Figure 2. Slender trypanosomes can complete the entire tsetse infection cycle, and a single parasite is sufficient for tsetse passage.

The flies were infected with different numbers of either stumpy or slender trypanosomes. Slender^ES, Slender^SIF, and monomorphic trypanosome cell lines were cultivated with regular dilution and a maximum population density of 5x10⁵ cells/ml, in order to avoid SIF accumulation. Stumpy development was induced by expression site attenuation (ES), or SIF-treatment (SIF). Stumpy^ES induction was performed by ectopic overexpression of VSG121 for 56 hr, in the absence of SIF. SIF-mediated stumpy transition (Stumpy^SIF) was induced by incubating slender trypanosome populations in the presence of SIF-containing conditioned medium for 48 hr. The expression of the stumpy marker PAD1 was checked before fly feeding. (A) Percent of the total fly infections is shown. MG, midgut infection; PV, proventriculus infection; SG, salivary gland infection; TI, transmission index (number of SG infections divided by MG infections); n, number of independent fly infections. (B) Graphical visualisation (beeswarm plot) of the data shown in panel A, colour-coded according to cell population used. MG, midgut; PV, proventriculus; SG, salivary gland; n, number of independent fly infection experiments.

Figure 2—figure supplement 1. Number of trypanosomes per milliliter (ml) of blood correlating to how many trypanosomes would be found in a tsetse bloodmeal.

This calculation was done using the average tsetse bloodmeal of 20 µl of blood per meal. For a number less than one per bloodmeal, such as 0.2 trypanosomes/bloodmeal, this equates to roughly every 5th fly getting a trypanosome, or a 20% chance of a trypanosome getting taken up by a fly.

Figure 2—figure supplement 2. Slender trypanosomes can complete the entire tsetse infection cycle, and a single parasite is sufficient for tsetse passage.

The flies were infected with either stumpy or slender trypanosomes. Stumpy trypanosomes were generated by induction of expression site attenuation (ES), or SIF-treatment (SIF). ES-attenuation consisted of seeding slender cells cultured at 5x10⁵ cells/ml and inducing the overexpression of VSG121 for 56 hr. SIF stumpy induction was performed by seeding a slender trypanosome culture at 5x10⁵ cells/ml and harvesting after 48 hr, without dilution. All slender and monomorph cultures were harvested at or below 5x10⁵ cells/ml to avoid SIF induction. Background PAD1 expression was checked for every culture. This table shows the total numbers of each infection, as seen graphically in Figure 2B. MG, midgut infection; PV, proventriculus infection; SG, salivary gland infection; TI, transmission index (number of SG infections divided by MG infections); n, number of independent fly infection experiments. Appendix 1. Original findings of the Nobel laureate, Robert Koch, in both original German and translated into English (Kleine, 1909). He found that very few trypanosomes were found in blood samples of his patients. From this, it can be roughly calculated that just 1–2 trypanosomes would be found in a tsetse bloodmeal taken from his patients, giving our data some broader perspective.