40 RUPERT W. JACK. 



to the north, travelled down the Gwanyika River and down the Umniati to ascertain 

 if any movement of the fly had taken place. No fly were encountered until the spot 

 was reached where its southern limit on the Umniati River was marked in 1914. 

 From the Umniati the return journey to Gokwe was made via the Mtanka River, 

 all the country traversed, as on previous occasions, proving free from fly. 



The following points are noteworthy in connection with the cases cited above : — 



(1). In all cases the outbreak attained its height during the spring and summer 



months (October-April). 

 (2). In all cases, in connection with which sufficient time has elapsed for such 



observation, there has been no recrudescence of the disease in the following 



season, although susceptible animals have still been kept in the same locality.* 

 (3). In all cases infection occurred only amongst animals that were herded 



together. 



From the foregoing it appears probable : — 



(a) that the transmitting agent or agents are abundant in the spring and summer 

 and disappear or decrease greatly in number in the winter ; 



(6) that the disease is not readily transmissible under Southern Rhodesian 

 conditions by these agents from chronic cases that live over until the rains 

 or recover ; 



(c) that the agents concerned in this method of transmission are not capable 

 of perpetuating the disease indefinitely, or are only capable of doing so 

 under exceptional circumstances ; 



(d) that the method of transmission is of a mechanical and not of a cyclical 

 nature ; 



(e) that the segregation of infected animals (on showing temperature) would 

 in the absence of tsetse effectively check the spread of the disease. 



Allowing for the moment that the deduction given tentatively under (a) is correct, 

 very few species of blood-sucking Diptera are eliminated by this statement, as, apart 

 from Glossina morsitans and certain species of Hippobosca, all the common blood- 

 sucking flies are very much scarcer in winter than in summer. Moreover, observa- 

 tions as to the prevalence of possible transmitting agents in the locality of the out- 

 breaks furnishes a number of different species as likely subjects for experiment. 

 The jnost probable agents are included in the following : — Tabanus fuscipes, 

 T. taeniola, Haematopota pertinens, Haematopota spp., Stomoxys calcitrans, Lyperosia 

 and mosquitos. Of these it must be admitted that S. calcitrans and mosquitos are 

 the only agents that are everywhere abundant throughout the season over which 

 infection extends. It is, however, by no means necessary to infer that the power 

 of mechanical transmission is confined to one species or to one family ; and in view 

 of Mitzmain's; failure to associate S. calcitrans with the transmission of surra in the 

 Philippines, it appears unlikely that this species is solely responsible in regard to 

 the present .disease. The whole matter calls for careful experiment and investiga- 

 tion. 



* This is, of course, contrary to the experience in Barotseland and Portuguese East 

 Africa 



