80 THE ECOLOGY OF ANIMALS 



every human disease can be paralleled. Thus tsetse 

 flies carry the trypanosome that causes nagana in 

 cattle and horses in Africa, and this occurs also in 

 game but does not harm them. Red-water fever is 

 caused by a protozoan that is carried by ticks, which 

 are also responsible in the tropics especially for the 

 carriage of many other diseases. Surra, which kills 

 camels by trypanosomiasis, is carried by flies. In 

 South America horse trypanosomiasis is associated 

 with periodic epidemics among capybaras — large 

 water rodents (Joan, 1930). In Scotland, louping 

 ill, a serious disease of sheep, is carried by a tick 

 which also occurs on many wild animals and birds. 

 The severe epidemics resembling encephalitis which 

 occur among Arctic sledge dogs in the Arctic regions 

 of Canada and Greenland are apparently associated 

 with and sometimes derived from similar disease in 

 wild Arctic foxes (Elton, 1931). Gapes, a round- 

 worm disease of turkeys and fowls, is also harboured 

 by starlings and rooks, and the latter frequently die 

 of the disease in nature (Elton and Buckland). An- 

 other roundworrn is carried to horses by the agency 

 of a horsefly. The liver-fluke of sheep is carried by a 

 freshwater snail. 



Turning to pests of agriculture the black list is 

 found to be very long. The pests are partly indi- 

 genous ones such as the cabbage white caterpillars 

 in England and partly introduced ones such as the 

 Colorado potato beetle in France. Locusts are one 

 of the biggest problems in the tropics and the desert 

 locust is said to have caused damage amounting to 

 £6,000,000 during the last outbreak. 



For a full account of agricultural pests the reader 

 is referred to Wardle (1929), who gives useful sum- 

 maries of the chief pests of each region of the world — 

 including also notice of forestry and disease -carrying 

 pests. The accounts given in this and other text- 

 books leave an impression of world-wide struggle 

 between growers of cotton, coffee, tea, fruits, vines, 



