IO PRESIDENT S ADDRESS. 



tence of a reservoir in the shape of a different species of 

 animal, harbouring the parasite of the disease in its blood. 

 Such an animal may be cold-blooded or warm-blooded, a bird 

 or a mammal. Here, again, we have not yet been able to make 

 further progress. We enter on a different branch of research 

 ]t will be interesting work for our zoologists to point out to 

 us the geographical distribution of any such animals, coinciding 

 with the distribution of the disease. Then we might have more 

 hope of proving the theory than there is at present, where we 

 have to work more or less in the dark. It is this theory which 

 justifies the hope that within the districts of the reservoir those 

 diseases will be suppressed one day. Recently an assistant of 

 mine, Mr. Walker, found a parasite in the blood of young 

 ostrich chicks known under the name of leucocytozoon and related 

 to the trypanosomes. Whatever the practical outcome of this 

 discovery will be, one conclusion we are entitled to make now, 

 and that is the parasite is transmitted by insects ; and should it 

 prove to be the cause of the mortality observed in chicks, the 

 way to combat it is indicated by this conclusion. Whilst on the 

 subject of suppressing disease, I wish to refer to some other 

 well-known observations made by farmers, the correct interpre 

 tation of winch has led to important applications. They are in 

 connection with immunity. When horses or sheep recover, 

 they are said to be salted against the disease, viz., to 

 be immune. We expected this to be so by comparison 

 with other diseases of a similar nature, but caused by 

 visible organisms. To this latter group belong those against 

 which modern science introduced methods of preventive 

 inoculation, and by analogy we were entitled to anticipate 

 that a similar possibility would exist in connection with those 

 under discussion. Tt proved to be the case, and on recognised 

 principles, methods of inoculation for mules as well as for 

 sheep were worked out, which proved to be successful. In the 

 case of horses, however, great difficulties were experienced, 

 inasmuch as these animals showed a much higher susceptibility 

 than mules, a fact which can only be explained by inherited 

 immunity from their sires, which, although susceptible to the 

 disease, have, at least in my experience, never been found to die 

 The methods in use for mules proved useless for horses. Here 

 the observations of the farmers came to the rescue ; they led 

 to deductions which proved to be applicable in the practice. 



Long ago farmers had the experience that the so-called 

 salted horses may break down in immunity. Thev called these 

 relapses, or " aanmanings." Subsequently our experience proved 

 the same observations to be correct. Some of the mules and 

 horses which were undoubtedly immune broke down when 

 exposed to natural infection. The virus from such cases was 

 collected, and in several instances it was shown that breakdown 

 in immunity could be produced in almost any salted animals. 

 The experiments showed that there was no actual loss of 



