Vector-borne diseases constitute a major problem affecting human and animal populations . The Haemosporidians (Phylum Apicomplexa) is a group of vector-borne parasites requiring both vertebrate and insect hosts to complete their life cycle. Avian haemosporidians are the largest group of haemosporidians by species number, which include, among others, the malaria parasite Plasmodium and the closely related genera Haemoproteus and Leucocytozoon[2, 3]. These parasites require the intervention of blood-sucking insects (Diptera) during their sexual and sporogonic phases along with an intermediate vertebrate host for the merogony phase and the development of gametocytes . Plasmodium and Haemoproteus parasites can infect a broad range of insects showing, however, a certain degree of specificity. Avian Plasmodium species are transmitted by blood-sucking mosquitoes (Culicidae), while Haemoproteus are transmitted by biting midges (Ceratopogonidae) and louse flies (Hippoboscidae) [1, 4].
Nowadays, the vast majority of avian malaria studies focus on the interaction between blood parasites and vertebrate hosts [5–8]. However, considering the large number of studies on avian haemosporidians, there is a lack of information targeting the parasites infecting insect vectors (for a recent review ). The use of molecular techniques has significantly improved the capacity to identify the networks of avian blood-parasite transmission and has led to a new era on the research of parasite–insect interactions. However, despite the recent increase in interest toward studies on insects as major vectors in the transmission of avian malaria parasites, the role of vectors on population dynamics of avian malaria parasites in natural ecosystems has been poorly studied. In addition, most studies on potential vectors have been focused on particular insect groups, mainly on mosquitoes [10–14] and black flies [15–17]. In contrast, other groups, as is the case of the biting midges Culicoides Latreille (Diptera: Ceratopogonidae) have been comparatively poorly studied.
Biting midges Culicoides are a diverse and widespread genus with more than 1400 species in the world , with at least 81 of them present in Spain . Biting midges transmit pathogens with sanitary importance, including the Bluetongue virus and the emergent Schmallenberg virus [18, 20], in addition to the majority of avian Haemoproteus (subgenus Parahaemoproteus) parasites. To fully understand the role of biting midges in the transmission network of blood parasites is essential to reveal the interactions between insects, avian hosts and the blood parasites harboured by the insects. Different studies have revealed the avian hosts of biting midges by identifying the blood meal origin [21, 22] or the insect species attracted to birds [23–25]. However, to our knowledge, only three molecular studies have screened the blood parasites harbored by biting midges. In one study, authors identified the blood parasite lineages from individual whole parous females  while, in another, Haemoproteus lineages were isolated from pools containing the head-thorax of blood-fed females . Additionally, Santiago-Alarcon et al.  isolated blood parasite lineages from the abdomen of blood-fed Culicoides. Therefore, further studies on the role of Culicoides species on the transmission of blood parasites are necessary, as this information is currently missing for the vast majority of Haemoproteus species. This is especially relevant in light of the fact that that these parasites play a key role on the health status and survival probability of avian species [26–28].
Here, using a molecular approach, we identified: i) the Haemoproteus and Plasmodium lineages harboured by the biting midge Culicoides circumscriptus (Kieffer) captured in different areas around the Doñana National Park and ii) the Haemoproteus and Plasmodium lineages infecting wild birds in the studied area in order to identify some nodes of the transmission networks connecting parasite lineages, potential insect vectors and avian hosts. We focus our study on the biting midge C. circumscriptus because this species: i) has an ornithophilic behaviour [22, 23, 25], ii) has a broad distribution covering most of Europe and North Africa being considered the most abundant ornithophilic species in southern Spain  and iii) harbours different blood parasite lineages . Altogether, these studies suggest that C. circumscriptus may play a key role on avian blood parasite transmission in the wild.