The history of sleeping sickness
Crop cultivation and rearing of domestic animals was a means to be less dependent of seasonal fluctuations in the food supplies. In Africa this started about 5000 years BC. The agriculturists lived in relatively dense aggregations of several hundreds of people, considerably larger grotups than the hunter-gatherers. This favoured man-to-man transmission diseases like malaria and small pox for instance date from that time. Also the human pathogenic trypanosomes could adapt to man as a regular rather than an incidental host and evolved into the less virulent T.b. gambiense. Typical sites of transmission of T.b. gambiense are the riverbanks in West and Central Africa where people were exposed to tsetse whilst collecting water, washing and fishing. The transmission of T.b. rhodesiense in East Africa remained mainly animal to man transmitted in the savannah as still is the case at present in low endemic situations. Paradoxically the older of the two human pathogenic trypanosomes remained the less adapted and the more virulent parasite of the two.
The numbers of millions of years are not of too great importance here but may give an idea of the sequence in evolution and the time spaces in-between the origin of each: trypanosomes that existed long before animal hosts and they on their turn long before the prototypes of tsetse. Nature has given us a lesson in modesty when one realises that prototypes of trypanosomes and tsetse emerged hundreds of million years before the bi-ped prehominids became part of the complex cycle of transmission.
The geographic distribution of sleeping sickness is limited by the geographic distribution of the tsetse flies. Their exclusive environmental and climatic requirements are: humidity, shade and temperatures between 20°- 30°. Since the desertification of the Sahara (4000 BC) the infested areas range from 15° N till 29° latitudes, roughly between the Sahara and the Kalahari desert.