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Barbara Waldeck, Günter A. Schaub

• Investigating parameters influencing natural infections with \(\textit {Trypanosoma cruzi}\) via the skin, the diameters of mouthparts of different stages of triatomines vectors were measured to determine the size of the channel accessible for \(\textit {T. cruzi}\) during cutaneous infection. The mean diameters of the skin-penetrating mandibles of first to fifth instar nymphs of the vector \(\textit {Triatoma infestans}\) increased from 18 to 65 \(\mu\)m. The mean diameter in fourth instar nymphs of \(\textit {Dipetalogaster maxima}\) was 86 \(\mu\)m. Different numbers of isolated vector-derived metacyclic trypomastigotes (10–10,000) were injected intradermally into mice. Prepatent periods, parasitemia and mortality rates were compared with those of mice obtaining 10,000 metacyclic trypomastigotes that are usually present in the first drop of faeces onto the feeding wounds of fifth and fourth instar nymphs of \(\textit {T. infestans}\) and D. maxima, respectively. After injection of 50–10,000 \(\textit {T. cruzi}\), in all 42 mice the infection developed. An injection of 10 parasites induced an infection in 8 out of 15 mice. With increasing doses of parasites, prepatent periods tended to decrease. The level of parasitemia was higher after injection of the lowest dose. Except for one mouse all infected mice died. After placement of 10,000 metacyclic trypomastigotes onto the feeding wound of fifth or fourth instar nymphs of \(\textit {T. infestans}\) and \(\textit {D. maxima}\), respectively, the infection rates of the groups, prepatent periods and the levels of parasitemia of \(\textit {T. cruzi}\) in mice indicated that about 10–1,000 metacyclic trypomastigotes entered the skin via this route. For the first time, the present data emphasise the risk of an infection by infectious excreta of triatomines deposited near the feeding wound and the low number of invading parasites.