THE PROBLEM OF HORSE SICKNESS. 69 



malarial fever and yellow fever in many, that not all mosquitoes 

 become infected. 



By analog-y with the fly-transmitted trypanosome diseases,, 

 a direct transmission of the d'.sease by a winged insect is 

 feasible, in which case a definite host would not even be required. 

 If the transmission is effected by the same adult insect which 

 sucks the blood, then the proof will be forthcoming; one day. , 

 Should, howexer, the xirus go throug'h the progeny of the insects, 

 as is the case in the tick-transmitted diseases, then the question 

 becomes more difficult, since it is very difficult or imi>ossible to 

 breed some species (in particular of the sub-family Anophelinas) 

 in captivity. Notwithstanding- our failures, we are so convinced 

 of the correctness of the insect theory, that we feel sure that 

 one day the exact proof will l)e forthcoming. 



3. The \Trus Reservoir. 



The third question to be answered is : Where does the insect 

 host obtain its infection? Once the horse-sickness season has 

 started, every equine suffering- from the disease will in turn 

 infect the sucking insects. It is an experimentally well-estab- 

 lished fact that, besides horses and mules, donkeys also contract 

 the disease, but seldom die from it. It has also been experi- 

 mentally proved that dogs can comparativeh^ easily be infected 

 with horse-sickness, either by injection with virus, or by feeding 

 on carcases of horses that died from the disease. In our ex- 

 periments on 54 dogs utilised for virus, nine died of the disease 

 and 30 showed symptoms and recovered. It is noteworthy that 

 we succeeded in transmitting the disease in dogs through 49 

 generations, and that, notwithstanding tlie adaption to the canine, 

 the virulency in no way abated for the equine. Angora goats 

 occasionally develop a typical fever to the injection of virus, 

 and blood obtained during the reaction i)roved to be virulent for 

 dogs. A curious fact in connection herewith was ol^served. 

 The blood of the injected goat would ])rove to be infective for 

 a dog and for a goat, but not for a horse, and only the blood 

 of the dog would serve again as a virus for the horse. This 

 transmission was, however, only ])Ossible for a few generations. 

 It is likely that other animals as well are susceptible to horse- 

 sickness, and serve as carriers of the virus. iVe tried sheep 

 and cattle, but failed. It is an important fact that the immune 

 animal no longer contains the virus in its circulatory blood. All 

 attempts to this eft'ect have failed ; once the horse has recovered, 

 the ])lood is no longer infective.- The same holds good as far 

 as dogs and goats are concerned. From an enzootological 

 point of view, and in analogy to other diseases, we require an 

 animal from which the insect obtains its infection as a virus- 

 carrier other than those mentioned. To demonstrate this, I 

 wish to mention that horse-sickness exists in localities where, 

 perhaps on account of horse-sickness, there are no equines. and 

 for various reasons there are no other domesticated animals, 

 and in which localities the disease can be contracted at almost 



