TICKS AND THE ROLE THEY PLAY IN THE 

 TRANSMISSION OF DISEASES 



By F. C. Bisiiopp 

 Principal Entomolofiist in Charge, Division of Insects Affecting Man and 

 Aniwals, Bureau of Entomolofiy and Plant Quarantine, United States Depart- 

 ment of Agriculture 



[With 9 plates] 

 INTRODUCTION 



Ticks rank among man's worst enemies. Some of the most deadly 

 diseases of the human race and of domestic animals are borne by 

 these creatures. Furthermore, all ticks are parasitic and weaken 

 their hosts by sucking their blood and also irritate them by their 

 bites. 



While generally regarded as insects, ticks are not true insects, but 

 are more closely akin to the mites and spiders. They constitute a 

 separate superfamily, known as the Ixodoidea, almost world-wide 

 in distribution. In this country there are about 40 species, nearly 

 half of which are known to be of more or less economic importance. 

 The diseases now known to be carried largely or wholly by ticks 

 make a formidable array. Four different types of disease organisms 

 are known to be conveyed by ticks, namely spirochaetes, piroplasms, 

 rickettsias, and bacteria. They also carry certain parasitic worms 

 and virus diseases such as heartwater of sheep, cattle, and goats and 

 Nairobi sheep disease, and other maladies of man and animals the 

 causative agencies of which are unknown, such as tick paralysis and 

 certain tick fevers. Apparently ticks are of relatively little im- 

 portance in the transmission of bacterial diseases, although in tula- 

 remia they appear to play a rather important part. The Protozoa of 

 the family Babesiidae are ordinarily transmitted solely by ticks. 

 Fortunately the diseases due to this group of organisms do 

 not appear to attack man, though many species of mammals are 

 affected by them. Among the rickettsia diseases Rocky Mountain 

 spotted fever of man is carried exclusively by ticks, and apparently 

 this is true of Rickettsia i^rriinantium of cattle in South Africa. A 

 number of spirochaetoses of fowls, mammals, and men are tick-borne. 

 Tick bites are annoying to most people, and certain individuals are 

 especially sensitive to them. Furthermore, they often serve as points 



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