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It is obvious, however, that in order to grasp the mechanism of infection fully, one must distinguish between insects which are able to make a wound and those which are not. The latter may be left out of consideration for the present. The mechanism of infection by non-suctorial insects will be discussed under a separate heading. Such insects should they harbour pathogenic bacteria, can only become dangerous in an indirect way.
(a.) Suctorial Insects.-Numerous species of insects suck the blood of man and animals. Each country has its collection of such animals and it is quite out of the province of the present manuscript to deal with the actual species of insecte biting man. Mosquitoes, fleas, bugs, gnats, ants, etc. are found all over the world, and if suctorial insects are the means of spreading infectious diseases directly by their bite, it becomes a difficult problem to sift out those which are most culpable. Mosquitoes may be dismissed in a sentence. I have examined numerous mosquitoes, caught in the Kennedy Town Plague Hospital, during an epidemic of plague and at a time when the wards were practically full of cases of plague, but in every instance I have failed to find the B. pestis. Many of the mosquitoes were caught under the nets of beds containing plague patients, yet, although these had sucked a considerable quantity of blood from the patient, no plague bacilli could be found. Emulsions of these mosquitoes were injected into rats with a negative result. It will be well to mention here, that even although plague bacilli had been found in their stomachs, it is difficult to explain how they could convey the infection to another individual in the absence of some special mechanism. Allow- ing the faces of the mosquito to contain plague bacilli, then, should the insect defacate on the individual's skin during the act of sucking, the subsequent scratch- ing by the individual might possibly inoculate the puncture wound. Such an infection, however, must be regarded as distinctly secondary, through a wound in the skin and not directly due to the action of the insect itself. Its occurrence must be rare. A puncture wound, such as made by a suctorial insect, becomes closed almost immediately after its infliction owing to the reactive changes which occur at once around the wound.
My absence to find plague bacilli in mosquitoes is in accordance with the views already expressed by the members of the Austrian and German Plague Commis- sions. Again if mosquitoes played an important role in the direct dissemination of plague infection, doctors, nurses, and attendants in our Plague Hospital, where such insects abound, would have little chance of escaping infection.
Fleas.-These insects have been the objects of much investigation and particularly in regard to role played by them in the spread of plague. As a result of this, the genus has been thoroughly worked out and the various individual species tested for their plague carrying powers. It is asserted by BATTLEHUER that at least 60 to 80 different species of fleas exist and that each species is restricted to a definite animal. That is to say we have rat fleas, dog fleas, etc. These fleas are not supposed to bite man. According to GALLI VALERIO (Cent. f. Bakt. Bd. 27, 1900) the human flea is different morphologically from animal fleas, especially the rat flea.
The human flea, however, is cosmopolitan. It prefers the blood of man, but in the absence of that, it can accomodate itself and feed upon the blood of other animals. Again, it must be remembered that there undoubtedly exists a predis- position on the part of certain individuals to fleas. Certain human beings are almost immune to fleas, e.g., fleas do not bite them; other persons are decidedly susceptible, and are attacked and bitten whenever an opportunity presents itself. Probably the same obtains in regard to different animal species.
During the past few years, in fact ever since the rat theory of the spread of plague became prominent, many experiments have been undertaken in order to determine whether plague infection in the rat is conveyed to man by way of rat fleas. The experimental evidence which has been obtained is so far of a most unsatisfactory nature. SIMOND (Anual. Pasteur, Bd. 12, 1898), who has given a great deal of attention to this subject, came to the conclusion that Indian rat fleas bite man and that rat fleas spread plague from one rat to another and also to the human species.
At this point it will be well to remember that the dead bodies of rats while still warm, are infested with fleas, but when the body becomes cold and stiff, these insects migrate. So that so far as fleas are concerned, the dead bodies of rats