Immunity in Tropical and Sub-tropical Diseases. 



The term "tropical" or "sub-tropical" disease is applied in this paper 

 to such raaladies of stock as are principally encountered in Africa, and 

 more particularly in South Africa. 



Those of any economic importance are, so far without exception- 

 either due to protozoa (Piroflasmoses, Trypanosomiases, Sfirochaetoses 

 theileria) or to a specific group of ultravisible organisms {CMamydozoa) 

 which, in the same way as the protozoa, are dependent for their propa- 

 gation on a host in the form of a tick or an insect. (Heartwater, 

 horse-sickness, blue-tongue.) 



In South Africa other infectioiis diseases also exist, such as anthrax, 

 quarter-evil, glanders, strangles, etc., caused by specific bacteria, and not 

 so many years ago epizootics such as pleuro-pneumonia, foot and mouth 

 disease, and rinderpest have played considerable havoc amongst stock. 

 These latter two are also due to ultravisible organisms, but are spread by 

 direct contact with the sick animals and their morbid products. 

 These and the mentioned diseases caused by bacteria do not require 

 hosts as propagators, and in this fact lies the important difference between 

 those designated as " tropical " or " sub-tropical." They are also encoun- 

 tered in colder regions and have been known for a considerable length of 

 time, whereas the tropical ones are more or less new to science. 



Ever since I have undertaken the study of the diseases of South Africa, 

 I have laid particular stress on the question of immunity. A possible 

 apphcation of any immunity for the purpose of protecting the greatest 

 number of animals exposed to infection must bear immediate results, and 

 for this reason, wherever possible, it forms the best weapon under present 

 conditions for successfully combatting these maladies. 



It is my object in this paper to compile the facts known concerning 

 the immunity in tropical diseases and to compare them with those known 

 in other diseases, selecting for examples such as are known or have been 

 observed in this sub-continent. 



Immunity is spoken of as general or specific. In the first instance 

 is usually meant the absolute insusceptibihty of a certain animal or a 

 class of animals to a disease. When we apply this to some of the tropical 

 diseases, it would, for instance, express that cattle and sheep are immune 

 against horse-sickness ; cattle, sheep, and dogs are immune against the 

 piroplasmosis of horses ; sheep and dogs immune against the piroplasmosis 

 of the ox ; cattle immune against blue-tongue of sheep : horses immune 

 against heartwater, etc. 



