174 INSECTS AND MAN 



and poultry can all be artificially infected with caderas, 

 naturally, it is almost exclusively a disease of horses ; 

 mules and asses are also attacked, but they are more 

 resistant than horses, while cattle are said to be completely 

 resistant. The incipient stages of the disease are accom- 

 panied by profuse weeping; later, the most pronounced 

 symptom is that of progressive anaemia, accompanied with 

 partial paralysis of the hind quarters. The disease is 

 uniformly fatal. Although some of the symptoms of 

 nagana and mal de caderas are similar, the two diseases are 

 quite distinct ; the morphological differences between Try- 

 panosoma brucei, causing nagana, and Trypanosoma 

 equinum, causing mal de caderas, are constant, and again, 

 horses immunised against nagana are still susceptible to 

 mal de caderas. 



The actual vector of the disease is unknown, and, in this 

 regard, it is of importance to remember that the transfer 

 of the trypanosomes may be effected in two ways, either 

 indirectly or directly. Indirect transfer of blood parasites 

 takes place when malaria mosquitoes carry infection, and 

 the process is indirect, because the parasites, perforce, pass 

 through certain developmental stages within the body of 

 the mosquito before they are capable of again producing 

 the disease. Direct transference takes place where the 

 fly simply carries the germs of disease on its mouth organs, 

 and so infects the next host from which it takes a meal, 

 and, in this respect, it is of interest to note that the house 

 fly, which is incapable of biting, can transmit various 

 trypanosomes, and, among them, those of mal de caderas, 

 by alternately sucking sores on an infected and a healthy 

 animal. The house fly, however, cannot be considered as 

 the natural carrier of this disease, but rather must we look 

 to the biting flies, Stomoxys calcitrans and Stomoxys 

 nebulosa, known in the Argentine as " mosca brava " ; indeed, 

 virulent mal de caderas trypanosomes have been found in 

 the stomach of the former fly, after feeding on an infected 



