TRANSMISSION OF TROPICAL DISEASES. 9 



thought not to do so in man, as relapses in man are practically- 

 unknown (at least after any long period), but it is still just 

 possible that the blood may be infective even though the 

 patient is well, though against this is the fact that transmission 

 experiments only succeed during the first two days of the fever. 

 The other alternative is that the disease is transmitted through 

 the larvae of the flies. Our knowledge is at present accordingly 

 incomplete, and it is so, largely owing to the fact that these 

 minute flies, which easily pass through a mosquito net, only 

 survive captivity for about ten days, and so experimental 

 work with them is difficult. 



Sleeping Sickness. 



Is due to two different species of trypanosomes, viz., 

 Trypanosoma gambiense and T. rhodesiense. The former 

 trypanosome is the cause of sleeping sickness in most parts of 

 Africa where the disease exists, while T. rhodesiense is confined 

 to Rhodesia, Nyasaland, and a few other adjoining territories. 

 One peculiarity of the latter form of the disease is that the 

 number of known cases is few, say about a hundred, but 

 on the other hand the disease is even more deadly, or at any 

 rate more rapidly fatal than that due to T. gambiense. The 

 trypanosomes in each case exist in the blood and the disease 

 is transmitted by tsetse flies, T. gambiense by Glossina palpalis 

 and T. rhodesiense by Glossina morsitans. The mode of 

 transmission is not simply mechanical, but a developmental 

 cycle takes place in the fly, for a fly after biting a sleeping- 

 sickness patient is only capable of transmitting the disease 

 to a healthy person some twenty to thirty days later. The 

 mode of transmission of T. gambiense is the following. The 

 trypanosomes which are sucked into the gut disappear more 

 or less completely in about a week, to reappear later as short 

 stumpy crithidial forms. These eventually find their way to the 



