Page:Tropical Diseases.djvu/275

XII] human or other warm-blooded victim on which the infected insect may subsequently feed. The irritation caused by the bite of the insect provokes scratching and consequent inoculation of the deposited spirochætes.* The parasite is not introduced by the mouth parts of the insect.

In the case of O. moubata the hereditarily infected nymph can communicate the disease (Nuttall). The tick itself has been shown to be infective up to a year and a half after the initial feed on spirochæte containing blood.

It is undoubtedly the case that in one part of the world the spirochæte of man is transmitted by one species of blood-sucker, and in another part of the world by another species of blood-sucker; that the various relapsing fever spirochætes exhibit slight morphological differences ; that their serum reactions differ; and that the fevers they cause are in some respects different clinically. It is not justifiable, how ever, to conclude on these grounds alone that the various strains of pathogenic spirochætes are specifically distinct. It has been ascertained that as regards some of the strains their usual transmitters are interchangeable. It may well be that the continued passage of a given strain through many generations of the same species of insect transmitter may confer on the spirochæte concerned special morphologic and pathogenic qualities, without at the same time transmuting it specifically. Indeed, it would seem, that opportunity as supplied by the presence of an efficient insect transmitter, the habits and circumstances of the inhabitants, and the introduction of a pathogenic spirochæte are the principal circumstances that determine the endemicity or epidemic prevalence of relapsing fever; and that the insect transmitter has much to do with the pathogenic qualities of the organism that has been cultivated in its alimentary canal for countless generations.