Human health effects from DDT at low environmental doses are unknown. Following exposure to high doses, human symptoms can include vomiting, tremors or shakiness, and seizures. Laboratory animal studies showed effects on the liver and reproduction. DDT is considered a possible human carcinogen.
DDT-resistant mosquitoes were first detected in India in 1959, and they have increased so rapidly that when a local spray program is begun now, most mosquitoes become resistant in a matter of months rather than years.
DDT is classified as "moderately toxic" by the US National Toxicology Program (NTP) and "moderately hazardous" by WHO, based on the rat oral LD 50 of 113 mg/kg. Indirect exposure is considered relatively non-toxic for humans.
Mass spraying of DDT led mosquitoes to develop resistance. Nor has there been much incentive for companies to develop new mosquito-killing insecticides, which could be used in tandem with existing ones to slow the development of resistance
By using the protein determination techniques such as X-ray crystallography, scientists observe that the protein coded by the gene described exactly how the mutation vested resistance, by opening up the 'active site' where DDT molecules bind to the protein, so more can be broken down.
This signifies that the mosquito can survive by breaking down the toxin into non-toxic substances.
Mosquitoes usually become resistant to pyrethroids through the mutation of a sodium channel gene that controls the movement of ions across cell membranes.
Mutations in a single gene are enough to make mosquitoes almost completely resistant to the level of pyrethroids used in insecticides.
Might be this is how mosquitoes have come to develop this resistance.
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