ECONOMIC PROBLEMS 79 



America (hares, especially), the Volga region (water 

 voles), Norway (hares), and other countries, but has 

 not yet turned up in England. Plague kills most of 

 those it attacks, and in the thirteenth century it 

 killed more than half the population of Europe. 

 Tularaemia (caused by a small bacterium) usually only 

 kills less than one in twenty. Kats are now known 

 to be the reservoirs also of spirochaetal jaundice, rat- 

 bite fever, and certain forms of tropical typhus. 

 Japanese river fever is a virus carried by a harvest 

 mite {Trombicula) which has as chief host a vole, 

 but also occurs on several other animals and on birds. 

 Then Rocky Mountain spotted fever kills people by 

 the bite of a tick that harbours the virus, the tick 

 itself being dependent on wild rodents, etc., for its 

 maintenance and bloodmeals (Parker, 1933). The 

 great problem of Africa is skeping sickness whose 

 trypanosome is carried by Tabanid flies [Glossina) 

 that bite man. The trypanosome can live also in 

 wild game animals, while the fly can obtain blood 

 from many wild animals such as game animals, 

 also hippopotami, crocodiles, etc. Many parasites 

 causing disease in man have only an insect as alter- 

 nate host, e.g. malaria protozoa and filaria round- 

 worms and yellow fever virus in mosquitoes, and 

 kala azar protozoa and the sandfly fever virus in 

 sandflies [Phlebotomus) . Schistosomiasis is caused by 

 a flat-worm whose alternate host is a snail, while the 

 giiihea worm is harboured in the larval stage by a 

 Copepod, as also is the tapeworyn, Diphyllobothrium 

 latum. For a summary of part of this subject see 

 Hull (1930). 



Vi will be noticed that the alternate hosts are 

 s<in(' limes actualty carriers of the disease organism, 

 and sometimes only hosts of the insect or acarine 

 vector. In all cases the numbers of each member of 

 the complex: are important in maintaining a complete 

 qjqXq in winch disease is possible. When we turn 

 to domestic animals we find a similar story. Almost 



