Science against dengue-fever mosquito
MEET "AEDES AEGYTI", THE MOMENT'S PUBLIC ENEMY NUMBER ONE: SCIENCE AGAINST DENGUE-FEVER MOSQUITO
In 1787 botanist Jean Poiret first published the description of a mosquito that he had studied during an expedition to North Africa. It has its body all covered with silver stripes and it seemed so beautiful to him that often he left it bite him for the pleasure of observing it. But what he did not know was that the insect would later become famous as the "Aedes aegypti", or "the odious of Egypt", which transmits dengue fever.
The first record of a dengue-fever epidemic appeared two years later in the United States of America, when physician Benjamin Rush published his book "Medical inquiries and observations." In it he described this unpleasant, debilitating febrile illness that struck Philadelphia in the summer and fall of 1780 with these words: "its more general name among all classes of people was, the break-bone fever."
The name by which we know this ailment today has been since 1801 in the Archives of the Royal Palace of Madrid, Spain, in a series of letters in which Queen Louise makes known that both her and her minister Manuel Godoy were sick. One of the last letters, dated 12 June, reads: "I am better, since it has been the cold in vogue, called dengue". Apparently, this word derived from the African language Swahili, from the phrase "ki denga pepo", which means "sudden spasm attack by an evil spirit."
But more than a century would pass before this "evil spirit" would become better understood.
In 1903, researcher H. Graham explained in the scientific publication "Journal of Tropical Medicine" a series of experiments in Lebanon that indicate that dengue fever is transmitted by a mosquito. Soon after, in 1907, Percy Ashburn and Charles Craig described in "The Journal of Infectious Diseases" experimental investigations in the Philippines into the cause of dengue fever, by way of which were discovered that the "evil spirit" is actually a virus. Finally, in 1924 Joseph Siler, M. W. Hall and Arthur Hitchens published in the "Journal of the American Medical Association" various experiments with diverse species of mosquitoes. They noted how one of them feeds on people suffering from dengue, gets infected and then transmit the infection to others. This species proved to be the "Aedes aegypti".
Today, the World Health Organization reports that dengue is present in most continents and estimated that 2500 million people (including 1000 million children) are at risk of infection. The worst situation occurs in Asia and Pacific countries, although in the Americas it got much worse in recent years. Unplanned urbanization, lack of mosquito control, population growth of cities in tropical countries and rapid travel of infected people made available by modern transportation have contributed to the sharp rise in epidemics.
Amateur naturalist Roland Mortimer wrote for "Micscape Magazine: the monthly online magazine of Microscopy UK.", No. 30, April 1998, an account of his observations of "Aedes aegypti" in Rio de Janeiro: the males do not bite humans or any other animal, but feed on fruit. Only the females bite, looking for blood, which they need to mature their eggs.
The "Aedes aegypti" is very domestic: it prefers to bite humans rather than other animals, and do not venture into forests, as other mosquitoes do. It lives very close to houses, usually no more than a city block away.
Its flight is much quieter than other mosquitoes, so much that usually it does not wake up their victims [but it prefers to attack by day, in the shade, or at dawn and dusk]. It attacks from behind and below, normally under tables and chairs, and bite mainly in the ankles and feet. It flies very fast (unless it is full of blood) and it is very difficult to catch.
Unlike other species, the "Aedes aegypti" lays its eggs in clean water, where no other species are present. It can live very well in chlorinated water. The eggs can survive for long in times of drought, about 1 year. They do not place these eggs in groups, like other mosquitoes, but deposit them well apart.
At first the eggs are white, but quickly turn black. Young larvae feed on bacteria in the water and grow rapidly, shedding their skin while they are developing. After a few weeks, or even faster in summer, the larvae reach the pupal stage [encapsulated]. This state is usually short, and finally the top of the pupa opens like a can, releasing the adult mosquito. This mosquito is small compared to others, but it can live for months if not disturbed.
Dengue virus [or, in some cases, the one of yellow fever] remains in their salivary glands, from where, unbeknownst to the mosquito, infects humans.
