The maintenance of human sleeping sickness and nagana across sub-Saharan Africa
depends on cyclical transmission of trypanosomes through tsetse flies. Infection rates in
tsetse are normally very low as most parasites ingested with a bloodmeal die in the fly
gut. Infections which successfully establish in the fly midgut may subsequently mature
into mammalian infective trypanosomes in the salivary glands. However, these
processes are not automatic and involve tsetse, symbiont, trypanosome and
environmental factors.Previous work showed that the symbiotic bacterium Sodalis glossinidius was involved in
susceptibility to trypanosome infection. Streptozotocin (a toxic analogue of the
bacterium's main food source) has been recently shown to decrease trypanosome
infection rates in the offspring of treated tsetse. In the present work streptozotocin did
remove S. glossinidius from the offspring of treated flies but it was not possible to
generate a line of tsetse free from 5. glossinidius infection.Other potential factors involved in acquisition of trypanosome infection were then
examined. A range of antioxidants or cyclic GMP were shown to prevent trypanosome
death in the tsetse midgut. The process was shown to be independent of protein synthesis
as D-cysteine (an unphysiological isomer of L-cysteine) also enhanced midgut infection
rates. Further experiments showed that cGMP could significantly inhibit trypanosome
death when fed up to 96 h post-infection, whereas antioxidants only functioned for 48 h
post-infection. Moreover it was found that maturation of established midgut infections
could be regulated by environmental stimuli as well as by antioxidants. Cold shock of
infected flies as well as addition of L-cysteine but not D-cysteine to the bloodmeal
resulted in significant increases in maturation rates, while nitric oxide synthase
inhibitors reduced maturation rates.It is concluded that reactive oxygen species play a major role in killing trypanosomes
entering the tsetse midgut and that cysteine containing proteins and/or nitric oxide are
essential for differentiation of established midgut infections into mammalian infective
salivary gland infections
