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Reducing Dengue Disease
Using Biological Agents

For "Kids of All Ages," especially those above 5th Grade

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What's it all about?

Dengue is an infection that is carried by mosquitoes and can cause severe disease in people. It can cause headaches, pain, and rash, and, in more severe forms, can cause internal bleeding and death. Currently, there is no vaccine for dengue, and the most widely used methods of control are chemical agents. These insecticides sometimes can harm the environment as well as people. For this reason, scientists are looking for better ways to prevent the spread of dengue.

Various living things, also known as biological control agents, can be used to prevent infections carried by mosquitoes in place of chemicals that may hurt the environment.

What's the problem?

People seek long-term and environmentally friendly relief from dengue outbreaks.

Dengue is a disease that occurs in the tropics, in cities as well as in villages. A few people have been infected who live in southern Texas in the United States.

The disease is called "breakbone fever" because of the pain that may occur in a personís joints. The internal bleeding that sometimes follows repeated infection can be fatal.

Programs made to prevent dengue by trying to kill the adult stage of the mosquitoes that carry this infection frequently fail because the adult stage of Aedes aegypti, the main kind of mosquito that carries dengue, stays in villages and towns where they are difficult to find and kill. Old tires and trash are places where these mosquitoes reproduce. Also, the containers of drinking water, another place where these mosquitoes live, must not be contaminated with potentially harmful chemicals.

What can be done?

For these reasons, people seek to use biological control agents that can get into hard to find places, will not hurt people, and do not cost too much.

"Biological control" means that people use different kinds of living things, called organisms, and other biological products, many of which can spread by themselves or are not harmful to the environment. This information is here to explain the best ways to use this kind of intervention to keep people from getting dengue.

Ways to control dengue
by controlling mosquito populations

ANTIADULT AGENTS (Agents that would kill the adult mosquito)

A biological agent that would most effectively reduce risk of dengue would act against the adult mosquitoes. Some chemicals that have been used in houses destroy other predatory arthropods such as spiders and ants. The mosquitoes, then, lose their enemies, and can reproduce and live. The spraying of chemicals might therefore ultimately increase the transmission of dengue. Needed are agents that act directly on the length of life, the feeding patterns, and the ability of an adult to spread the disease.

Zooprophylactic agents are animals that might slow the spread of dengue by attracting mosquitoes that might otherwise feed on people. For example, some scientists found that a different type of mosquito in Hawaii frequently feeds on cats and remarked that the presence of cats would slow down dengue transmission. This early finding showed that a large range of different animals and plants, biological diversity, might protect people against dengue infection. The idea needs further testing.

ANTILARVAL AGENTS (Agents that would kill the mosquito in the larva stage)

Aside from destroying the larval stages of A. aegypti, the ultimate objective of a public health program using this type of agent is to reduce the number of the adults of this vector mosquito. However, a smaller number of larvae do not always reduce the number of adults (Agudelo-Silva and Spielman 1984; Washburn et al. 1990). Destruction of a portion of the larvae in a site may increase risk of human disease because sometimes the death of some of the mosquitoes makes the other mosquitoes stronger.


Aedes aegypti (female),
primary vector of dengue-hemorrhagic fever
around the world.

FISH

Chinese health authorities have used various kinds of fish to exclude A. aegypti mosquitoes from breeding in large cisterns or other containers of drinking water (Lu 1989). "..Small fishes, such as Claris fuscus, Tilapia nilotica, and Macropodus spp., have...been used in many regions to eliminate the larvae in the domestic water containers with considerable success." The use of catfish appears to be particularly effective (Neng et al. 1987). Such fish may have potential elsewhere in the world where the introduction can be maintained and where the human population does not object to the presence of these animals.

TADPOLES

Omnivorous tadpoles constitute another potentially beneficial predator of mosquitoes (Spielman and Sullivan 1974). The larvae of the giant Cuban tree frog (Hyla septentrionalis) destroy aquatic insects and algae growing in water containers. They rarely are found in containers that hold less than several liters of water and seem to have a small impact on Aedes mosquitoes. These frogs may somewhat slow down the force of transmission of dengue. Of course any introductions of an organism beyond its natural range must be done with the utmost reluctance and caution.