These impressions are supported by scientific studies:
In his book "The physiology of mosquitoes," Macmillan, 1963, Alan Clements confirms that only the females bite. He describe that male mosquitoes feed primarily on nectar from flowers, while female mosquitoes require a blood meal to produce eggs. They feed usually every 3-4 days; in a single meal, a female mosquito typically consumes more than its own weight in blood.
It is well known that the "Aedes aegypti" thrives in hot, humid climates. In November 2005, Richard Russell, Cameron Webb and Neil Davies described in the "Journal of Medical Entomology" experiments on an island in French Polynesia. They found that the frequency of bites by "Aedes aegypti" increases notoriously if we go from the end of the dry season until the end of the rainy season. They also confirmed that this species dwells in urban areas: in the northern part of the island, where the ports and airports are and local people and tourists concentrate, the frequency of bites was significantly higher.
In laboratory studies detailed in 1965 in the journal "Mosquito news", R. Fay and A. Perry found that females prefer to lay their eggs in rough, dark water containers, usually in the shade.
As an important side note, all people at risk areas should be informed that when one of these diseases attacks, the whole neighborhood is at risk. José González Ordóñez and collaborators detailed in the "Journal of the American Mosquito Control Association", of 17 July 2001, experiments in northeastern Mexico, where 400 females, marked and unfed, were set loose in a field, where 100 sticky traps were positioned in an area of 300 m in diameter. After a period of 19 days, they discovered that the maximum mosquito dispersion occurred at a distance of 120 meters.
Something remarkable is that, indeed, this species of mosquito developed a preference for human blood. Wannapa Suwonkerd and colleagues described in the "Journal of Vector Ecology", December 2006, a test in which several experimental huts were prepared in Thailand. Some were left empty, but in others human volunteers were positioned as "baits", and in some others dogs were placed. 800 marked mosquitoes were released, and it was found that half of the mosquitoes left the empty cabins and those with dogs. (The peak hour of exit was between 14 and 17 hours.) But in the cabins with people inside, more than 95% of the mosquitoes preferred to stay. It was also seen that the mosquitoes come from outside to inside the cabin between 10 and 12 hours of the day, if the cabin is empty or with a dog inside, but if it is occupied by a human they often come well before, between 8:30 and 11:30 hours.
The irony of our current situation is that, as an illustration prepared by Duane Gubler shows ("Trends in Microbiology", Vol 10, No. 2, February 2002, p. 100), if we look to the past we see that by the decade of 1970 we were able to control epidemics of dengue hemorrhagic fever and urban yellow fever in many of the countries of the Americas, thanks to successful programs of eradication of "Aedes aegypti". From that date on, however, the Inter-American program was disbanded and the "odious" mosquito invaded again most of the countries of the region. So we know what we have to do, again.
The U.S. Environmental Protection Agency (EPA for short) provides a list of products tested and registered to kill mosquitoes:
It seems obvious to focus on adult insects. Malathion (chemical formula C10 H19 O6 P S2) is a broad-spectrum organophosphate first registered in 1956 and was last reassessed in 2006. It has numerous commercial applications in agriculture, industry, government and households. Currently about 7-million kilograms per year are used. In mosquito control programs conducted by the authorities, malathion is applied by truck-mounted sprayers or by airplanes. It is applied as an ultra-low volume spray: sprinklers disperse very fine aerosol droplets that remain suspended in the air and kill mosquitoes when touching them. The ultra-low volume applications require small quantities of the pesticide's active ingredient in relation to the size of the treated area, which minimizes exposure and risks to people and the environment. A similar adulticide is Naled, formula C4 H7 Br2 Cl2 O4 P.
Another option is to kill mosquito larvae: larvicides are products applied to habitats that can serve as breeding grounds. Destroying the larvae before they emerge as adults can reduce or eliminate the need for spraying insecticides to kill adult mosquitoes, thus reducing the total amount of poisons to be used in a program to control this pest.
A striking technique is to use other living things to kill the larvae. The "Bacillus thuringiensis israelensis" is a bacterium that occurs naturally in soil and is authorized to control mosquito larvae. It was first registered as a pesticide in the United States in 1983. Mosquito larvae eat the industrial product, which is made of dormant bacteria in the form of spores and a pure associated toxin. The toxin disrupts the mosquito guts by sticking to receptor cells present in insects but not in mammals. Another microorganism that serves a similar function is the "Bacillus sphaericus", which occurs naturally in many parts of the world. It was initially registered in the United States in 1991, for use against various kinds of mosquito larvae.
