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Dancing with the Enemy

Dung Hoang

Dancing with the Enemy

The malaria parasite’s life cycle relies on humans and mosquitoes. Could we use the mosquito to stop the parasite?

The small sign on the door designating the space as the Johns Hopkins Malaria Research Institute’s Core Insect Facility doesn’t do justice to the utter creepiness of what lies waiting inside. Carved into the sterile white walls are doors marked with the names of mosquito species: A. stephensi, A. gambiae, A. funestus. Within these rooms is a disconcerting nursery.

Tiny, tadpole-like larvae float in water in plastic bins on shelves. They are sustained by a single, bloated cat-food pellet. The bins holding older pupa are topped with a layer of mesh held down with lead weights, anticipating their transformation into flying and biting adults. A grad student stands in the corner with an adult-filled bin. Carefully, he uses a vacuum hose to transfer the bugs into mesh-sided cubes, where a dense array of mosquitoes line the walls and buzz around inside. A bug zapper near the ceiling shines blue light—ever ready to catch any escapees.

 As an anxious visitor to the A. stephensi room, I rub my bare arms and wish I’d worn long sleeves. Warily eyeing the bug-filled boxes, I tell insectarium director Marcelo Jacobs-Lorena, PhD, MS, Molecular Microbiology and Immunology (MMI) professor, that the room’s hundreds of mosquitoes are really disturbing. “That’s okay,” he laughs. “If you feel that way, then it gives you even more pleasure to dissect them.”

It’s the fate that awaits many of these Anopheles mosquitoes. That’s because these insects are one of JHMRI’s greatest resources in the fight against the disease. The bugs host the malaria parasite, Plasmodium. That makes mosquitoes an obvious target for efforts to stop malaria.

Several researchers at JHMRI are concentrating on this very goal, focusing on various ways to use the mosquito to block transmission: either by decreasing mosquitoes’ own defenses against Plasmodium, shortening their life span so they die before passing on the disease, or developing an innovative vaccine for people that blocks Plasmodium infection in mosquitoes. To solve malaria, they say, we have to partner with the very vector responsible for transmitting the disease.

The Clues Genes Hold

As I lean back in a comfortable chair in Jacobs-Lorena’s office, warmed by late afternoon sun and safely distant from any six-legged intruders, he explains in a musical accent that reveals his Brazilian heritage that targeting the mosquito is far from a novel idea. As soon as people realized that malaria sprouted from the proboscis of this tiny insect, scientists have been working on strategies to kill off mosquitoes or minimize contact with these bugs to prevent bites. The two most effective strategies for preventing malaria thus far have been widespread use of insecticides and bed nets (both plain and treated with insecticides).

However, these approaches have drawbacks. Heavy use of insecticides, most notably DDT, has led to whole populations of mosquitoes that are resistant to these poisons. Insecticides also aren’t the most eco-friendly of interventions, and they require constant follow-up—often unsustainable in poor locales. Bed nets—which are easily ripped and wear out over time—are also far from perfect.

Consequently, scientists will need to find new ways to interact with the other half of malaria’s host dyad to slow or prevent infection rates. But before we form a close relationship with an arthropod, it will be crucial to know exactly who we’re working with, says Douglas Norris, PhD, MS, MMI associate professor.

In many places in which scientists would like to target mosquitoes, they’re still pondering lots of basic questions: Which species are actually present in a region? Do they bite just humans, or do they take their meals primarily from other animals? If humans are their food source of choice, are they biting many people sequentially, or do they get a full meal from just one at a time? How many of these human blood suckers are infected with Plasmodium and currently infectious?

To address such questions, Norris and his colleagues are going straight to the insect source, using high-tech genetic methods to derive answers. They have partnered with colleagues in Macha, Zambia, and elsewhere to gather mosquitoes for study. These collaborators ensnare mosquitoes using a variety of methods—one of the most popular is installing a light trap right above a sleeping human, who’s safely covered with a bed net—then freeze them for later study.

Potentially important species are identified through a number of steps, first of which is visual inspection under the microscope. Then, the scientists pop off the insects’ head and thorax (which have salivary glands that contain infectious Plasmodium parasites) and their abdomens (which may contain blood if a mosquito has recently fed) and place them in a small tube with a dollop of silica gel and a cotton plug. Norris explains the procedure as he hands me a tiny plastic tube rattling with dried insect pieces mummified by the silica gel.

When the researchers are ready, they can run a variety of tests on the heads and thoraxes. Since many mosquito species and subpopulations look alike, researchers can examine the mosquito’s own genetics to determine exactly which one is in the tube. Such knowledge aids efforts to create and disseminate genetically altered mosquitoes—which requires transformed lab mosquitoes carrying the “anti-Plasmodium” genes to breed with existing populations.

“Researchers working on genetically altered mosquitoes are operating on the assumption that the target population for each species is one big, happy breeding population. But what if, instead, this group is structured into subpopulations that don’t breed with each other?” Norris asks. For example, he says, work with his collaborators at the University of California-Davis has shown that a population of Anopheles arabiensis in central Africa is separated into groups that don’t interbreed. “If the subpopulations don’t interact, then you’ll only get your new gene into one population. That one subpopulation will be converted, but you’ll still potentially have malaria transmission by the other populations—if they’re not interbreeding or the gene isn’t introduced to each population, your whole strategy may fail.”

Understanding if mosquito populations are interbreeding can also tell you whether or how speedily they could be swapping genes, Norris adds. That’s particularly important for strategizing against insecticide resistance, a trait that mosquitoes can acquire through random genetic mutations. If mosquito subgroups are heavily interbreeding, then it’s practically a given that the mutation for insecticide resistance will sweep through the population quickly. However, if the subgroups are giving each other the cold shoulder, then researchers may have more time to develop an action plan to control insecticide resistance.

The desiccated insect body parts can yield answers to other basic questions as well. Any blood that remains in a mosquito’s abdomen from a recent meal can tell researchers which animal it’s recently fed on. If it’s human, Norris’ group can also search deeper to see if multiple people’s blood is present in the same insect. Moreover, they can look for Plasmodium genes or antigens in the mosquito’s salivary glands, a dead giveaway that the insect was infectious at the time of its death.

Gaining such genetic information has already proved useful, Norris says. In a recent study, he and his colleagues examined human blood isolated from mosquitoes caught in Macha to see whether it came from male or female victims. Sex markers in the blood showed that women were bitten most often—perhaps because they get up earlier and go to bed later than men in this particular area, leaving them vulnerable during mosquitoes’ prime biting hours. This information could help researchers target an education plan to encourage women and others to protect themselves from mosquitoes if they’re awake while the insects are active.

Comments

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  • saswati datta

    india 04/30/2011 10:15:23 AM

    very informative, very interesting

  • Sief Mahagoub

    Lincoln, Nebraska 02/04/2012 06:47:24 PM

    Great article. I was born and raised in Sudan, Africa. I can relate to this study. Thanks for sharing.

  • Gebremeskel Gebremariam

    Addis Ababa University, Ethiopia 03/06/2012 10:06:15 AM

    I am highly facinated the dimension identified to rollback Malaria, one of the top three most killer diseases in developing countries like Ethiopia. This research may clear our fear or atleast may reduce the nightmare to live in mosquito endemic areas. I am eager to read the next thank you?

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