OTHER MOSQUITOES: PREDATION

Various larval mosquitoes are adapted as predators of other mosquitoes, including certain

Psorophora and Culex (Fuscus) species. Toxorhynchites, a kind of mosquito with cannibal larvae, have attracted much attention as biological control agents. Certain kinds of toxorhynchites mosquitoes are good for control of A. aegypti because they breed in the same kinds of containers.

There are problems with the use of toxorhynchites mosquitoes for control of dengue. The technique began to lose luster when it was recognized that it increased prey density. This type of interference effect became evident when the prey was present in excess (Hubbard et al. 1988). Certain tests and trials have shown this to be true.

Other predatory insects have been used to reduce the abundance of A. aegypti mosquitoes. Dragonfly larvae, in particular, may be useful for this purpose (Sebastian et al. 1990). In an experiment in Myanmar (=Burma), target A. aegypti larvae virtually disappeared immediately after 2 dragonflies were placed in each container, and the density of target adults declined about 6 weeks later. The experiment is considered as a success. These observations suggest that a predator that is satisfactory for use in tanks of potable water must be large enough to consume many larvae in a short time, cling to the sides of the container so that it will not be lost when water is removed, develop slowly enough that it need not be replaced frequently and be able to survive for months when no larval mosquitoes are available as food. This model dragonfly release effort may be too expensive to serve as a basis for operational anti-dengue programs, particularly where our "throw-away society" provides alternative breeding sites that may be too small, numerous and difficult to identify.

OTHER MOSQUITOES: DISPLACEMENT

Certain mosquitoes, most notably A. albopictus, appear to establish relationships with A. aegypti that eliminate A. aegypti from parts of their range (Gilotra et al. 1967). Indeed, the former species has displaced the latter from large portions of the southern United States, and only within a few years (Rai 1991). A. albopictus is a highly competent carrier of dengue as well as a variety of other viruses and is a pest species.

Aedes albopictus
Aedes albopictus (female), the Asian Tiger Mosquito,
a recent invader mosquito into North and South America,
Africa and Europe; a rural, secondary vector of dengue.

In Asia, A. albopictus and A. aegypti rarely are found in the same sites. On Taiwan, for example, A. albopictus is found in the dengue-free north and A. aegypti in the south, where dengue is endemic (JC Lien, personal communication). The area of overlap there is exceptionally small. In India, A. aegypti seems to displace A. albopictus in urban sites, while the reverse occurs in rural sites (Gilotra et al, 1967).


Frontal poses of male and female each
of Aedes aegypti (left) and Aedes albopictus (right)

A. aegypti is similarly related to another mosquito that breeds in small containers, A. bahamensis (Spielman and Feinsod 1978).

Has the invasion of these exotic mosquitoes partially freed Florida and certain other portions of the southern United States from the threat of epidemic dengue? A. bahamensis is not a good carrier a variety of viruses (Llewellyn et al. 1970). Indeed, although these mosquitoes are capable of feeding on vertebrate hosts, they rarely do so and appear never to do so before laying their first eggs (O'Meara et al. 1992b). A. albopictus, on the other hand is a highly competent host for dengue and is a pest.

Sexual interaction may provide another mode of displacement. In the event that the males of one kind of mosquito were to mate with, but not effectively fertilize, the females of another, the fertility of those females might be impaired (Spielman and Kitzmiller 1967). Then, the population would be unable to continue to produce more mosquitoes.

OTHER MOSQUITOES: GENETICALLY MODIFIED STRAINS

A discussion of "vector control" should also mention the potential of genetics for use against mosquito-borne infection (Curtis 1990). The development of methods to modify genes of mosquitoes is frequently believed to provide the best hope for intervening against malaria and dengue. The present efforts to genetically control mosquitoes are based on the techniques of molecular genetics. The main objective is to modify a laboratory strain of mosquitoes in order to reduce its ability to serve as a carrier of disease and eventually to release these mosquitoes in nature such that the wild mosquitoes will eventually be replaced. However, there are several problems with this approach, and prospects for this technology seem dim.

