66 THE PROBLEM OF HORSE SICKNESS. 



connected with the resistance of the horse. AUhough the 

 impossibility of ciUtivating the virus in artificial media un- 

 doubtedly represents a great obstacle, it does not debar us from 

 studying its character or from utilising it for the purpose of 

 immunisation. It is a peculiar fact that horse-sickness virus 

 retains its virulence in defibrinated blood for a considerable 

 length of time, even four years or more, and frequently under 

 the conditions of a polluted medium, the pollution giving rise 

 to abscesses and gangrene in the injected horse before the 

 disease develops. On the other hand, a virus ma}^ lose its 

 virulence at any time, and particularly under the influence of 

 some specific contamination not yet clearly elucidated. With 

 proper care the virus will maintain a definite virulency for at 

 least three years. Virus is onl}- \irulent as long as it is in a 

 liquid medium. Of such a virus less than t-^o-q of a cubic 

 centimetre is recjuired to i^roduce the disease when injected 

 into a susceptible horse, whereas when virus is given through 

 the mouth comparatively large doses of e\en loo or 150 c.c. or 

 more are necessary. Taking this fact into consideration, it 

 becomes exident that the froth discharged from the nostrils 

 shortly ])efore or after death, and which contains the viru^, 

 cannot be made responsible for the maintenance or the propa- 

 gation of the disease outside an animal's body, neither the dead 

 iDody itself. 



2. The Transmission of the Disease. 



The (|uestion arises here : In which way does \irus gain 

 access into the body of a suscejjtiljle horse? Here ]j()siti\e 

 l^roofs are lacking, but facts so far collected allow of onl\' one 

 interpretation, vi.":., that the disease is communicated to animals 

 by means of winged insects. This view was in the first instance 

 suggested l)y the fact that malarial fever in man and horse- 

 sickness in equines are usually found under identical telluric 

 and climatic conditions. It is true that in bad seasons horse- 

 sickness appears before malaria, and has a wider range, s])rea(l- 

 ing more ra])idly and breaking out at higher altitudes. It was 

 evident that the sinfilarity was coincidental; it led to the con- 

 clusion that l)oth diseases are pro]jagated in an identical manner. 

 Malaria iii man is spread b}' mosquitoes of the genus Anopheles, 

 and likewise horse-sickness must be spread by some insect, but 

 not necessarily by the same species or even genus. Indeed, 

 seeing that horse-sickness has a wider range, this fact should be 

 interpreted to mean that a difl:'erent insect was responsible. It 

 is true that the analogy between the two diseases only holds as 

 far as the transmission is concerned, and not to the cause. In 

 the case of malarial fever the micro-organism is well known. 

 It is a Plasmodium, and its cycle within the hosts has been 

 studied in detail. There, are. however, other ecjually well- 

 studied diseases in man carried by winged insects which are due 

 to ultravisible micro-organisms, such as the yellow fever trans- 

 mitted by the genus Stegojiiya and the Pa]:)patacci fever trans- 



