Bentley has recently noted that, with improvement in agricultural methods and utilization of marshy lands, malaria tends to disappear as much from the physical improvement and thereby greater resistance of the people as from the destruction of mosquitoes by the draining of the swamps. The resulting greater prosperity makes better food and shelter obtainable.
1. Destruction of Mosquitoes.
Such measures may be directed either toward the larva or fully developed insect.
(a) Measures against larvae. When practicable permanent measures should be preferred to temporary ones and when agricultural development goes along with drainage of swamps the cost is repaid.
The doing away with mosquito breeding places may be accomplished by filling in pools or by making ditches with smooth sloping sides to carry away the water. These ditches require a great deal of attention to prevent their filling up with tropical vegetation and thereby adding to breeding places. Subsoil drainage with tiled drains is better. Care should be exercised that public works operations do not raise the level of the subsoil water.
Anophelines tend to breed in sluggishly moving streams or in stagnant pools especially where there is a luxuriant growth of weeds or grass, and are not apt to be found in rapidly flowing streams, hence the necessity for constant care of ditches and the like to prevent their becoming obstructed by vegetation or silt. When filling in or drainage is not practicable the method of oiling the surface of the pool with crude petroleum is to be recommended. One uses about ½ pint for every 100 square feet of surface and the process should be repeated every two weeks.
In places where oil is not effective, Barber recommends Paris green mixed with dust and so used as to form a scant surface deposit. Anopheline larvae, being surface feeders, ingest it and are killed. It does not affect Culex larvae. On account of its ease of transportation, and adaptability to weedy places where oil does not penetrate, Paris green dust will doubtless prove a valuable selective larvicide. Mayne and Jackson recommend cresol as the best larvicide. In 1 to 1,000,000 parts it is an effective larvicide, and even in 1 to 1,000,000,000 it is destructive to young larvae.
Mixtures of soft soap and petroleum are better than petroleum alone.
Winds are apt to blow away the surface coating of oil and it is difficult to oil the surface of a pool filled with grass. Wise recommends crude carbolic acid, using 1 ounce to 16 cubic feet of water.
In using any larvicide it is well to introduce it along the banks of water collections with a long-spout can and mix it thoroughly with a stiff reed broom.