PROC. ENT. SOC. WASH., VOL. 20. NO. .i, MAY, 1918 95 



Quite a number of the disease complexes associated in this 

 group contain an unknown factor. The vertebrate hosts are 

 known and the insects known, but the organism or disease prin- 

 ciple is unknown. Here belong the filterable viruses of Rocky 

 Mountain spotted fever, typhus fever, papatasi fever, dengue, 

 yellow fever, etc. Various claims have been made for discoveries 

 of the causative factors in these diseases but none have so far been 

 generally accepted. 



Types of Transmission. 



Among the organisms which are known to pass through insect 

 hosts we find many types of transmission. 



1. There are those organisms which pass a certain phase of their 

 life cycle in the insect and are reinoculated into other hosts by 

 means of the insect's proboscis, as the malaria and plague parasites. 



2. Others carry out their life cycle in the invertebrate, passing 

 into its egg and are transmitted by the bite of the second genera- 

 tion, as the organism of Texas cattle fever, Babesia bovis (Piro- 

 plasma bigeminuni) in the cattle tick, Boophilus annulatus. 



3. Another group finish their development in the malpighian 

 tubules and are voided with malpighian secretions from the anus 

 while the invertebrate host is feeding, and by means of coxal or 

 other secretions are washed into the wound. Such is the method 

 of transmission of Spirochaudinnia duttoni (Spirochaeta) , the cause 

 of West African relapsing fever, which is transmitted to man by 

 the tick Ornithodoros megnini. 



4. Still another mode of transmission is that in which the or- 

 ganism passes through the insect and is voided in its excrement and 

 is then scratched into the host, as just proven possible in the case 

 of trench fever carried by lice to men, and also proven the method 

 of entrance of Spiroschaudinnia berbera, the cause of North African 

 relapsing fever, another louse borne disease of man. In both of 

 these cases the crushing of the louse over an abrasion will give rise to 

 the disease. At this point it is interestinatto note that Futaki found 

 Spiroschaudinnia exanthematotyphi, which he believes to be the 

 cause of typhus fever, in lice. Various authors have called atten- 

 tion to certain similarities between typhus fever and trench fever. 



5. Another interesting mode of transmission is found in Ilaemo- 

 grcgarina muris, cause of mouse anemia, which passes part of its life 

 cycle in the mite, Laelaps (rhidiiinus. The mite is finally eaten by 

 the mouse and thus the parasite enters the vertebrate host. 



6. In all of the cases quoted above the parasite passed from 

 vertebrate to invertebrate through the sucking of blood by the 

 latter. There are also many disease organisms which leave the 

 vertebrate in its feces, and are taken up by an insect feeding on 



