Crataerina melbae louse flies are competent vectors of Trypanosoma sp. associated with nestling mortality in Alpine swifts (Tachymarptis melba).

Hippoboscidae diptera or louse flies are widely distributed blood-feeding ectoparasites that can transmit blood parasites to their host. Four species are recognized within the Crataerina genus, which are parasites of the bird orders Apodiformes and Passeriformes. Alpine swifts (Tachymarptis melba) are frequently parasitized by Crataerina louse flies during the nesting season. In Switzerland, an increase in Alpine swift nestling mortality has been associated with a Trypanosoma sp. infection. In this study, fixed and native specimens of Craterina louse-flies collected from Swiss Alpine swifts were analysed to reveal the species identity based on morphotypes and PCR-sequencing, and examined for the presence of Trypanosoma sp. using microscopy and PCR. Morphologically, all specimens belonged to the genus Crataerina. Based on wing types, both C. melbae and C. acutipennis, as well as mixed morphotypes, were recorded. Sequencing based on the cytochrome-oxidase 1 (COI) gene of 24 flies confirmed the presence of previously defined haplotypes of C. melbae and suggest that the morphotypes C. melbae and C. acutipennis likely constitute a single species. One louse-fly specimen was processed by scanning electron microscopy and elongated organisms resembling trypanosomes were observed on the ventral abdominal surface. Dissected intestines and abdominal surface swabs resulted positive by Trypanosoma sp. PCR. Microscopy of native specimens evidenced motile and actively multiplying trypanosomes in the midgut and rectum. All Trypanosoma sp. sequences from louse flies were identical to those obtained from tissues of infected Alpine swifts. This study suggests that C. melbae is a single species infecting Alpine swifts, regardless of the wing morphotype or sequence type, and that C. melbae acts as a competent vector for Trypanosoma sp. associated with nestling mortality. Further studies should be focused on environmentally sustainable ways to control C. melbae, with the goal to minimize transmission and the impact of trypanosomiasis in Alpine swift populations.