The ability of any insect transmitted infection to gain a foot hold and maintain itself requires at least two conditions: (r) That the definitive insect host exist in a certain degree of abundance, and (2) That intermediate mammalian hosts harboring the transmitting stage of the' parasite be accessable to the insects. Thus the eradication of malaria can be accom plished without absolutely exterminating all anophelines in an area, by reducing their numbers to such a point that very few if any anophelines will have an opportunity to bite a gamete carrier.
2. Mechanical Insect Transmission.—Transfer of infective agents by this means may either be through the assistance of biting or sucking insects. The diseases in which the mech anism is more or less clearly understood are all due to bacterial infective agents. The relationship of the insect and parasite is not specific, that is the parasite can be transferred from man to man by other agencies and frequently those other agencies are of greater importance in the propagation of the different diseases than the insects. The presence of infective agents in or on the insect is more contaminative in character, the insect relationship does not play any essential part in the life cycle of the parasite and commonly we find that the insect can imme diately transfer infection after having had an infective meal. In other words an extrinsic incubation period is not observed. Temperature and humidity exercise a similar influence upon the development of the insect vectors of this group, and their influence is thus exerted and manifested in the seasonal and climatic distribution of these diseases, or else influence the bacterium within or upon the' nsect. The diseases whose infective agents may be transmitted by this means are plague, cholera, typhoid, and both bacillary and amoebic dysentery. Fleas are concerned in the transmission of plague, and flies, principally the house fly, in the transmission of the others. In the case of the former, the plague bacillus leaves the flea's body both by the probocis and the feces; in the case of the latter group, the flies' probocis, its excreta, regurgitated material and the exterior of its body surface may afford means of convey ance. In the former instance the infective agents are directly reintroduced by the insect into a new host, in the latter trans mission is effected through the contamination of food by the insect.
3. The Activity of the House Fly.—The house fly (Mitsca
donzestica) (Fig. 67) is an ubiquitous insect with a world wide distribution, living in close association with man. Its ova are preferably deposited in manure piles, from these hatch the familiar maggots (Fig. 68), which burrow through the pile and feed on the decaying organic matter. In about a week in the summer these leave the manure and burrow in the soil where they pupate. In a few days the adult fly emerges. The entire life cycle may be completed under favorable circum stances in from ro to 14 days. The dangers arising from this insect are due to its promiscuous feeding habits and its intimate association with man. It will walk over and feed upon the fecal deposits in a privy, the refuse in a garbage can, the pas tries in a bake shop and the food in the kitchen and pantry with equal impartiality and in turn, conveying infective agents secured from the excreta to food during its journey. There is absolutely no question but that in unsewered cities and towns or in rural areas where fly tight privies are not used, flies are a great menace and an actual source of danger. They are un doubtedly a considerable factor in the autumnal increase in typhoid. A very marked reduction in the incidence of typhoid fever was secured in Jacksonville, Florida following systematic efforts at fly elimination through prevention of breeding, the extension of the sewer system and the substitution of fly proof provies for open closets. Prior to 1910 there were about 8,500 privies in Jacksonville, which accommodated about 4o per cent. of the population. They were distributed uniformly over the entire town. None were fly proof in the slightest.
The typhoid cases annually reached a maximum in the fly season, while water and milk could be eliminated as a cause. In 1910 there were 329 reported cases with 62 deaths. Late in 1910 an ordinance was passed requiring the fly proofing of all privies, a requirement that was completely met before the season of 1911. During 1911 there were 158 reported cases and 4o deaths and in 1912 87 cases with 16 deaths. Of the 1911 cases 88 received infection outside of Jacksonville, i.e., were imported and of the 1912 cases 48 were imported. Later years continued to show the same results. This recalls the experience with fly borne typhoid in the southern army camps during the Spanish war, in which, however, the role of the flies was not recognized until serious losses occurred.