470 



NATURE 



[December 26, 1912 



that a development must have followed. It having 

 been demonstrated that the malady was inoculable, it 

 formed the subject of much speculation to explain the 

 ob>;ervations which the farmers had been collecting 

 ever since they knew it, and which principally apply 

 to the climatic and tellurical conditions under which 

 it appears. You have probably all heard that the 

 farmer interpreted his observations to the effect that 

 the dew is the cause. There is nothing ridiculous in 

 this theor)'. Remember that our knowledge of micro- 

 organisms as causes of disease is practically only a 

 science of yesterday ; remember that the English 

 translation of the name "malaria" for the disease of 

 that name means "bad air," and it is only a few 

 years back that science admitted of such a theory as 

 the probable cause ; that is just as our farmers have 

 done and are still doing for horse-sickness. 



The observations of the farmer are correct in details. 

 We give them the right interpretation when we sub- 

 titute for the name "dew" the name "blood-sucking 

 night insect." Under the conditions under which dew- 

 is formed horse-sickness and blue-tongue appear most 

 frequently, and these conditions are most favourable 

 for the breeding of mosquitoes and other blood-sucking 

 insects. This being so, the question might be put to 

 us, "But are there any direct proofs to this effect?" 

 If we had all the proofs, we would no longer speak 

 of a theory, and we must speak of a theory until the 

 actual blood-sucking insect has been demonstrated and 

 until the experiments have been made under such 

 conditions that no doubts are left any longer. In- 

 directly, the theory has been so well founded that the 

 only missing link is the insect itself. The reason why 

 this link has not been demonstrated yet is the fact 

 that we do not know sufficient of all the nocturnal 

 blood-sucking insects of South Africa, of which various 

 genera and many species exist ; we do not yet know 

 how to breed and handle them for such delicate experi- 

 ments as are required to bring the proofs with horse- 

 sickness and blue-tongue. Notwithstanding this, the 

 theory has its practical value, inasmuch as it shows 

 in which way protective measures can be adopted, and 

 what has been said about the destruction of mos- 

 quitoes in connection with human malaria applies 

 equally well to the diseases under discussion. 



The theory goes still further. Seeing that flying 

 insects must be accepted as being the transmitting 

 agencies, we conclude that there also must be a reser- 

 voir somewhere from which these insects obtain the 

 virus. This is perhaps the most interesting point. 

 The horse alone in the case of horse-sickness, and the 

 sheep in the case of blue-tongue, are not sufficient to 

 represent that reservoir. When recovered, the blood 

 of these animals no longer contains any virus. 

 Furthermore, horses, when introduced into a wild 

 country where before there had never been any equines, 

 are liable to contract the disease. ."Xgain, the almost 

 "explosion-like" expansion when climatic conditions 

 arc suitable does not allow us to conclude that the sick 

 animal alone is responsible, and we naturallv ask, 

 "Where does the virus come from?" By analogy 

 with tsetse and human malaria we accept the exist- 

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

 animal, harbouring the parasite of the disease in its 

 blood. Such an animal mav 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. It 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 

 (he theory than there is at present, where we have to 

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



\0. 2252, VOL. 90] 



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 \oung 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 interpretation of which 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 preven- 

 tive inoculation, and by analogy we were entitled to 

 anticipate that a similar possibility would exist in 

 connection with those under discussion. It 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 observation 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. 

 They called these relapses, or " aanmanings." Subse- 

 quently 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 immunitv in the animal 

 affected, but the relapse was due to the different nature 

 of the virus. This means from a biological point of 

 view, the ultravisible micro-organism will also follow 

 the laws of other organisms, viz. that of variability 

 or mutability, but which can show itself to our eyes 

 only by a different virulency in the animal it attacks. 



Accordingly more than one variety of horse-sickness 

 organisms exist, and although from a pathological 

 point of view we onlv recognise one disease, yet there 

 are as many diseases as there are varieties of ultra- 

 visible organisms. At one time we thought that the 

 variation was simplv due to the influence of environ- 

 ment, but, based on a number of experiments, we came 

 to the conclusion that the cause of the variability of 

 a particular strain lies in the horse from which it is 

 collected. The host represents, so to sav, its environ- 

 ments. The passage through a horse determines 

 whether there will be a decrease or an increase in 

 virulency. This fact established, the further conclusion 

 was made that there must be certain strains or varie- 

 ties of which the virulency would not be so pronounced, 

 and accordinglv that a greater number of animals 

 would recover when infected. This, indeed, proved to 

 be the case. The variability of the organism has now 

 been made use of for the inoculation of horses in 

 connection with the method as applied to mules. The 



