Diseases Acquired From Animals - Mosquitoes Carrying Tropical Diseases
( Originally Published 1936 )
Mosquitoes are probably the most dangerous insects on earth. One or another species carries malaria, yellow fever and tropical diseases such as dengue fever, filariasis, and possibly African sleeping sickness. There are some 60 species in the United States, but only three of them are known to carry malaria and one yellow fever.
The malarial mosquito occurs all over the United States in numbers sufficient to infect thousands of people. This statement is vouched for by entomologists, but the malarial mosquitoes must be quite scarce in the northern parts: In a large hospital service in the middle west I saw last year only one doubtful case of malaria. In pioneer days malaria occurred over very much larger areas of the United States than obtains today. The settlers in the Ohio valley and in Illinois, Indiana and Missouri, were infected by the hundreds. These areas are almost totally free of the disease today. This is due probably to the clearing away of forests, underbrush, and the consequent draining of pools where the mosquitoes breed, and the screening of houses. This, however, does not entirely account for the disappearance of malaria in the Missouri and Ohio valleys, because malaria is still found in the lower Mississippi valley and in Florida, where screening and other precautions are used. Possibly the presence of salt or brackish water accounts for this, as mosquitoes breed particularly well along ocean marshes.
The life history of the mosquito points the way to methods of control of mosquito-borne diseases. During the winter, adult mosquitoes hide away in cellars, barns and outhouses and remain there in a dormant condition. This applies more to the nonmalarialmosquitoes.
The females that survive the winter become active in the early spring and deposit their eggs in a pool of water, in roadside ditches, hollow stumps, rain barrels, cisterns, tin cans, water troughs, etc.
The egg masses look like small pieces of soot floating on the water. Depending on the weather they hatch within 24 hours to several days, in the form of larvae or wrigglers. The third stage is called the pupae, which develops into the adult mosquito. The entire period of development, from the egg to the adult of the mosquito, is about 16 days.
The larvae and pupae, while they lead an aquatic existence, have to breathe air, and come to the surface to obtain it. This is the reason for the effectiveness of spraying the surface of pools with kerosene oil in destroying them. The oil floats on the surface of the pool and prevents the larvae and pupae from forcing their breathing tubes above the surface. Since the intermediary stage of mosquito life lasts 16 days, it follows that pools and possible breeding places should be sprayed every 16 days in order to be perfectly safe.
In the Belgian Congo, I understand, little attempt is made to spray pools, but cementing up hollow stumps has reduced the incidence of malaria about 50 per cent. Malarial mosquitoes in the United States seem to prefer ground water, and usually moving ground water, as breeding places.