INFEOTIOTJS DISEASES. 573 



mitted to cattle, buffaloes, sHeep, goats, rabbits, guinea pigs, rats, 

 and monkeys. No birds, reptiles, amphibia (frogs, etc.), or fish are 

 known to suffer from it. It attacks both male and female animals, 

 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 somewhat; in experimental cases it is from 2 to 75 (usually 

 6 to 8) days, according to conditions. The duration varies with 

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

 average duration in the horse is reported at 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 indi- 

 cate that surra is strictly a wound disease, namely, that the parasite 

 may enter the body only through a wound of some kind. Appar- 

 ently 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 before biting 

 the healthy animal. Crows may also transmit the infection by peck- 

 ing 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 become 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, infec- 

 tion does not take place. Thus dogs and cats may contract the dis- 

 ease 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 evi- 

 dence to support this view. Probably the correct interpretation of 

 the facts cited in support of this theory is that biting flies are numer- 

 ous around stagnant water and in inundated pastures; hence, that 

 a great number of possible transmitters of the disease are present 

 in these places. 



