18 PARASITES IN GENERAL 



sites of the blood or lynipliatic systems usually rely on biting 

 arthropods (insects, ticks and mites) to transmit them from host 

 to host, and it is in this capacity, i.e., as transmitters and inter- 

 mediate hosts of blood parasites, that parasitic arthropods are 

 of such vast importance (see p. 322). 



Geographic Distribution. — - The geographic distribution and 

 dispersal of parasites is another subject which has received 

 much fruitful attention in recent years. Parasites, like other 

 organisms, are dependent upon certain physical conditions of 

 their environment in order to thrive. One of the most important 

 limitations on the dispersal of a parasitic disease is the distri- 

 bution of suitable hosts. Some parasites can live with ap- 

 parently equal vigor in a large number of hosts, others are 

 confined to a few or to one only. A double limitation is placed 

 on parasites which require two hosts in order to complete their 

 life historj^; they obviously cannot exist beyond the territory 

 where both hosts exist together. The local as well as geographic 

 distribution of the hosts is, of course, effective in limiting the dis- 

 tribution of the parasites. In the case of human parasites, the 

 alternate host is practically the only limiting factor. The geo- 

 graphic distribution of human sleeping sickness is coincident 

 with the distribution of certain species of tsetse flies; the distri- 

 bution of yellow fever nowhere extends beyond the range of a 

 certain species of mosquito; Rocky Mountain spotted fever is 

 geographically limited by the distribution of a certain species of 

 tick. The accidental or gradual extension of the range of one of 

 these intermediate hosts is likely to be followed by an extension 

 of the disease carried by it. It sometimes happens that a strain 

 of a certain parasite establishes itself in a new host, thus often 

 greatly extending the territory which it affects, and this is a 

 possibility which must alwaj's be remembered and watched 

 for. The trypanosome of Rhodesian sleeping sickness, for in- 

 stance, is very possibly a race of Trypanosoma brucei, which is 

 common in domesticated and wild game animals in a large 

 portion of Africa. Some slight alteration in the nature of the 

 parasite has made it possible for it to affect human beings and 

 thus give rise to a new disease. A somewhat different situation 

 is presented in the case of Rocky Mountain spotted fever, the 

 parasite of which has not yet been discovered. In nature ap- 

 l)arently only one species of tick acts as an intermediate host, 



