possibility would have been realized already if it were likely to occur, for cases of this common European fever must undoubtedly have been imported.
We have already referred to ticks as carriers of spotted fever in this country. Another important disease, or group of closely similar diseases, known as relapsing fever, is known to be tick-borne. This malady is due to a very small spirochete, a protozoan organism known as Spirochæta recurrentis. In acute human cases of this disease these are present in the peripheral blood from whence they may be withdrawn by ticks. Within the alimentary tract of at least some species of ticks the parasites undergo a sort of development which is not well understood, entering the Malpighian tubules or other parts of the body and later assuming a somewhat different spirochete form. Infection of another person may then occur from a subsequent bite by the infected tick, the virus not passing into the body from the salivary glands or mouth, but entering the wound after having been excreted by the attached tick. As occurs with the tsetse-fly carriers of sleeping sickness, only a part, in this case about one third, of the ticks feeding upon a person with relapsing fever become infected themselves. Of those which do, however, some may transmit the infection to their offspring, which are then capable of infecting man with the virus thus received. Relapsing fever is very widely distributed, mainly in warm countries, although in Europe it has occurred in epidemic form as far north as St. Petersburg. The etiology and method of dissemination of the African type appear to be best known. A common tick, Ornithodoros moubata, was first found to act as vector and was until very recently believed to be the only carrier. Now it has been demonstrated that other ticks may act in the same way, and there is a possibility that other carriers may exist, probably in the form of blood-sucking insects.
Relapsing fever has been occasionally reported from the United States, but has never become established.
Among the less important insect-borne diseases is a very widespread tropical fever known as dengue which occasionally spreads into temperate regions in epidemic form. This is due to the presence in the red blood corpuscles of a protozoan smaller than the malarial parasite, probably a spirochete of some sort. However this may be, we know from experimental tests that dengue may be spread through the agency of certain mosquitoes. The widespread Culex fatigans is capable of transmitting the infection and there is good evidence to incriminate the yellow fever mosquito, Stegomyia calopus. Dengue is quite common in the southern United States, where in the minds of many people it is confused with malaria. It is less serious, however, although an even more unpleasant ailment to endure.
The same Culex fatigans has been shown to be at least partly respon-