THE SIMPLEST ANIMALS 315 



Those who have read the " Travels " of Livingstone and other 

 African explorers will recall how their journeys were so often 

 delayed by death of their oxen. The disease also caused much 

 trouble among horses during the South African war a few years 

 ago. The fly does not develop the parasite, as mosquitoes 

 do in the case of the malarial organism, but simply transmits 

 the parasites from diseased to healthy animals. It is believed 

 that the blood of some large game animals may infect the flies. 



The surra disease, which destroys large numbers of horses, 

 camels, and cattle in India and the Philippines, is due to a 

 similar parasite transmitted by bites of flies. 



276. Other Protozoan Diseases. Dangerous diseases, 

 caused by protozoan parasites which are introduced into the 

 blood by the bites of ticks ( 319), affect cattle, dogs, horses, 

 and sheep. The parasites enter the red blood-cells. Texas 

 fever is one form of the cattle disease, and it has caused 

 enormous financial losses to stockmen in the United States 

 and elsewhere. 



A common disease of the liver in rabbits, diseases which 

 seriously affect fish, the silkworm disease made famous by 

 Pasteur's studies, these are some of the more important 

 cases in which parasitic one-celled animals have great eco- 

 nomic importance. 



There are some suggestions that yellow fever, scarlet fever, 

 smallpox, and hydrophobia are caused by protozoan parasites 

 similar to those which cause malaria ; but there is yet no con- 

 vincing proof. In the case of yellow fever, it has been proved 

 that a mosquito (not the species concerned with malaria) 

 transmits the unknown germ of the disease ; and so at pres- 

 ent the only known way of checking the spread of yellow 

 fever is to check the mosquitoes. This has been very suc- 

 cessful in Cuba and in Panama. 



Fortunately, most of the protozoans which live in water 

 in lakes, rivers, and sea are unable to live in the bodies of 

 animals, and hence are entirely harmless. 



