Helicopter gunships may have slain their thousands, and B-52bombers their tens of thousands, but hinged-wing marauders the sizeof a little fingernail are killing their human victims by the millions.
They are of course mosquitoes, and one of the nastiest goes by thename of Aedes aegypti. If A. aegypti were a military aircraft insteadof a pathogenic insect, its fuselage would carry the names of itsvictories _ yellow fever and dengue fever.
The former disease is controlled in much of the world by vaccination,but for dengue _ a.k.a. breakbone fever _ there is no vaccine, notreatment. And so far, almost no one dies of dengue, though at thepeak of its febrile crisis, many victims wish they would.
Of late, though, a new and deadly form of the disease has emerged _dengue hemorrhagic fever (DHF), a.k.a. as dengue shock syndrome,which can kill infants and young children bitten repeatedly by A.aegypti.
"DHF is a new phenomenon," observed molecular biologist KenOlson of Colorado State University's Arthropod-Borne andInfectious Diseases Laboratory, in Fort Collins. "It's new in the sensethat prior to the early 1980s, it was generally not reported in theWestern Hemisphere. However, since that time the number of caseshas increased dramatically."
Olson, and others, surmise that DHF results from sequential mosquitobites, which inject its victims with two or more differentimmunologic versions of the dengue virus.
DHF differs from ordinary dengue fever in two main respects: it'smarked by internal bleeding, and it's often fatal. The classicalaffliction is bad enough: For the first three days, its victim feelsincreasing pain in body, joints and limbs, particularly excruciating inthe knees and behind the eyes.
A bright red characteristic rash, between measles and scarlet fever inappearance, breaks out over much of the skin, and the face may turndeep purple.
These symptoms clear up around the fourth day, but like a cat playingwith a mouse they return around day seven for another week oftorture. Knee or ankle arthritis may hang in for months or years.
In the current issue of The Lancet, dated May 11, 1996,epidemiologist Alan Lifson, at the University of Minnesota inMilwaukee, maps and charts the spread of dengue fever (DF) through14 countries of South and Central America, including Mexico and theCaribbean. The numbers add up to 184,063 cases of DF plus 5,723 ofDHF.
As for the U.S., Olson said, "There were a few cases in southernTexas toward the end of the last mosquito season, most of themimported. In general, it `s not a problem." But it is a problem, and agrowing one, around the world's tropical and subtropical latitudes ofAfrica, Asia and the Americas.
A Mosquito That Loves Human Homes, Blood
A. aegypti is no jungle mosquito; it prefers the urban haunts ofhuman, and hangs out within 100 yards of a house. Where else couldone find such made-to-order breeding spots as tin cans, cooking pots,discarded tires, coconut shells, cut bamboo, sagging eaves, vases incemeteries, bilges of ships, old beer bottles . . . .
As with other disease-bearing mosquitoes, eradication efforts havefailed in many developing countries. "Novel control strategies arenow being considered," wrote Olson and his co-authors, in thecurrent issue of Science, dated May 10, 1996, "including the geneticalteration of mosquito vectors, to control the rapid proliferation ofarthropod-borne pathogens."
Their paper bears the title: "Genetically engineered resistance todengue-2 virus transmission in mosquitoes."
"A general problem that molecular biologists face in dealing withmosquitoes," Olson told BioWorld Today, "is getting expression ofdesired genes in the insect. We don't have any good DNAtransformation of transient expression systems right now that workwell in mosquitoes."
What did work well was a transducing construct based on theunrelated but also mosquito-borne Sindbis virus. It makes two typesof RNA within a single cell, one genomic, the other subgenomic. Thelatter, Olson explained, "is a messenger that's translated to form thestructural proteins of the virus.
"So," he continued, "we took this engineered Sindbis, which makestwo subgenomic messengers. One includes the information needed togive the viral structural proteins. The other drives the antisensecDNA encoding the dengue virus's premembrane protein, which isimportant in assembling that virus in infected cells."
Co-author Stephen Higgs inoculated half a microliter _ one two-millionth of a liter _ of a suspension containing this Sindbis packageinto the thoraxes of female A. aegypti mosquitoes, chilled to makethem sit still under the microinjection microscope.
"What happens," Olson continued, "is that it gets into the salivaryglands of these aegypti, and pumps out large quantities of thisantisense sequence, directed toward the mosquito's cargo of denguevirus. What we've reported in Science," he said, "is a very first stepin showing that we can actually deliver some antiviral gene, or in thiscase antisense RNA, to a mosquito salivary gland, knock outreplication of the dengue virus, and stop its transmission to humans."
Getting From Lab To Spaying Mosquitoes In Real Life
Since submitting the paper to Science last January, the Colorado teamhas repeated this experiment with yellow fever virus, which A.aegypti also carries, as well as with La Crosse virus _ delivered by adifferent mosquito _ which causes bunyavirus encephalitis in theMidwestern U.S.
In an editorial accompanying the Colorado paper, molecular biologistAnthony James, at the University of California, Irvine, commented:"The new Sindbis viral constructs are not permanently integrated intothe mosquito genome, so the resistance to dengue virus cannot yet bepassed down to new generations of mosquitoes. But when this hurdleis overcome, these dengue-resistant mosquitoes can be tested in fieldtrials for effectiveness in real-world situations."
Olson agreed, and foresees a way over that hurdle: "It's alltheoretical at this point, but there is a paradigm for driving antiviralgenes into insect populations. It may be like the transposable geneticP-element, which is identified in Drosophila melanogaster. Prior to1950, it was found in wild-type fruit fly populations, but by 1980 inall fruit flies.
"This is very long-term research," he pointed out. "Our paperbasically shows that we can engineer the mosquito to where it willnot transmit a given virus. By doing that, the idea is to break themaintenance cycle of the virus in nature." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.