Natural Infection. §19 



brane. Trypanosomes at times in varying numbers are present 

 in these for months even before the appearance of the symp- 

 toms, and also in the later course of the disease, even when the 

 animals otherwise show no symptoms or at most only a very 

 moderate swelling of scrotum or vulva. During the periodical 

 aggravations of the local manifestations, an increase of their 

 number may usually be demonstrated. 



In the rare cases in which the disease appears in horses 

 not used for breeding, sometimes even in geldings, the inf et^tion 

 is probably conveyed by sponges or by the hands of the at- 

 tendants. 



The possibility of transmission by stinging flies cannot be discarded 

 entirely. Lingard has already indicated such a possibility, and Sieber & 

 Gonder have observed the infection in a horse which stood in a stable 

 alongside of an artificially infected animal. Schubert & Kuhn also 

 succeeded in an intentional transmission with the aid of the stomoxys 

 calcitrans. Further than this Eabinowitsch has infected laboratory 

 animals with dourine trypanosomes through lice and fleas, while TJblen- 

 huth & Woithe, Manteufel, also Zwick & Fischer, have proved in rats 

 the possibility of infection through the intact skin. As however try- 

 panosomes occur at least in northern countries, only rarely in the blood 

 ' of horses, and even then but few in numbers, stinging flies have only 

 slight opportunity for taking up the parasites (most likely from the 

 fresh plaques). Therefore such a mode of transmission is apparently 

 of very minor importance as is also confirmed by experience in practice 

 (according to Sieber & Sender the trypanosomes remain viable only for 

 three hours in the stomachs of flies) . 



In former times an important part in the extension of the disease was attrib- 

 uted to heredity (Eoloff, Jessen). With the experience acquired in its eradica- 

 tion, and with the establishment of the etiology of the disease, this conception is no 

 longer tenable. 



In general the susceptibility of solipeds for infection is 

 considerable, but it varies individually, as not all mares covered 

 by an infected stallion become affected. This may however be 

 explained by a temporary disappearance of the trypanosomes 

 from the secretions of the genitals. 



According to Eoll about 66% of mares which are exposed to the infection 

 become affected. EodloflC in Sierakowo observed in all of 107 mares which were 

 covered by an infected stallion, mild or severe symptoms occurring later o" (54 ot 

 these died), while in the experiments of Prince & Lafosse only 10 out of 15 mares 

 became infected under similar conditions. Kern reports 24 cases m 105 mares. 

 which were covered by an affected stallion. 



Pathogenesis. The trypanosomes which penetrate into the 

 mucous membrane of the external genital organs first multiply 

 at the point of entrance where they produce a serous and m 

 part a cellular inflammation. Later they enter the blood and 

 with it pass to distant parts of the body. This mode of dis- 

 semination is indicated not only by the possibility of demon- 

 strating at times the parasites in the blood, but also by the tact 