These larvicides are used in outdoor areas such as irrigation ditches, flood water, standing ponds, woodlands pools, grasslands, wetlands and areas for stormwater retention. The duration of its effectiveness depends primarily on environmental conditions, product formulation and water quality.
The development of resistance to bacterial larvicides by mosquito larvae can be prevented by using an organophosphate pesticide known as temephos, registered in the United States in 1965 and that is the only organophosphate used as larvicide. Temephos is most commonly applied by helicopter, but it can be applied by airplane, truck or backpack sprayers, both in granular or liquid form.
Another control technique is to interrupt the growth of the larvae. Methoprene is a compound (first registered in the U.S.A. in 1975) that simulates the action of a hormone that regulates the growth of the insect, preventing the normal maturation of the larva.
Paradoxically, it is also possible to drown these odious in their own water, preventing them, by physical means, of getting the air they need to breathe. Low-toxicity pesticides known as monomolecular films are spread on the water surface as a thin film, making it difficult for mosquito larvae, pupae or emerging adults to stick to the water surface for breathing, causing drowning. The film may remain active for 10 to 14 days, in standing water.
Some specialty oils, derived from petroleum distillates, have been used for many years in the United States to form a coating over the water to drown larvae, pupae or emerging adult mosquitoes, just as monomolecular films do.
All of the above products are for use by trained professionals only, under the supervision of the health and environmental authorities.
There are also less complicated products: pyrethroids, which are synthetic chemicals that act in a similar manner to pyrethrins, poisons derived from certain chrysanthemum flowers. They are widely used to control various insects. Typically, they are mixed with a compound that lowers the mosquito defenses, such as as piperonyl butoxide.
Pyrethroids are used as very fine aerosol droplets that remain suspended in the air, killing the adult mosquito when in contact with them. The traditional mosquito coils do not seem to be the best choice for indoors: experimental data from a study conducted by Weili Liu and colleagues and published in "Environmental Health Perspectives", September 2003, showed that burning a single spiral releases the same amount of harmful fine 2,5-micrometers particles as burning 75 cigarettes or more.
The U.S. Centers for Disease Control and Prevention (CDC), based in Atlanta, recommends permethrin as the pyrethroid of choice for use by people from the general public. Permethrin is registered in the United States since 1977 and is sold in a number of household products against different pests. Like any synthetic pyrethroid, it kills insects by strongly exciting the nervous system. Permethrin causes them to remain hypersensitive to stimuli from sensory organs. Instead of sending a single pulse in response to a stimulus, the nerves exposed to permethrin send a pulse train. Permethrin alters the movement of sodium ions from the outside to the inside of the nerve cells, which causes excitation, as published by H.P. Vijverberg and J. van den Bercken in 1990, in the scientific journal "Critical Reviews in Toxicology".
As for the risks, the WHO reported in 1990 that reviewed three studies with mice: in 2 cases the data show that, in females, permethrin increased the frequency of lung tumors, therefore users should be careful in their application and always follow the instructions stamped on product packages word by word. If in doubt, a medical doctor should be consulted immediately.
Dr. Mark Fradin, in the journal "Annals of Internal Medicine" of 1 June 1998, gives a list of references for two decades of experiments that tested more than 20 000 different substances, from which emerged as champion repellent the "N, N-diethyl-3-methylbenzamide", also known as "N, N-diethyl-m-toluamide", "diethyl toluamide" or simply DEET. The data showed that this is the insect repellent currently available with longest-lasting protection and against the broadest variety of insects. It was discovered and developed by scientists at the Department (Ministry) of Agriculture of the United States of America and was patented by the U.S. Army in 1946. It was registered for use by the general public in 1957. By the time of its registration renewal in 1998, the EPA estimated that 30% of the U.S. population (about 90 million people) was using DEET.
