Finally, let us consider the bionomics of Anopheles walkeri in relation to the transmission of human malaria. This species is found in parts of the United States where malaria exists in endemic form, such as the southeastern states and the lower Mississippi valley. Perhaps of equal significance is its presence in northern regions from which the tides of malaria have ebbed, but where the brief, devastating epidemic is still a menace.
Anopheles walkeri exhibits a fastidious discrimination with regard to breeding places, but multiplies bountifully where favorable ones exist. The adults disperse for at least two miles in quest of food. They bite fiercely, even in bright daylight if near their moist resting places, and feed on man and domestic animals with equal avidity where both are available. The species appears to be considerably more phototropic than A. quadrimaculatus and its entrance into houses may be facilitated by the introduction of modern electric lighting into rural areas. Lastly, Anopheles walkeri has been shown to be a host of at least two species of human malarial parasite.
On the other hand, it frequently bites the more available domestic animals and does not enter dwellings as readily as A. quadrimaculatus. From the observations on the physiology, one sees that it is a delicate species and cannot long survive the conditions imposed upon most mosquitoes who choose to enter houses—and attempt to leave. Also to our knowledge, malaria is not yet known to exist where this species is the only anopheline. In areas of endemic malaria in this country where both A. walkeri and A. quadrimaculatus are found, the latter is apparently the important vector.
We have seen then that the effectiveness of Anopheles walkeri as a potential vector of human malaria depends on the maintenance of an exceptionally favorable environment and ample opportunity for contact with humans. These conditions are fulfilled at Reelfoot Lake. Where they have developed in regions formerly free from malaria, the introduction of the parasite by present shifting populations may reveal Anopheles walkeri as a vector of importance.
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