62 



REPORT 108, U. S. DEPARTMENT OF AGRICULTURE. 



Fig. 115.— Shield of Dermacentor 

 variabilis, female. (Original.) 



infestation from birth become immune to the disease, but if not raised in tick-infested 

 fields they are as susceptible to the disease as northern cattle. 



Texas fever occurs in most parts of the Avorld. and Marc/aropus is the disseminator. 

 Coincident with the discoveries connecting mosquitoes and other insects with disease 

 came similar discoveries in regard to ticks. These have come so fast and have been 



so novel and astonishing that ticks have become 

 one of the most prominent economic groups. 



Many of the statements are but suspicions; other 

 discoveries lack confirmation; but several promi- 

 nent diseases have been definitely connected with 

 certain ticks. In South Africa, Lounsbury has 

 shown that heartwater is transmitted by the "bont 

 tick," Amhlyomma hebraev.m Sa^dgny. Later he 

 has proved that malignant jaundice in dogs is due 

 to the attacks of Haemaphysalis leachi Koch, and that 

 African coast fever in cattle is carried by five dif- 

 ferent species of Rhipicephalus. The "moubata 

 bug," Omithodoros mouhata Murray, is the inocu- 

 lating agent of one of the most dangerous diseases 

 of tropical Africa, known as human tick fever, or 

 African relapsing feA^er. The discovery of the tick's 

 relation to the disease is due to Drs. J. E. Dutton 

 and J. L. Todd. 



In our country Dr. H. T. Ricketts connected 

 the Rocky Mountain spotted fever with Dermacentor venustus Banks. The Argas 

 winiatus Koch, has been shown to transmit spirochsetosis in fowls. 



Many other diseases are accredited to the bites of ticks. Thus louping-ill in sheep 

 appears to be carried by an Ixodes; carceag, a European disease of sheep, is supposedly 

 transmitted by Rhipicephalus bursa Neumann; a disease of turtles is laid to Hyalomma 

 aegyptium Koch; and this same Hyalomma is considered by Laveran to transmit a 

 blood parasite of the python snakes. In Russia Dermacentor reticiilatus Linn, carries 

 a piroplasmosis of horses. An undetermined Ceylonese tick is the supposed vector 

 of paranghi or "yaws." The tick is a most necessary host in the life history' of these 

 parasites, for in some cases (perhaps in all) the sexual conjugation of the parasite is 

 consummated within the body of the tick. 



From the known results, it is evident that the power to transmit disease-causing 

 organisms is not confined to any one genus or section of Ixodoidea,^ but common to all. 

 Moreover, in different countries extremely similar dis- 

 eases are carried by very different ticks. Therefore 

 the diseases have not originated in the ticks. Most, 

 if not all, of the species now acting as vectors of 

 disease to certain hosts were probably originally con- 

 fined to other hosts. To their original or natural 

 ho.st they brought no disease Certain low organisms 

 living in the blood of the host were transmitted by the ticks to other animals of the 

 same species with little or no serious danger. Rut when a tick containing the blood 

 parasites of one of its natural hosts becomes attached to a new and different kind of 

 host, then the blood parasite in this alien blood may originate a disease. The occasional 

 transference of a tick from one host to another may not be sufficient; but when a spetnes 

 of tick is compelled by the march of ci\ ilization and consequent extermination of 

 native animals to adapt itself to another host, a disease may result, ]irovided, of course, 

 that the ticks were commonly infected with a blood parasite of their old host. 



Fig. 116.— Tarsus IV of Drrma- 

 ccntor. (Original. 1 



1 It has been shown experimentally that the Rocky Mountain spotted fever may be 

 transmitted by several species of ticks of oven different genera. 



