Disease vectors

The United Nations Stockholm Treaty on persistent organic pollutants calls for the phase out of DDT but recognizes its efficacy as a deterrent to vector-borne diseases such as malaria and typhus. According to the treaty, the continued use of DDT is discouraged, but until effective economical alternatives are found, DDT use will be continued in countries with high rates of vector diseases. A number of developing countries still use DDT. It is applied primarily in the interior of homes to prevent malaria. Currently DDT is produced only in India and China, and current production volumes are unknown. 

The timing of Muller s discovery insured its early application in the Mediterranean and South Pacific theaters of World War II to eliminate mosquito-vectored diseases malaria, filariasis, dengue fever also typhoid fever, carried by lice and fleas. 

The efficacy of DDT in controlling morbidity and reducing mortality rates from pest-vectored diseases was probably a major factor in the world population explosion of the 70s (Metcalf, ref. 16). 

Sod-bome fungus. Polymyxa gmminis is the vector. Disease encouraged by hard winter. Symptoms appear from January. Found only in autumn sown cereals. May survive in soil for many years. 

The value of insecticides in controlling human and animal diseases spread by insects has been dramatic. It has been shown that between 1942 and 1952, the use of DDT in pubHc health measures to control the mosquito vectors of malaria and the human body louse vector of typhus saved five million hves and prevented 100 million illnesses (4). Insecticides have provided the means to control such important human diseases as filariasis transmitted by Culex mosquitoes and onchocerciasis transmitted by Simulium blackflies. 

Triorganotin compounds have also been used experimentally in controUed-release formulations to control the infective snail vector in the debiHtating tropical disease schistosomiasis (biHiarzia) and to control mosquitoes in stagnant ponds (103). As yet, the large-scale use of such methods has Htfle support in the host third world countries where these problems are most severe. Tributyltin chloride has been used to confer rodent-repeUent properties on wine and cable coatings (104). 

Malaria. Malaria infection occurs in over 30% of the world s population and almost exclusively in developing countries. Approximately 150 X 10 cases occur each year, with one million deaths occurring in African children (87). The majority of the disease in humans is caused by four different species of the malarial parasite. Vaccine development is problematic for several reasons. First, the parasites have a complex life cycle. They are spread by insect vectors and go through different stages and forms (intercellular and extracellular sexual and asexual) as they grow in the blood and tissues (primarily fiver) of their human hosts. In addition, malaria is difficult to grow in large quantities outside the natural host (88). Despite these difficulties, vaccine development has been pursued for many years. An overview of the state of the art is available (89). 

Vector-born infectious disease caused by Trypanosoma cruzi. 

Vector-born infectious disease caused by Trypanosoma btucei gambiense and Trypanosoma bracei rhode-siense. 

African trypanosomiasis (sleeping sickness) and American trypanosomiasis (Chagas disease) are caused by Trypanosoma brucei and Trypanosoma cruzi, respectively. Sleeping sickness results from being bitten by the insect vector, the tsetse fly. At first there is only local lymphadenitis but about a month later generalized malaise, fever, and systemic disease involving skeletal muscle is seen. 

The water-related or water-associated infectious diseases are typically arranged in four classes from the environmental engineering point of view, although more complex categorizations have also been proposed [14]. These categories are faecal-oral water-borne diseases, water-washed diseases, water-based diseases and diseases transmitted by water-associated insect vectors. Each type has different causes and potential solutions. Too often the term water-borne disease is erroneously used to name all of them without distinction. 

Hunter PR (2003) Climate change and waterborne and vector-home diseases. J Appl Microbiol 94 37S 6S... 

Selective toxicity is also important in relation to the development of resistance or tolerance to pollutants from two distinct points of view. On the one hand, there is interest among scientists concerned with crop protection and disease control in mechanisms by which crop pests, vectors of disease, plant pathogens, and weeds develop resistance to pesticides. Understanding the mechanism should point to ways of overcoming resistance, for example, other compounds not affected by resistance mechanisms or synergists to inhibit enzymes that provide a resistance mechanism. On the other hand, the development of resistance can be a useful indication of the environmental impact of pollutants. 

The cyclodiene insecticides aldrin, dieldrin, endrin, heptachlor, endosulfan, and others were introduced in the early 1950s. They were used to control a variety of pests, parasites, and, in developing countries, certain vectors of disease such as the tsetse fly. However, some of them (e.g., dieldrin) combined high toxicity to vertebrates with marked persistence and were soon found to have serious side effects in the field, notably in Western European countries where they were extensively used. During the 1960s, severe restrictions were placed on cyclodienes so that few uses remained by the 1980s. 

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