Stumpy forms are the predominant transmissible forms.

Comparison of (A) midgut and (B) salivary gland infection rates in teneral (T) Vs. adult (A) tsetse flies (batches of 50 flies) infected with 10 to 100 (circles, 3 independent experiments), 100 to 1,000 (squares, 4 independent experiments) or 1,000 to 10,000 (triangles, 5 independent experiments) parasites in the slender (SL in blue) or stumpy (ST in red) forms.