50 

 COMPLETE SUMMARY OF CONCLUSIONS. 



I. 



Two different types of immunity may be distinguished : — 



(1) Immunity whereby the recovered animal no longer acts as a 

 reservoir of virus. Immunitas sterilisans (East Coast fever and the 

 tropical diseases due to ultra visible organisms ; trypanosomiasis partially.) 



(2) Immunity whereby the recovered animal acts as a reservoir for 

 virus. This immunity may cease by intercurrent fevers ; breakdowns. 

 Immunitas non sterilisans. (All inoculable piroplasmoses and spiro- 

 chaetoses ; trypanosomiasis partially.) 



II. 



In speaking of immunity of a recovered animal, distinction must be 

 made between immunity generally without reference to any particular 

 strain of virus and immunity in particular with reference to definite 

 strains of virus. (For instance, immunity against horse-sickness can be 

 generally spoken of, or in particular the immunity obtained from the 

 Ordinary strain, or Tzaneen, or any other.) 



We can then conclude as follows : With the exception of East Coast 

 fever in South Africa no other tropical disease produces complete immu- 

 nity. The cause of the other diseases is not uniform ; there exist many 

 different strains or varieties of the species of organisms which all form 

 the cause of the disease. 



Concerning a particular strain, the following may be concluded : 



(1) An immunity against a. particular strain may be complete. 

 (Ordinary virus of horse-sickness ; some trypanosomiases ; heartwater.) 



(2) An immunity against a particular strain may be broken 



(a) by the injection of large doses of the same strain of virus 

 (blue-tongue) ; or 



{h) after a certain time has elapsed (blue-tongue) ; or 



(c) by a more virulent virus of the same strain (Tzaneen virus in 

 horse-sickness) ; or 



{d) by a different strain of virus (horse-sickness, heartwater, red- 

 water, and surra). 



(3) Immunity can vary with the species and with the breed of animals 

 susceptible to the disease (redwater, heartwater). 



III. 



The facts noted in South Africa concerning variations of immunity 

 may be explained as follows : — 



East Coast fever is a freshly imported disease, and the outbreak 

 throughout South Africa may all be due to one and the same source, 

 whereas, for instance, horse-sickness, blue-tongue, etc., are diseases 

 estabhshed since ages, and are localised ; the micro-organisms have 

 undergone and are still undergoing certain variations, the result of which 

 is the different immunity. 



