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MALARIA: Obstacles and Opportunities
use in indoor residual spraying, provided that the vector is susceptible. This chlorinated hydrocarbon remains one of the most effective insecticides for malaria control efforts in endemic countries. Compared with other available insecticides, it is inexpensive and, importantly, is nontoxic to humans. Few of the accidental poisonings each year can be attributed to DDT use in public health disease control programs. Parenthetically, the use of DDT as a residual indoor spray does not introduce DDT into the environment in amounts sufficient to enter the food chain, and thus this usage does not have adverse ecological consequences.
Innovative Vector Control Measures
Antimosquito Vaccines The idea that blood-feeding vectors may be damaged by mammalian antibodies directed against insect tissues is not new. Recently, scientists developed an antitick vaccine that protects cattle against tick infestations (Willadsen et al., 1989). The antibodies induced by this vaccine are directed against “concealed antigens” on tick gut cells that are not exposed during the blood-feeding process.
Preliminary studies using homogenized mosquito tissues or whole mosquitoes as the immunogen have demonstrated significant effects on mosquito survival, fecundity, and egg viability (see Chapter 9). The direct effects of antibodies produced by these methods on the sporogonic development of P. falciparum and other human malaria parasites are unknown, however, as are the mosquito antigens that might elicit the most effective human antibody response. Modern tools of immunology and molecular biology offer hope for the development of mosquito-derived antigens that could serve as candidate vaccines for malaria control.
Genetic Modification of Mosquito Vectors It is theoretically possible to replace populations of malaria-transmitting mosquitoes with genetically altered forms that cannot transmit the malaria parasite. The foundation for vector replacement strategies is based on continued progress and major advances in critical areas of vector biology. Already there have been successful attempts to insert foreign genes into the mosquito genome (Miller et al., 1987), and molecular probes for species identification have been developed and field tested (Collins et al., 1988).
Much research remains to be done before genetically engineered mosquitoes can be used for malaria control. Regardless of the long-term outcome of such research, however, valuable new techniques and approaches for identifying the genetic basis of malaria transmission will be developed along the way.
Environmentally Safe Biological Control Agents Natural predators and pathogens, ranging from viruses to nematodes, help regulate populations of both