OTHER INSECTS: HORMONOMIMETIC COMPOUNDS

A "third generation of insecticides" was designed in a classical program of work originally housed in the late Carroll Williams laboratory at Harvard University. Williams anticipated that active hormonomimetic compounds could be designed that would block development of particular kinds of insects (Williams 1967). The first such hormonomimetic insecticide was later modified to produce a more active and stable compound that is commercially available under the name methoprene or altocid (Staal 1975). Methoprene now promises to be the most environmentally acceptable chemical usable against mosquitoes and could possibly be used in drinking water.

CYCLOPOIDS

The ability of certain cyclopoid copepods to destroy larval mosquitoes was noted in 1938 (Hurlbut). These "water fleas" were seen preying on newly hatched larvae. Field experiments in Rongaroa (French Polynesia) later demonstrated that

Mesocyclops can be used in interventions against A. aegypti (Riviere et al. 1987). Cyclopoid copepods appear to limit the abundance of certain kinds of mosquitoes. However, not all copepods destroy all mosquitoes.

Field trials have been conducted to determine whether copepods can usefully destroy larval Stegomyia mosquitoes. Copepods (Macrocyclops albidus) were released into each of about 200 tires arranged in 2 stacks of about 100 discarded tires each located near New Orleans (Marten 1990a). A third stack remained untreated. Larval A. albopictus that were numerous in the treated tires at the beginning of the experiment virtually disappeared within 2 months. Adults disappeared about 1 month later and remained scarce for at least another year. These predators, however, did not reduce the abundance of Culex salinarius, another type of mosquito.


Discarded tires, a common urban habitat
of Aedes aegypti and Aedes albopictus

A series of other field trials are currently being conducted in different parts of the world to determine if this is a good way to prevent transmission of disease. Preliminary results have been encouraging. Cyclopoids now appear to offer high promise as biological control agents for A. aegypti mosquitoes. The technology is "appropriate" and costs appear to be modest. Little environmental risk is evident. However, certain drawbacks of a copepod release program have been identified. Although highly promising, the practicality of these biological agents in anti-dengue programs remains to be established.

PROTOZOA

Parasites provide fascinating prospects for the destruction of mosquitoes. Although parasites do not affect all kinds of mosquitoes, in some, as the larvae eat these parasites, they can cause death (Walker et al. 1987).

MICROSPORIDIA

Until recently, the microsporidian pathogens of mosquitoes appeared to hold little promise for use against A. aegypti mosquitoes because they did not appear to live well in similar habitats. Recent findings, though, show that this might be a possibility (Sweeney and Becnel, 1991). For a biocontrol effort against A. aegypti, microsporidian-infected eggs might be placed into natural breeding sites. Although many of the mosquitoes would escape the lethal effects of this agent, the infection should cause problems for those mosquitoes that escape.

BACTERIA

BTI is a proven, environmentally safe mosquito larvicide that is nontoxic for people. Bacillus thuringiensis israelensis (BTI) was discovered only in the 1970's (Goldberg and Margalit 1977). It has become commercially available under such names as Teknar, Vectobac and Bactimos. The beauty of this material is that an application destroys larval mosquitoes but spares any predators that may be present. The toxin, however, is destroyed by sunlight. This imposes a necessity for frequent application. Weekly visits to each breeding site may be required, thereby making it more expensive.

FUNGI

Of the numerous fungi that have been described, only Lagenidium giganteum seems to have the ability to possibly stop the spread of disease transmission through mosquitoes. Indeed, the federal Environmental Protection Agency registered several such formulations for use against larval mosquitoes on 16 August 1991. Oospores that can resist droughts can readily be produced in bulk (Kerwin et al. 1986). Oospores survive for many years in soil, but reactivate only about a month after flooding. In practice, the spores are activated by staying in water for 1-2 weeks before being sprayed onto the surface of the site to be treated. About half of the target mosquito population becomes infected by the oospores and larvae continue to die for the next several weeks. The infection may continue for a period of time. This material holds promise for use against dengue vectors breeding in potable (drinking) water; it should soon become commercially available.

