The Mosquito and the Itch

At their worst, mosquito bites kill. At their best, they're akin to a really bad memory.

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A scientist once told me that she wished for a safe way to obliterate mosquitoes from the Earth. I was kind of shocked, since it seemed like this would tinker with the natural order of the world. Don’t worry, she said: “They have absolutely no ecological value.” And just imagine the benefits: Without mosquitoes there would be no malaria and no dengue fever, also no yellow fever, no Rift Valley fever, no West Nile virus, no Japanese encephalitis, no St. Louis encephalitis virus. And of course, there would also be the far less consequential miracle of no more itchy bumps.

We’re lucky here in the United States that, with their nasty bites, our mosquitoes deliver just the small bump and itch. While I dread both — mosquitoes plague me all summer long — it could be a lot worse. In areas of Africa, Asia, and Central and South America, itchy little mosquito bites often bode sickness and death when those of certain mosquito species leave behind a parasite or virus. There are 350 to 500 million malaria cases and tens of millions of dengue fever cases worldwide each year, resulting in unquantifiable pain and suffering, loss of productivity, and more than one million deaths, mostly among young children.

Alas, as yet, there is no safe way to obliterate the mosquito. Since DDT was banned in many countries in the 1970s, efforts to stop mosquitoes from spreading diseases emphasized mostly bite prevention, notably through the use of bed nets and control and eradication practices. Research on mosquito-borne diseases was widely neglected until the past decade, when seriously increased funding — most notably from the Bill and Melinda Gates Foundation and the Global Fund for AIDS, Tuberculosis, and Malaria — spurred investigation and accelerated progress. Promising vaccines aimed at blocking malaria and dengue fever are now in development, as are new treatments. Emerging mosquito-control efforts also hold some promise, including the development of newer and safer insecticides, and techniques to get mosquitoes to ingest antimalarial and antiviral treatments that stop pathogens or even genetically sterilize the specific species that carry malaria.

Nevertheless, the mosquito problem is likely to get worse before it gets better, since the world is expected to get warmer and wetter due to climate change, and warm, wet conditions help mosquitoes thrive. Even among scientists who do not agree that climate change will spread disease-bearing mosquitoes into new territories (the issue is controversial), there is general agreement that other changes will. These include a growing population and increased poverty worldwide, urban crowding, deteriorated public health infrastructures, and mosquito resistance to insecticides.

Poorer, less developed areas of the world are likely to be hit hardest by the growing mosquito problem. But the United States and Europe are also likely to feel some effects. Malaria was once a significant problem here, and it could return; at the same time, the already present risk of West Nile Virus is growing. An emerging form of encephalitis called Chikungunya virus, associated with several recent large-scale epidemics of arthritic disease, holds potential to threaten the world over. Even dengue fever has popped up a few times in southern Texas over the past few decades.

Today we can only hope that scientific advances will one day block mosquito-borne diseases. But it seems unlikely that even then the insect will ever totally disappear and stop biting and bothering us. So I felt a bit of selfish relief when I learned that, in addition to more directed mosquito-related research that might hold the power to save me and millions of others from dangerous diseases, investigators are also working to understand the science of the itch.

Researchers at Wake Forest University Baptist Medical Center recently used functional magnetic resonance imaging (MRI) to see how people’s brains responded when their lower leg was scratched. They found scratching significantly lessened activation of the anterior and posterior cingulate cortex, areas of the brain associated with unpleasant or aversive emotions and memories. The more intense the scratching, the lower these areas were activated. The investigators hypothesized that scratching may suppress the negative emotional components of an itch to bring about its relief.

Their research holds serious implications that reaches beyond resolving the simple mosquito bite or other minor skin irritations. For some people, itchiness is a chronic condition that seriously diminishes overall health, as with eczema and the chronic moderate-to-severe itching that occurs in many kidney dialysis patients.

The MRI study was small, and the scratching occurred in the absence of itch, so more research is clearly needed. But scientists are closer to uncovering just why scratching an itch can sometimes be so hard to stop. Certainly, solving the riddle of why we itch does not rank on the scale of stopping malaria or dengue fever, but it is with little steps like these that great scientific discoveries are made. • 26 February 2008

Sources: “A Single Mutation in Chikungunya Virus Affects Vector Specificity and Epidemic Potential.” Tsetsarkin KA, Vanlandingham DL, McGee CE, & Higgs S. PLoS Pathog. 2007 Dec 7;3(12):e201. “The Brain Processing of Scratching.” Yosipovitch G, Ishiuji Y, Patel TS, et al. J Invest Dermatol. 2008 Jan 31.

 

Jennifer Fisher Wilson is the science reporter for Annals of Internal Medicine. Her stories are available at www.annals.org.

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