Prevention And Control

Nosema. Fungal pathogens include Coelomomyces, La-genidium, Culicinomyces, and Metarhizium. Viruses pathogenic to larvae include the iridescent viruses, den-sonucleosis viruses, polyhedrosis viruses such as the bac-uloviruses, and entomopox viruses. Generally, the above-mentioned parasites or pathogens of mosquito larvae are still in experimental stages of development, or they have limited effectiveness and have not been used in operational programs.

An exception is the bacterium Bacillus thuringiensis is-raelensis, or Bti, which has been developed into commercial formulations since its original discovery in 1975. It is used extensively in mosquito control programs. Larvae die when they ingest crystalline, proteinaceous toxins produced by the bacterial cells during sporulation. The bacterium B. sphaericus has a similar mode of action but is more specific. It is particularly effective against Culex larvae, and it is more persistent in water and more tolerant of water with a high organic content than is Bti.

Genetic control, a biological control category using a variety of genetic methods, has been successful against some pests; however, its use against mosquito vectors of disease remains experimental. There are two hypothetical approaches: release of sterilized males or incompatible strains, resulting in attrition of the natural population, and replacement of natural vector populations with species or strains that are poor vectors or are not susceptible to infectious agents. These methods have been reviewed by Rai (1996).

Chemical controlïs achieved with insecticides against either larvae or adults. Larvicides are placed in water where larvae develop or where water will accumulate and provide habitat for larvae. Formerly used larvicides included inorganic compounds such copper arsenate, fuel oil, and organochlorine chemicals such as dichlorodiphenyl-trichloroethane (DDT) and dieldrin. Currendy, categories of registered larvicides are light mineral oils, organophosphates, and insect-growth regulators. Rapidly degradable oils spread over the water surface, penetrating the tracheal systems of larvae and pupae and suffocating them. Organophosphates, such as temephos, malathion, and chlorpyrifos, function as nerve poisons. The insect-growth regulator methoprene is a mimic of juvenile hormone and interferes with metamorphosis and emergence. The specific kind and formulation (dust, powder, water-soluble liquid, emulsion, oil-soluble liquid, granule, pellet, briquet) of the larvicide recommended depends on the biology of the target mosquito, the ldnd and size of habitat, the method of application, the chemical composition of the water, and the presence of nontarget organisms that might be adversely affected. Some can be formulated for slow release from a carrier. These may be applied to dry ground, releasing the active ingredient when inundated.

Adulticides are applied to surfaces where adults will rest or in the air where they fly. Residual insecticides applied to resting surfaces may retain their toxicity for days to months. They were central to the global malaria eradication program, in which DDT spraying of the inner walls of human dwellings at 6-month intervals killed all mosquitoes landing on these walls before or after taking blood. In areas where the vectors bit humans primarily indoors, this effectively interrupted most malaria transmission until mosquito populations developed resistance to the insecticide or when programs were abandoned. This approach is still used widely in some areas. Residual adulticides also can be used outdoors on vegetation or structures that serve as harborages. They tend to have short-term effects, because sunlight, wind, and rain cause the insecticide to degrade.

Adulticides intended for direct contact between airborne droplets and the mosquito are of two types: thermalfogs and low- or ultra-low-volume (ULV) sprays. Both can be applied from hand-carried equipment, motor vehicles, or aircraft. Thermal fogging involves mixing an insecticide with a combustible liquid such as kerosene. The mixture is heated, creating a fog of insecticide that drifts through the area to be treated. The ULV approach involves special nozzles and pumps that dispense fine droplets of insecticide, forming a mist that passes through the target area. Currently, insecticides registered for use in fogs and low-volume sprays are organophosphates, carbamates, pyrethrins, and synthetic pyrethroids. Resistance to insecticides is an important consequence of their use and has developed in many mosquito populations. The mechanisms of physiological resistance have been well characterized biochemically and genetically. Behavioral resistance also can develop. This is typically a change in adult feeding or resting behavior, so that mosquitoes no longer contact insecticide residues. Resistance has been reviewed by Ferrari (1996).