But as nothing is perfect in this world, Ali Abdel-Rahman, Ashok Shetty and Mohamed Abou-Donia reported in the science journal "Experimental Neurology", November 2001, that rats in which DEET was applied to their skin for a long time presented brain damage. In 50 years there have been 14 cases of brain disease in humans (13 of them children). Of those, 11 recovered, but three died: one case detailed by C. Zadikoff in 1979, another detailed by H.M. Heick and collaborators in 1980 and another by J. de Garbino and A. Laborde in 1983.
But we have to compare these 14 cases with the 50-million to 100-million cases of dengue fever, 500 000 cases of dengue hemorrhagic fever and 20 000 deaths from dengue shock syndrome occurring in the world each year, according to WHO estimates. The American Academy of Pediatrics recommends that parents apply to their children products with DEET concentrations not exceeding 30%, preferably 10%.
The CDC list as alternatives Picaridin (KBR 3023), less sticky, and the unpronounceable "3-[N-Butyl-N-acetyl]-aminopropionic acid, ethyl ester", conveniently nicknamed "Insect Repellent 3535", sold already for over 30 years in Europe. But all of them are less effective than DEET.
For those who prefer natural products, the CDC cites an extract (para-menthane-3,8-diol, or simply PMD) of lemon-eucalyptus oil (scientific name: "Eucalyptus citriodora"), though it is very irritating to the eyes and should not be used in children under 3 years old. There is also the essential oil of citronella ("Cymbopogon nardus"), a plant of Indonesia. Dr. Jerry Butler, from the University of Florida, with funding from the company "International Flavors and Fragrances" (which provides ingredients for the cosmetics industry) formulated a herbal repellent which active ingredient is geraniol. But Angela Brammer, who was also in the same department of Dr. Butler, said that although the oil of many plants repel insects, the protective effect is usually significantly less than that of DEET.
Consumers should know that the same product but from different manufacturers may have different efficiencies. Anyway, users must always read and reread the product labels and follow the recommendations word by word. If you have questions or suspect an adverse reaction, you should consult a medical doctor immediately, showing the packaging.
A DENGUE-FEVER VACCINE?
Dengue virus was first isolated in 1943 by Japanese Sumusu Hotta and Ren Kimura. Months later, Albert Sabin (yes, the polio man) and Walter Schlesinger, working in Hawaii and New Guinea, isolated another type: DEN-2. In 1956, Bill Hammon, with colleagues from the Philippines and Thailand, isolated serotypes DEN-3 and DEN-4.
After infection, the human body creates resistance against the serotype acquired but not against the others. There is the possibility that a vaccine for one serotype lead to dengue hemorrhagic fever if later the person is infected with the wrong serotype. For these reasons controlling dengue will only be possible after an effective "4-in-1"-vaccine is developed.
A weakened-live-virus vaccine prepared by Sanofi-Pasteur, and tested in collaboration with the U.S. Walter Reed Army Institute of Research (WRAIR) in 78 children in Bangkok, showed 85% response to DEN-1, 78 % to DEN-2, 100% to DEN-3 and 76% to DEN-4.
A similar technique was chosen by the WRAIR and GlaxoSmithKline Biologicals, and in their initial human studies achieved 90% response to DEN-1, 60% to DEN-2, 60% to DEN-3 and 25% to DEN-4.
The ChimeriVax system, originally developed to build a vaccine against Japanese Encephalitis, has been applied to dengue by Acambis in the U.S.. Evaluated in 56 human volunteers, it resulted in a 100% response to DEN-2.
The development of these vaccines is financially supported by Bill and Melinda Gates.
Thus the eradication of dengue is finally taking priority, as it is becoming nothing less than the primary disease by mosquitoes in the world.
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Originally published in ABC Color, on 5 February 2007. Photo: Read instructions before applying any repellent to children. Make sure the container is in good condition and pay attention to the expiration date. Never apply over blisters, wounds, cuts or skin irritations. Neither apply to eyes, nose, lips, hands or directly in the face. Use just enough to cover exposed skin or clothing according to label instructions. Never apply repellent under clothing. If in doubt, consult your medical doctor immediately. Credit: U.S. Environmental Protection Agency.