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 <i>Crataerina</i> genus, which are parasites of the bird orders Apodiformes and Passeriformes. Alpine swifts (<i>Tachymarptis melba</i>) are frequently parasitized by <i>Crataerina</i> louse flies during the nesting season. In Switzerland, an increase in Alpine swift nestling mortality has been associated with a <i>Trypanosoma</i> sp. infection. In this study, fixed and native specimens of <i>Craterina</i> 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 <i>Trypanosoma</i> sp. using microscopy and PCR. Morphologically, all specimens belonged to the genus <i>Crataerina</i>. Based on wing types, both <i>C. melbae</i> and <i>C. acutipennis,</i> as well as mixed morphotypes, were recorded. Sequencing based on the cytochrome-oxidase 1 (<i>COI</i>) gene of 24 flies confirmed the presence of previously defined haplotypes of <i>C. melbae</i> and suggest that the morphotypes <i>C. melbae</i> and <i>C. acutipennis</i> 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 <i>Trypanosoma</i> sp. PCR. Microscopy of native specimens evidenced motile and actively multiplying trypanosomes in the midgut and rectum. All <i>Trypanosoma</i> sp. sequences from louse flies were identical to those obtained from tissues of infected Alpine swifts. This study suggests that <i>C. melbae</i> is a single species infecting Alpine swifts, regardless of the wing morphotype or sequence type, and that <i>C. melbae</i> acts as a competent vector for <i>Trypanosoma</i> sp. associated with nestling mortality. Further studies should be focused on environmentally sustainable ways to control <i>C. melbae</i>, with the goal to minimize transmission and the impact of trypanosomiasis in Alpine swift populations.