876 



THE ANIMAL CARRIERS OF DISEASES 



Therefore, in order that man may be the definitive host and the 

 blood-sucker a pathologically affected intermediate host means a 

 comparatively recent evolutionary change. Therefore an old 

 original life-cycle may still be discoverable, if sought for, and 

 may be of value in prophylaxis, which is the end and aim of the 

 study of the animal carrier. 



With these preliminary remarks we may now turn to the history 

 of the subject. 



Historical. — The history of the animal carrier of disease may be divided into 

 three periods by the dates 1878 and 1898 — i.e., by Manson's and by Ross's 

 discoveries — and these periods will be :— 



I. Early views. 

 II. Manson's period. 

 III. Ross's period. 



I. Early Views. — This period is characterized by gropings in search of 

 truth, mainly by suggestions and by theories, but also by the first experi- 

 ments with bacteria. 



In 1577 Mercurialis suggested that plague might be spread by house-flies, 

 and in 1666 Sydenham opined that the autumnal diseases of England were 

 due to the flies of summer. In 1769 Bancroft advanced the theory that 

 Frambcesia tropica was a fly-borne disease, and in 1808 Crawford believed 

 insects to be carriers of infection. In 1848 ISTott of Alabama brought forward 

 reasons to support the insect origin of yellow j ever. In 1853 Moore referred 

 to flies as possible carriers of cholera, typhoid, tuberculosis, anthrax, and leprosy, 

 to the last named of which Linnaeus had already invited attention, while 

 Raimbert in 1869 performed the first actual experiments to try to prove that 

 flies carried anthrax. 



In 1853 Beauperthuy argued that mosquitoes spread yellow fever. 



II. Manson's Period. — This opens with the publication of Manson's epoch- 

 making discoveries of the carriage of Filaria bancrofti by mosquitoes, and it 

 must never be forgotten that he grasped at once that this was a new infection 

 of the mosquito, and that water was the original method; and thus he dis- 

 covered two great truths, of which so far only one has been properly appreci- 

 ated, because the insect-carriage laid the foundation of Ross's great work. 



In 1 881 Findlay definitely accused mosquitoes as being the transmitters of 

 yellow fever, and conducted experiments in which he is considered to have 

 been successful in transmitting the disease experimentally by their bites. 



In 1883 King formulated the theory that malaria was spread by mosquitoes, 

 and in the same year Thomas demonstrated the carriage of Fasciola hepatica 

 by snails, while Grassi and Stiles showed that parasitic worms were carried 

 by arthropods. 



In 1895 Bruce discovered that Trypanosoma brucei was spread by a tsetse- 

 fly, a fact which led eventually to clearing up of the carriers of trypanosome 

 diseases. 



During the closing years of this period Ross in India was hard at work 

 dissecting mosquitoes in the face of much difficulty, thus opening the way for 

 the last period of our history. 



III. Ross's Period. — Just twenty years ago (1898) the new stage of research 

 into the animal carrier in disease was opened by Ross's most important, 

 careful, and laborious work into the causation of malaria, and the carriage 

 of its parasites by the anopheline mosquitoes. 



This discovery was followed in 1899 by the classical paper by Nuttall, on 

 ' Insects as Carriers of Disease,' while in 1900 Reed, Carroll, and Agramonte 

 showed that Stegomyia {esdes) calopus was the carrier of yellow fever. 



The resv of this period has been referred to in the opening chapter of this 

 book, and need not be recapitulated, and we will now pass on to study the 

 results of this work, by considering the parasites in the zoological order 

 followed in the preceding chapters. 



