250 GLOSSINA AND TRYPANOSOMIASES [CH. 



Glossina and Disease. 



The more common members of this genus have all been 

 proved to be capable of serving as the invertebrate hosts of 

 various species of trypanosomes, and in addition may carry 

 infection directly from one animal to another, when there is no 

 long interval between the bites. Some species of trypanosomes 

 seem to be mainly spread by only one species of Glossina, as in 

 the case of sleeping sickness {T. gambiense), which seems to be 

 restricted to regions where G. palpalis is present. Nevertheless, 

 G. morsitans has been shewn experimentally to be capable of 

 transmitting T. gambiense and it is difficult to see why such 

 transmission does not take place in nature. Fortunately, this 

 trypanosome seems to be unable to readily adapt itself to 

 development in more than one species of Glossina, but some 

 of the cattle trypanosomes, e.g. T. cazalboui and T. dimorphon, 

 seem to be able to develop in any species of Glossina, though 

 not with equal facility in all of them. 



Probably the most important factor restricting the spread 

 of trypanosomiases is the difficulty with which the tsetse-flies 

 become infected. When several Glossince are fed on an animal 

 containing trypanosomes in its blood, only a relatively small 

 proportion of the flies become infected, the number depending 

 on a variety of conditions which are not thoroughly under- 

 stood. 



Attention may be called to Miss Robertson's experiment 

 with T. gambiense and G. palpalis, reproduced in tabular form 

 on page 309, from which it appears that the percentage of 

 flies in which the trypanosomes develop, depends to some 

 extent on the stage of the infection in the vertebrate host. 

 Yet another important condition is the interval that elapses 

 between an infective and the subsequent feed. In the case of 

 T. gambiense and G. palpalis, the trypanosomes that may have 

 developed in the gut of the fly after the first feed are frequently 

 swept out by the next influx of blood and thus no infection 

 is produced. Strange as it may seem, from Miss Robertson's 

 experiments there can be little doubt that when flies are fed 

 every two or three days there is much less chance of them 



