For the development of sustainable control of tick-borne diseases, insight is needed in biological factors that affect tick populations. Here, the ecological interactions among Ixodiphagus hookeri, Ixodes ricinus, and two vertebrate species groups were investigated in relation to their effects on tick-borne disease risk. In 1129 questing ticks, I. hookeri DNA was detected more often in I. ricinus nymphs (4.4%) than in larvae (0.5%) and not in adults. Therefore, we determined the infestation rate of I. hookeri in nymphs from 19 forest sites, where vertebrate, tick, and tick-borne pathogen communities had been previously quantified. We found higher than expected co-occurrence rates of I. hookeri with deer-associated Anaplasma phagocytophilum, and lower than expected rates with rodent-associated Borrelia afzelii and Neoehrlichia mikurensis. The prevalence of I. hookeri in nymphs varied between 0% and 16% and was positively correlated with the encounter probability of ungulates and the densities of all life stages of I. ricinus. Lastly, we investigated the emergence of I. hookeri from artificially fed, field-collected nymphs. Adult wasps emerged from seven of the 172 fed nymphs. From these observations, we inferred that I. hookeri is parasitizing I. ricinus larvae that are feeding on deer, rather than on rodents or in the vegetation. Since I. hookeri populations depend on deer abundance, the main propagation host of I. ricinus, these wasps have no apparent effect on tick populations. The presence of I. hookeri may directly interfere with the transmission cycle of A. phagocytophilum, but not with that of B. afzelii or N. mikurensis.

Vector‐borne diseases often originate from wildlife and can spill over into the human population. One of the most important determinants of vector‐borne disease transmission is the host preference of mosquitoes. Mosquitoes with a specialised host preference are guided by body odours to find their hosts in addition to carbon dioxide. Little is known about the role of mosquito host preference in the spillover of pathogenic agents from humans towards animals and vice versa. In the Republic of Congo, the attraction of mosquitoes to primate host odours was determined, as well as their possible role as malaria vectors, using odour‐baited traps mimicking the potential hosts of mosquitoes. Most of the mosquito species caught showed a generalistic host preference. Anopheles obscurus was the most abundant Anopheles mosquito, with a generalistic host preference observed from the olfactory response and the detection of various Plasmodium parasites. Interestingly, Culex decens showed a much higher attraction towards chimpanzee odours than to human or cow odours. Human Plasmodium parasites were observed in both human and chimpanzee blood, although not in the Anopheles mosquitoes that were collected. Understanding the role of mosquito host preference for cross‐species parasite transmission provides information that will help to determine the risk of spillover of vector‐borne diseases.

Tick-borne encephalitis virus (TBEV) is one of the most important arthropod-borne viruses in Europe and endemic in large parts of Eurasia. Each year, between 4,000 and 9,000 cases of tick-borne enc...

Usutu virus (USUV) is an emerging flavivirus that is maintained in an enzootic cycle with mosquitoes as vectors and birds as amplifying hosts. In Europe, the virus has caused mass mortality of wild birds, mainly among Common Blackbird (Turdus merula) populations. While mosquitoes are the primary vectors for USUV, Common Blackbirds and other avian species are exposed to other arthropod ectoparasites, such as ticks. It is unknown, however, if ticks can maintain and transmit USUV. We addressed this question using in vitro and in vivo experiments and field collected data. USUV replicated in IRE/CTVM19 Ixodes ricinus tick cells and in injected ticks. Moreover, I. ricinus nymphs acquired the virus via artificial membrane blood-feeding and maintained the virus for at least 70 days. Transstadial transmission of USUV from nymphs to adults was confirmed in 4.9% of the ticks. USUV disseminated from the midgut to the haemocoel, and was transmitted via the saliva of the tick during artificial membrane blood-feeding. We further explored the role of ticks by monitoring USUV in questing ticks and in ticks feeding on wild birds in the Netherlands between 2016 and 2019. In total, 622 wild birds and the Ixodes ticks they carried were tested for USUV RNA. Of these birds, 48 (7.7%) carried USUV-positive ticks. The presence of negative-sense USUV RNA in ticks, as confirmed via small RNA-sequencing, showed active virus replication. In contrast, we did not detect USUV in 15,381 questing ticks collected in 2017 and 2019. We conclude that I. ricinus can be infected with USUV and can transstadially and horizontally transmit USUV. However, in comparison to mosquito-borne transmission, the role of I. ricinus ticks in the epidemiology of USUV is expected to be minor.

Monitoring adult mosquito populations provides information that is critical for assessing risk of vector-borne disease transmission. The recently developed Suna trap was found to be a very effective trap when baited with an attractive odor blend. A modification of this trap was tested to improve its function as a tool for monitoring mosquito populations, including Anopheles coluzzii (An. gambiae sensu stricto molecular form M), Aedes aegypti, and Culex pipiens. The modified Suna trap (Suna-M) was altered by changing the position of the catch bag and the inclusion of a holding chamber in attempts to increase trapping efficacy and enhance the survival of mosquitoes. Each adaptation was tested in a dual-choice setup in a climate-controlled room against the original Suna trap and against 4 standard monitoring methods: the BG-sentinel (BGS), Centers for Disease Control and Prevention (CDC) light trap, Mosquito Magnet X (MM-X) trap, and human landing catch (HLC). No differences in trapping efficacy were observed between the original Suna trap and modified version; however, a version in which the funnel was extended with a box and supplemented with moistened cotton wool increased mosquito survival from 6.5% to 78.0% over 24 h. The HLC and BGS trap outperformed the Suna-M trap, whereas the MM-X and commonly used CDC light trap performed significantly less well than the Suna-M trap in the dual-choice setup. The performance of the Suna-M trap equaled the performance of the original Suna trap and could therefore be used for monitoring purposes. Although the HLC and BGS trap achieved higher catch sizes, the Suna trap has the advantage that it is standardized, does not place humans at risk, and is weather resistant. Field studies should be conducted to confirm that the Suna-M trap, baited with the odor blend, is an efficient and standardized tool to measure both indoor and outdoor disease transmission risk for a range of vector-borne diseases.

Ticks are obligate blood-feeding parasites associated with a huge diversity of diseases globally. The hard tick Ixodes ricinus is the key vector of Lyme borreliosis and tick-borne encephalitis in Western Eurasia. Ixodes ticks have large and repetitive genomes that are not yet well characterized. Here we generate two high-quality I . ricinus genome assemblies, with haploid genome sizes of approximately 2.15 Gbp. We find transposable elements comprise at least 69% of the two I. ricinus genomes, amongst the highest proportions found in animals. The transposable elements in ticks are highly diverse and novel, so we constructed a repeat library for ticks using our I . ricinus genomes and the genome of I . scapularis , another major tick vector of Lyme borreliosis. To understand the impact of transposable elements on tick genomes we compared their accumulation in the two Ixodes sister species. We find transposable elements in these two species to be drivers of genome evolution in ticks. The I . ricinus genome assemblies and our tick repeat library will be valuable resources for biological insights into this important ectoparasite. Our findings highlight that further research into the impact of transposable elements on the genomes of blood-feeding parasites is required.