330 , DR. W. YORKE ON 



matter was so inadequately discussed when Dr. Yorke read his 

 paper to the Society that it seems desirable to consider it here in 

 somewhat greater detail. In his published papers Dr. Yorke 

 hfVS throughout assumed the disease to be a new one, and gives 

 no sign of having even contemplated the other alternative. 

 "When, at the meeting referred to, I suggested that the disease 

 is probably endemic, and that in consequence immune natives 

 may prove to be a reservoir of the trypanosome, he rejected the 

 suggestion on the grounds (1) that the disease had only recently 

 been discovered, and it was incredible to suppose that so 

 distinctive an organism as a trypanosome had been previously 

 overlooked by medical men ; ancl (2) that the virulence of the 

 disease was so great in all the cases of infection investigatsd 

 (there being no recoveries) that he found it impossible to believe 

 that human beings could harbour this organism with impunity. 



As one who resided in Southern Rhodesia for nearly thirteen 

 years, I cannot seriously accept the first of these arguments. 

 The methods of blood-examination which are now matters of 

 every-day routine for any young doctor trained in our modern 

 Schools of Tropical Medicine, were certainly not practised in 

 Rhodesia until quite recently, and the chances of the disease 

 being coj-rectly diagnosed, at least up to 1906, would have been 

 extremely remote. With regard to the second contention, 

 the mere fact that a disease is highly virulent in susceptible 

 persons is no valid evidence as to the non-existence of immune 

 individuals. It is now generally admitted that the natives of 

 West Africa are very largely immune to Trypanosoma gamhiense, 

 and there seems to be no good reason for assuming that a similar 

 power of resistance to T. rhodesiense cannot exist in East Africa, 

 Indeed, the more virulent the disease, the more rapid will be 

 the development of a general immunity ; and further, the more 

 deadly the parasite, the less likely are there to be intergrades 

 between complete immunity and fatal susceptibility in the host. 



It is true that a new disease will usually exhibit what is known 

 as primary virulence ; but this virulence is essentially in relation 

 to population, and not merely in relation to the individual. In 

 other words, we may reasonably assume a disease to be newly 

 introduced if there is a veiy high percentage of cases and a low 

 percentage of recovery ; but if the percentage of cases is small 

 (as it certainly is in Rhodesian trypanosomiasis), the individual 

 severity is no proof that the disease is new. To take an example. 

 The endeixiicity of yellow fever in West Africa is now hardly 

 disputed, yet in the epidemics in Accra and Sekondi in 1910 to 

 1912, out of 23 Europeans attacked no fewer than 22 died. 

 Another aspect is perhaps worth consideration. There can be 

 no question that Sleeping Sickness was a new disease in Uganda, 

 and there was a marked primary virulence in its true sense ; yet 

 in spite of this, the individual course of the malady, though 

 eventually fatal, was less severe and much less rapid than in 

 Rhodesian trypanosomiasis. 



