The nature of the establishment barrier for Trypanosoma brucei in the gut of Glossina pallidipes

The low survival rate of Trypanosoma brucei in tsetse flies is interpreted as in part at least the result of an establishment barrier. This barrier appears to be less active in young flies than in older flies. The growth rate of the peritrophic membrane is 1 mm./hour during the first 30 hours after emergence, and also after feeding. The distribution of the blood meal in relation to the growth of the peritrophic membrane makes it appear unlikely that the membrane itself is the barrier mechanism. A postulated adjustment period for trypanosomes in the crop of young flies (with an incomplete peritrophic membrane) is supported by cytochemical evidence concerning an enzymic transformation within 1 hour of feeding. This phenomenon implies a double transformation for successful host transfer: one (well established) in the vertebrate host sometime before transfer, a second (new) in the fly, immediately after transfer. The destruction of non-transformed trypanosomes in the mid-gut (after leaving the crop) of mature tsetse flies is considered to be the main establishment barrier.