In this infectious disease of lower animals the blood shows the presence of the Tr. brucei. It is transmitted to healthy animals by the bite of a fly - the Glossina morsitans.

Sleeping sickness or human trypanosomiasis is an infection by the Tr. gambiense which is conveyed by a fly - the Glossina palpalis. The organisms are present in the blood for a long time before entering the cerebro-spinal fluid and giving rise to the characteristic symptoms, increasing weakness, emaciation, lethargy, coma and death.