INSECTS AS AGENTS IN SPREAD OF DISEASE 547 



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 Spirochceta 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 are present in the peripheral blood from whence they may be withdrawn 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 under- 

 stood, 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 at- 

 tached 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. 

 Eelapsing 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, OrnWiodoros 

 mouhata, 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 tem- 

 perate 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- 