Surveillance, which is at the core of effective mosquito control programs, determines mosquito distribution and abundance and degree of pathogen activity. The goal is to provide data so that control agencies can take action to prevent mosquito-related problems from occurring. Unfortunately, there have been few control programs establishing action thresholds for mosquito density or infection rate, the levels of threat at which controls should be initiated. More often, action is based on human perception of a pest problem, conditions similar to past experience with disease outbreaks, or first detection of pathogen activity. Surveillance strategies and techniques for mosquito-borne encephalitis viruses have been presented by Moore etal. (1993) and Moore and Gage (1996). Bruce-Chwatt (1980) and Sasa (1976) reviewed traditional techniques for detecting malaria and filarial parasites, respectively, and several new ones are in use.

Although all methods of reducing vector populations can lower the incidence of mosquito-borne disease, quantitative models of the dynamics of disease transmission have become important tools for setting realistic control objectives. They allow programs to focus efforts on parts of the pathogen-transmission system most vulnerable to attack. Useful references on this subject are Ross (1911), Macdonald (1957), Molineaux and Gramiccia (1980), Fine (1981), Koella (1991), and Dye (1992). The Ross-Macdonald equation describes the case reproduction rate, or the total number of new cases of a disease arising from a single infective case in a totally susceptible population. The vectorial capacity equation expresses that function on a daily basis using entomological parameters. Although the vectorial capacity measure is not epidemi-ologically comprehensive, it allows a comparison of the relative importance of different vectors and provides estimates of critical vector density, the adult mosquito density below which the case reproduction is less than 1 and the disease should die out. It also illustrates, mathematically, that even small changes in the interval between bites on susceptible hosts (which is squared) or in the longevity of vectors (which changes exponentially) cause large changes in transmission rates. The latter relationship has been critical in mounting effective disease-control operations, which target older females, rather than just female density in general. More complex models sometimes show good agreement between predicted and observed results in extensive field studies of malaria ( Koella, 1991; Molineaux and Gramiccia, 1980) and may become useful in establishing action thresholds. A simple and direct measure of transmission is the entomological inoculation rate (Onori and Grab, 1980), which is the product of the vector's human-biting rate and proportion of vectors that are infective.

Vaccines and drugs are important tools in protecting or treating humans and other animals susceptible to mosquito-borne disease. They serve not only to protect the individual but also to reduce transmission to others. Vaccines are available for several arboviral diseases, including YF and JE for humans, and eastern, western, and Venezuelan encephalitis for equids. These vary in the duration of protection they provide. An experimental human vaccine against eastern equine encephalitis has been produced. None currently exists for the DEN viruses. Human malaria vaccines are under development, and some field trials have achieved limited success, but their wide-scale efficacy remains uncertain. The three kinds of malaria vaccines being considered use antigens from sporozoites, blood stages, or gametes; the last kind is called a transmission-blocking vaccine because the human antibodies take effect against stages that form within the midgut of the mosquito. Among drugs, there exists a wide spectrum of antimalarials used for prophylaxis, therapy, or both. The most commonly used chemo-prophylactic is chloroquine, against which there is now widespread resistance in R falciparum and some P. vi-vax populations. Mefloquine is prescribed for areas with resistant populations. Strickland (1991) presented a detailed review of malaria chemotherapy and chemoprophy-laxis. For lymphatic filariasis, diethylcarbamazine (DEC) is the standard chemotherapy, which reduces microfilaremia but does not kill adult worms. Advanced disease manifestations (e.g., elephantiasis) cannot be reversed, except by surgery, but sustained mass treatment of human populations can drive transmission to zero. This was achieved in parts of China in 1 year by the use of DEC-fortified cooking salt. Owing to the longevity of adult worms, mass treatment for 5—10 years is necessary to completely break the infection cycle in a community. Ivermectin and albendazole are two other drugs showing efficacy in lymphatic filariasis cases. Both DEC and ivermectin are used as chemoprophylaxis against dog heart-worm infections.

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