Although these preparations seem to persist for a long time in water, there are certain additional problems. It can be difficult and costly to apply. The fungus infects and kills only a part of target larval mosquitoes, as well. The level may not be large enough to decrease risk of human dengue infection.

Various other species have been considered for use against mosquitoes, but there have been problems with these also (Whistler et al. 1974, Chapman 1985, Lacey and Undeen 1986).

Other fungi, such as Leptolegnia chapmani (Zattau and McInnis 1987) and Toplocladium cylindrosporum (Ravellec and Riba 1989), present some potential for anti-dengue programs.

PLANT EXTRACTS

The uses of various plant extracts to decrease the transmission of dengue are very traditional (Williams 1967). Chrysanthemum flowers, the neem tree, marigolds, Swartzia madagascariensis -- a variety of an African plant, and the Ethiopian soap-berry vine all create elements that can be used as insecticides (Spielman and Lemma 1973, Minjas and Sarda 1986, Green et al. 1991). Local traditions provide guides to the identification of such plants. Thus, many plants contain things that destroy mosquitoes; they can be useful in village-level self-help anti-dengue programs where there are a large number of people willing to help and where the plants are cheap.

ALGAE

Certain algae seem to fill the guts of larval mosquitoes with a mass of material that they are not able to digest, and this is said to hurt growth (Marten 1986: 1987). The promise of such material against the mosquitoes carrying the dengue virus, however, seems limited because these mosquitoes usually breed in sites that are too dark for algae.

ATTRACTANTS

Louis Roth's classical 1948 study on the role of sound in A. aegypti looking for mates created a highly productive series of research efforts that focused on the possibility of gathering mosquitoes by attracting them to a source of odor or sound. These efforts continue to this day

MISCELLANEOUS

A broad range of predators and parasites provide potential for developing anti-dengue agents. Different types of worms and spiroplasms could decrease the ability of mosquitoes to breed and carry dengue, as well (Case and Washino 1979, Kerwin and Washino 1985).

COMMENTARY

This case study is another in a long line of reviews of the potential of biological agents for decreasing transmission of one or another vector-borne agent, and, like these others, concludes with a set of recommendations.

The major requirement of a program that will help to stop transmission of dengue is the ability to get into obscure bodies of water scattered within and around groups of humans where A. aegypti mainly breeds. An untended guard-dog or a hidden automobile tire presents an obstacle to any effort in a society that respects privacy. Only biological agents carry the potential for overcoming this difficulty, and the most likely agents are those represented by closely related organisms. Toward this end, we require a program of biological research aiming toward an understanding of the factors that limit the number of mosquitoes.

A second requirement for an anti-dengue program is the potential for use in potable (drinking) water. Household sites for collecting and storing potable water generally can be visited in a way that will permit an effort to introduce biological agents. Biological agents can avoid the requirements of toxicity and absence of odor that such an application requires. Toward this end, we require a program of biological research aiming to develop methods for formulating BTI such that the material will not rapidly sediment.

The most vulnerable point in the dengue transmission cycle lies in the length of its incubation period. Toward this end, we require an understanding of the factors that influence the length of life of these vectors.

Nearly as sensitive in the continuation of dengue transmission is the requirement for a vector-host relationship (a relationship between a mosquito and the host where it feeds) that focuses on mammals that have the ability to maintain the basic reproductive rate of the thing that causes the disease. Toward this end, we require an understanding of the host-seeking behavior of these mosquitoes.

The requirement for a long-term, sustainable effort makes cost an important factor in any public health effort. For this reason, intervention agents cannot generally be produced in living biological systems. Production should be possible in certain tanks, and a minimum number of steps should be required for preparation of the final formulation. Extended shelf-life is crucial, as is the ability to kill all mosquitoes that have been treated. Pesticide development efforts should aim toward these objectives.

Most critical in an approach toward suppressing or containing any vector-borne infection is the ability to select research directions on the basis of practical matters. Any anti-dengue campaign that is designed should pursue attainable objectives that are worthwhile and sustainable. Failure to satisfy any of these criteria may result in the worst public health harm, an ultimate increase in the disease.

Activities and Puzzles

Crossword Puzzle - Reducing Dengue Disease.

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