29 Aug 2015
by MVI Director Ashley J. Birkett, PhD

Lab-raised mosquitoes are critically important to figuring out how to stop their wild cousins from spreading deadly diseases like malaria.

August is often a time when people travel to where the weather is warm, whether to visit family or vacation in tropical climates. They may come back complaining about the mosquitoes, and some travelers may even be grateful they returned home without any of the diseases that mosquitoes can carry, like malaria, dengue fever, yellow fever, and chikungunya.

For the people who live in the tropics, however, mosquitoes and their deadly diseases are a never-ending menace.Most everyone would be surprised to learn that the practice of raising mosquitoes in labs is critically important in the fight against these diseases. And not enough people celebrate World Mosquito Day, established to commemorate the 1897 discovery by Sir Ronald Ross that malaria in people is transmitted to and from mosquitoes.

Malaria alone kills more than half a million people every year and, according to the World Health Organization’s latest estimates, there were approximately 198 million cases of malaria in 2013.

Even though millions of mosquitoes are buzzing around in tropical regions of the world every day, scientists can’t just capture them in a jar and bring them inside for scientific testing on the many diseases that they carry. Instead, mosquitoes must be cultivated in a sterile, air-conditioned lab. They must be born and raised there.

In the case of malaria, once the mosquitoes mature in the lab, they are fed a blood meal containing the parasite that causes malaria. In this way, researchers can test whether a vaccine or a drug might work to stop the parasite. It might seem odd to breed and infect mosquitoes with malaria-causing parasites, but it’s a vital part of the process toward development of lifesaving new tools.

Preventing mosquitoes from getting infected

In addition to the development of new cures, efforts are also focusing on how to prevent people from getting malaria in the first place. One type of approach now in early stages of development essentially uses a vaccine in humans to prevent mosquitoes from getting infected with malaria-causing parasites, thus stopping them from spreading malaria back to humans.  The vaccine would transform people from carriers of the malaria parasite to road blocks.  The antibodies produced by a transmission-blocking vaccine would be transmitted to the mosquito when she bites and thereby interrupt the life cycle of the parasite, preventing it from being passed “back” to humans later.

It would not stop the disease in its tracks. Like all vaccines, for such a concept to work well, an entire community would need to be vaccinated. Over time, and especially if used with other malaria interventions, progressively fewer people would carry the parasites, resulting in progressively fewer infected mosquitoes co-inhabiting communities. Eventually, the parasite would be eliminated from the vaccinated community.

To test the transmission-blocking vaccine approach, vaccine developers must feed laboratory-reared mosquitoes with blood that contains the parasite and the vaccine-induced antibodies.

For such trials to demonstrate success, scientists need to control both the mosquitoes and their food supply so that the mosquitoes aren’t somehow compromised by other agents. This means keeping a steady supply of standardized lab-raised mosquitoes, despite the hurdles faced in cultivating them.

Although scientists and technicians can micromanage every aspect of the insects’ life cycle to help them breed, mosquitoes do not thrive in captivity like they do in the wild. In the lab, for example, the water in which they lay their eggs must be changed regularly and the tiny eggs must be washed to protect against contamination with any of the fungus or bacteria found indoors. Outside, any pool of standing water can be a suitable place to lay eggs.

Potential transmission-blocking vaccines in early development

Once a transmission-blocking vaccine is proven to be effective using lab-raised mosquitoes, the vaccine would progress to a series of larger-scale field trials to confirm safety and determine whether it works under natural conditions of malaria transmission. The vaccine would be evaluated by looking at malaria transmission rates in vaccinated communities. Success would be demonstrated if fewer mosquitoes end up carrying the malaria parasites and if fewer people get infected.

There are potential transmission-blocking vaccines in early development including those that the PATH Malaria Vaccine Initiative are sponsoring in collaboration with partners including Fraunhofer USA Center for Molecular Biotechnology. The National Institutes of Health is also testing at least one vaccine approach in early-stage trials. These efforts are part of a bold quest to rid the world entirely of malaria. The malaria vaccine technology roadmap has called for vaccines that reduce transmission by 2030 to help eradicate malaria.

Though we’ve made extraordinary progress over the past decade in reducing malaria deaths, the malaria parasite is rapidly becoming resistant to some of our best tools – drugs and insecticide sprays. Another tool to break the cycle of transmission could help tip the balance against malaria, and history tells us that a disease is unlikely to be eradicated without a vaccine.

While mosquitoes will never quite be at home in a temperature-controlled laboratory, lab-raised mosquitoes are critically important to figuring out how to stop their wild cousins from spreading deadly diseases like malaria. If successful, the only thing that people will get from a mosquito bite one day is an itch that needs to be scratched. And that too would be a day worth celebrating.