephen Dobson, a professor of entomology at the University of Kentucky, is the scientist who developed this technology where they first infect captive mosquitoes with a specific bacteria. "The idea is to use this bacterium as a way to control medically important mosquito species. The way this would be done is to release lots and lots of male mosquitoes, which don't bite. But they would fly around and mate with females that naturally occur in the environment. And because of this bacterium, its presence when they mate with these females in the field, their eggs don't hatch. So the hope is we can use this to actively suppress and even eliminate mosquito populations."

• Research paper: Open Release of Male Mosquitoes Infected with a Wolbachia Biopesticide: Field Performance and Infection Containment

The other group, backed by the Bill & Melinda Gates Foundation, is trying a different approach, but with the same bacteria. Their strategy is not to eliminate mosquitoes, but to remove their ability to carry disease. Guest host Sonya Buyting spoke with Dr. Scott O'Neill, the director of the Eliminate Dengue Program and the Institute of Vector Borne Diseases at Monash University in Australia.

Related links:

• Scientists scouring southern Canada for Zika-carrying mosquitos
• The virus outbreak - from Aedes to Zika

This interview has been edited for length and clarity.

Sonya Buyting: What is the key to your approach?

Dr. Scott O'Neill: What we're doing is we're working with a naturally occurring bacterium, called wolbachia, that when we put it into the mosquito population, the mosquitoes can no longer transmit viruses that cause these terrible diseases. The presence of the bacteria stops virus replication. And if the virus can't replicate within the mosquito, it can't be transmitted between people.

SB: What happens when you release the male and female mosquitoes with wolbachia in the wild?

SO: So this bacterium is quite unusual in that it doesn't spread infectiously, like a cold or something between the mosquitoes, it gets transmitted vertically. That means that the female mosquito, if it has the bacterium, passes it through its eggs to its children.

Wolbachia has a little trick that enables it to be able to spread into the insect population. If that wolbachia infects a male mosquito, which then mates with a female mosquito that doesn't have the bacteria, her eggs will die. But if that same male mosquito mates with a female that does have wolbachia, her eggs will survive and will all contain the bacteria. And so this provides an indirect mechanism for wolbachia to spread into a population.

In our program, we released a small number of mosquitoes, both males and females that contain wolbachia. They mate with the wild population and over a short amount of time, wolbachia spreads into all of the mosquitoes in the area. And because it stops the mosquitoes from being able to transmit viruses, we see a collapse in virus transmission.

SB: How does those bacteria prevent the viral transmission?

SO: When wolbachia is in the insect, we think it's doing two different things. The first is that it's boosting the natural immune system of the mosquito, but we also think wolbachia is competing with viruses for some key molecules, like cholesterol. And the wolbachia wins and as a result, the virus cannot replicate or be transmitted.

SB: What kind of results have you seen when you introduce these mosquitoes carrying the wolbachia into the field?

SO: We're currently working in five countries. What we found from those first places that we released is that wolbachia has been able to establish in those populations mosquito populations. And the other thing we found is that not only does wolbachia maintain itself in the mosquito population, but those mosquitoes are no longer able to transmit viruses. And indeed, in all of the locations now around the world where we've established wolbachia, we're seeing local transmission of the dengue virus, in particular, has stopped completely.

SB: The Google-backed program that's releasing 20 million male mosquitoes in California is also using wolbachia, but instead of establishing it within the mosquito population to reduce viral transmission, like your groups, they're only releasing males to suppress the mosquito population. What do you see as the relative strengths and weaknesses of each of your programs?
 

SO: The goals and the way we are approaching it are really quite different. For example, once we have deployed wolbachia, it maintains itself in the mosquito population, so we don't need to reapply. So our strategy is a one-off application, if you like. Whereas the other suppression approach requires continual application, almost like the application of insecticide. Also, many more mosquitoes need to be released in the suppression approach, than the way we're doing it, so the cost is expected to be much, much higher.

Click 'listen' at the top of the page to hear the full interview.