8 



SCIENCE 



[N. S. Vol. XXXVI. No. 914 



example of the latter type of infection is 

 the African tick fever, which is closely 

 identified with the ordinary relapsing 

 fever. The active carrier in this case is 

 the tick, Ornithodorus mouhata, which not 

 only can transmit the disease directly, bnt 

 can also do so through its otiEspring. A 

 spirochetal disease of chicken is similarly 

 transmitted by another tick, Argas mini- 

 atus. 



Our knowledge regarding the change 

 which the spirochete undergoes in the tick 

 and in the egg is by no means complete. 

 Thus, while some workers, such as Koch, 

 described the presence of the spiral organ- 

 ism in the internal organs and in the eggs, 

 other observers have failed to demonstrate 

 their presence. Leishman, a most careful 

 worker, was, as a rule, unable to detect 

 recognizable spirochetes later than the 

 tenth day after ingestion by the ticks. 

 Instead he observed clumps of chromatin 

 granules which were also invariably pres- 

 ent in the eggs, larvae and nymphs derived 

 from infected ticks. The spirochete origin 

 of these granules is uncertain, especially 

 since similar granules were found in 

 nymphs derived from ticks fed on normal 

 blood. 



The infection of a healthy animal by a 

 tick may occur, either by the injection of 

 spirochetes with the salivary secretion, or 

 by regurgitation of infective material from 

 the gut, but neither of these modes of infec- 

 tion can be considered as common. In- 

 stead, it appears from the work of Leish- 

 man and of Hindle, that infection is the 

 result of excretion of infective material 

 from the Malpighian tubiales and gut, 

 which enters the open wound caused by the 

 tick's bite. This contaminative wound in- 

 fection is therefore similar to that already 

 noted in connection with the flea and bu- 

 bonic plague. 



The Texas fever of cattle, as is well 

 known, was the first disease in which trans- 

 mission through the agency of an insect or 

 arthropod was demonstrated. The facts 

 presented by Smith and Kilborne twenty 

 years ago hold to-day. Thanks to that 

 work which served to open up the entire 

 field of invertebrate carriers, we know that 

 the disease is not transferred directly by 

 the tick which has fed upon an infected 

 animal, but indirectly through the young 

 ticks which hatch from its eggs. The 

 pathogenic agent in this case is a typical 

 intracellular protozoon, the Piroplasma. 

 Similar piroplasmatic diseases are met with 

 in a variety of domestic animals and, in all 

 such cases, transmission is effected by ticks 

 at one stage or another of their develop- 

 ment. 



In addition to the foregoing types of 

 organisms which are transmitted by ticks, 

 passing mention may be made of the com- 

 mon trypanosome infection of cattle, al- 

 ready referred to, which in all probability 

 owes its presence to this group of ectopara- 

 sites. 



The recognition of mosquito-borne dis- 

 eases marks one of the most important ad- 

 vances in modern times. Thanks to the 

 work of Eoss, Grassi and many others, we 

 learned that malaria was transmitted solely 

 through the bite of the mosquito. The life 

 history of the parasite in the Anopheles has 

 been traced, most completely, from the 

 moment when it enters the stomach with 

 the ingested blood, until it leaves the insect 

 by way of the salivary secretion. 



Barely a decade has passed since yellow 

 fever was shown by Reed and his co-work- 

 ers to be similarly transmitted, though by 

 another genus of mosquito, the Stegomyia. 

 The cause of the disease escaped their 

 search, and for that matter is still un- 

 known, but the practical results which fol- 

 lowed from their work culminated in the 



