SUEEA. 555 



young and old. Australian breeds of horses and white and gray 

 mules are said to be more susceptible than animals of other breeds 

 and color. 



Surra in equines and camels is said to be an invariably fatal dis- 

 ease, but cattle occasionally recover from it. There is no history of a 

 definite onset of the disease, and the condition is progressive, usually 

 with a number of relapses. The period of incubation may vary some- 

 what; in experimental cases it is from two to seventy-five (usually 

 six to eight ) days, according to conditions. The duration varies with 

 the species of animals attacked, their age, and general condition. The 

 average duration in the horse is reported as less than two months, 

 though some cases may terminate fatally in less than one to two 

 weeks. 



METHOD OF INFECTION. 



All evidence now available seems to indicate that surra is strictly a 

 wound disease, namely, that the parasite may enter the body only 

 through a wound of some kind. Apparently by far the most common 

 method is through wounds produced by biting flies, whose mouth 

 parts are moist with the infected blood of some animal bitten by the 

 same flies immediately previous to biting the healthy animal. Crows 

 may also transmit the infection by pecking at sores on a diseased 

 'animal, soiling their beaks with blood, and transferring this infected 

 blood to a healthy animal. Likewise, if a scratch is made on a horse 

 and then infected blood is rubbed on the scratch, the horse will be- 

 come diseased. If, in experiment, infected blood is fed to a healthy 

 animal, the latter may contract surra in case it has an abraded or 

 wounded spot in the mouth; but if no part of the lining of the 

 alimentary canal is wounded, infection does not take place. Thus 

 dogs and cats may contract the disease by wounding the lining of the 

 mouth (as with splinters of bone) while feeding on the carcasses of 

 surra subjects. All available evidence indicates that under normal 

 conditions of pregnancy the disease is not transmitted from mother 

 to fetus. 



There is a popular view that surra may be contracted by drinking 

 stagnant water and by eating grass and other vegetation grown upon 

 land subject to inundation, but there is no good experimental evidence 

 to support this view. Probably the correct interpretation of the facts 

 cited in support of this theory is that biting flies are numerous around 

 bodies of stagnant water and in inundated pastures; hence that a 

 gi'eat number of possible transmitters of the disease are present in 

 these places. 



Symptotns.'^ — The invasion of this disease when contracted natu- 

 rally is usually marked by symptoms of a trivial character; the 



o This summary of symptoms is based upou work by Liugard. 



