CONTROL OF ANIMAL PARASITES 259 



Donimmune man with the same virus. The man took smallpox as usual ; 

 James did not. Crude methods at first, making inevitable mixed inocu- 

 lations with other germs, raised violent objection to vaccination, but 

 at that time the disease itself was so much more serious than any such 

 complications, that the practice spread rapidly over the world. Modern 

 bacteriological methods have made the virus safe, so that countries 

 like Germany, in which vaccination under two years, with revaccination 

 between ten and twelve, is compulsory, have reduced smallpox to the 

 vanishing point. In England, however, the old opposition has persisted, 

 and this has resulted in many serious local epidemics. The same is 

 true of our own country and Canada. A new difficulty has also arisen. 

 The disease has been so nearly exterminated that even the most con- 

 scientious people are saying : " Why vaccinate our children against a 

 disease to which they wil-l never be exposed?" This argument is suf- 

 ficiently answered by the many local epidemics of recent years. Study 

 carefully the history of at least one such epidemic. 1 



No less than eighteen other cities and towns in New York State, and 

 several more in other states, were infected with smallpox from Niagara 

 Falls in 1914, and Canada was obliged to quarantine against the city. 

 Js it right for one person, or one city, to endanger the safety of others 

 in this way? Look up the prevalence and mortality of smallpox, and 

 iftethods of "inoculating" from mild cases, before 1800, and compare 

 with present conditions. Study also the story of the introduction of 

 smallpox into America by the Spaniards. It is said to have killed 

 o,500,000 natives in Mexico. 



The trypanosomes (trypanon, "auger"; soma, "body"). This genus 

 ( ontains about sixty known species, which live as free-swimming para- 

 sites in the blood plasma of many vertebrates, from fishes to man. Their 

 ] irimary hosts are probably bloodsucking flies, which, at any rate, act as 

 carriers. Surra, a disease of cattle, horses, and camels in India and the 

 .Philippines, is caused by T. euansii; and nagana, or tsetse-fly disease, 

 which long made impossible the introduction of European cattle, horses, 

 and sheep into East Africa, is caused by a similar blood parasite, 

 1\ brucei. Nearer home a serious disease of horses, dourine, long known 

 in Europe, and more recently reported from western Canada, is caused 

 1>y T. equiperdum. This forms a notable exception among diseases of 

 ihis class in being spread exclusively by breeding, and has no known 

 -onnection with biting insects. 



1 Dr. L. R. Williams, ff Smallpox Epidemic at Niagara Falls,' 1 American 

 Journal of Public Health, Vol. V (1915), p. 423. 



