On Nay ana, or Tsetse Fly Disease. 103 



received the whole blood of another rabbit containing numerous 

 hsematozoa may survive longer than the minimal lethal period for 

 rabbits. We have not been able to define the conditions which deter- 

 mine these variations in susceptibility. Rouget has noted similar 

 variations in the lethal period. 



The ratio of the minimal to the maximal lethal periods is about 

 1 to 5 or 1 to 6 in rabbits and rats, and 1 to 9 in guinea-pigs. The 

 number of other animals inoculated has not been sufficiently great to 

 determine a satisfactory ratio. 



In our experiments dogs survived an infection 14 26 days, cats 

 22 26 days, rats 6 26 days, mice 8 25 days, rabbits 13 58 days, 

 And guinea-pigs 20 183 days; the average duration being for dogs 

 18 days, for cats 24 days, for rats 12 days, for mice 13 days, for rab- 

 bits 30 days, and for guinea-pigs 50 days. 



Since the commencement of these experiments, a large number of 

 animals have been dealt with, and thus an extensive series of cross- 

 inoculations has been carried out ; but we have found that the dura- 

 tion of the disease is not dependent on the kind of animal from which 

 the hasmatozoa are derived. No constant modification is, therefore, 

 effected by passages, either in the direction of attenuation or of 

 increased virulence. This statement is completely borne out by Bruce's 

 observations on wild animals, as well as African sheep and goats, for 

 he found that the hsematozoa of these animals were as infective as 

 those obtained from highly susceptible animals, such as dogs. 



III. Mode of Inoculation. 



Inoculations have been made with the blood of an infected animal, 

 subcutaneously, intravenously, or intraperitoneally, or by applying a 

 minute and often minimal quantity of infected blood to a superficial 

 scratch. Babbits have also been inoculated in the anterior chamber of 

 the eye, and rats directly into a lymphatic gland. 



Blood taken from diseased animals, although showing no hsematozoa 

 when examined microscopically, has frequently been proved to be fully 

 infective, so that it appears that a single hsematozoon, or at any rate a 

 very small number of them, successfully introduced, are capable of 

 producing the disease. At present no method of graduating the dose 

 appears possible, since a minute quantity is as effective as much larger 

 quantities, though the lethal period may be somewhat prolonged. It 

 is also possible that unrecognised forms are present in these cases, 

 though it should be added that in some instances where no hsematozoa 

 are found in simple films, we were able to detect them by means of 

 centrif ugalising the blood. 



Successful inoculations have also been made with lymphatic gland, 

 spleen, bone-marrow, aqueous humour, serous fluid, oedema transuda- 

 tion, and testicular juice. 



