The reference transcriptome of the adult female biting midge (Culicoides sonorensis) and differential gene expression profiling during teneral, blood, and sucrose feeding conditions

Evaluation of potential reference genes in the biting midge Culicoides sonorensis for real-time quantitative PCR analyses

Studies examining differentially expressed genes and gene silencing by RNA interference (RNAi) require a set of stably expressed reference genes for accurate normalization. The biting midge Culicoides sonorensis is an important vector of livestock pathogens and is often used as a model species for biting midge research. Here, we examine the stable expression of six candidate reference genes in C. sonorensis: actin, β-tubulin, glyceraldehyde 3-phosphate dehydrogenase (GAPDH), ribosomal protein subunit (RPS) 18, vacuolar ATPase subunit A (VhaA), and elongation factor 1-beta (EF1b). Gene expression was assessed under seven conditions, including cells treated with double-stranded RNA (dsRNA), 3rd and 4th instar larvae treated with dsRNA, six developmental stages, four adult female body parts or tissue groups, and females injected with bluetongue virus or vesicular stomatitis virus. Stable gene expression was assessed using RefFinder, NormFinder, geNorm, and BestKeeper. The ranked results for each analysis tool under each condition and a comprehensive ranking for each condition are presented. The data show that optimal reference genes vary between conditions and that just two reference genes were necessary for each condition. These findings provide reference genes for use under these conditions in future studies using real-time quantitative PCR to evaluate gene expression in C. sonorensis.

In Situ Hybridization (RNAscope) Detection of Bluetongue Virus Serotypes 10 and 17 in Experimentally Co-Infected Culicoides sonorensis

Bluetongue virus (BTV) is a segmented, double-stranded RNA virus transmitted by Culicoides biting midges. Infection of domestic and wild ruminants with BTV can result in a devastating disease and significant economic losses. As a virus with a segmented genome, reassortment among the BTV serotypes that have co-infected a host may increase genetic diversity, which can alter BTV transmission dynamics and generate epizootic events. The objective of this study was to determine the extent of dissemination and characterize the tropism of BTV serotypes 10 and 17 in co-infected Culicoides sonorensis. Midges were exposed to both BTV serotypes via blood meal and processed for histologic slides 10 days after infection. An in situ hybridization approach was employed using the RNAscope platform to detect the nucleic acid segment 2 of both serotypes. Observations of the mosaic patterns in which serotypes did not often overlap suggest that co-infection at the cellular level may not be abundant with these two serotypes in C. sonorensis. This could be a consequence of superinfection exclusion. Understanding BTV co-infection and its biological consequences will add an important dimension to the modeling of viral evolution and emergence.

Wolbachia wAlbB inhibits bluetongue and epizootic hemorrhagic fever viruses in Culicoides midge cells

Culicoides midges are hematophagous insects that transmit arboviruses of veterinary importance. These viruses include bluetongue virus (BTV) and epizootic hemorrhagic fever virus (EHDV). The endosymbiont Wolbachia pipientis Hertig spreads rapidly through insect host populations and has been demonstrated to inhibit viral pathogen transmission in multiple mosquito vectors. Here, we have demonstrated a replication inhibitory effect on BTV and EHDV in a Wolbachia (wAlbB strain)-infected Culicoides sonorensis Wirth and Jones W8 cell line. Viral replication was significantly reduced by day 5 for BTV and by day 2 for EHDV as detected by real-time polymerase chain reaction (RT-qPCR) of the non-structural NS3 gene of both viruses. Evaluation of innate cellular immune responses as a cause of the inhibitory effect showed responses associated with BTV but not with EHDV infection. Wolbachia density also did not play a role in the observed pathogen inhibitory effects, and an alternative hypothesis is suggested. Applications of Wolbachia-mediated pathogen interference to impact disease transmission by Culicoides midges are discussed.

An updated review on bluetongue virus: epidemiology, pathobiology, and advances in diagnosis and control with special reference to India

Bluetongue (BT) is an economically important, non-contagious viral disease of domestic and wild ruminants. BT is caused by BT virus (BTV) and it belongs to the genus Orbivirus and family Reoviridae. BTV is transmitted by Culicoides midges and causes clinical disease in sheep, white-tailed deer, pronghorn antelope, bighorn sheep, and subclinical manifestation in cattle, goats and camelids. BT is a World Organization for Animal Health (OIE) listed multispecies disease and causes great socio-economic losses. To date, 28 serotypes of BTV have been reported worldwide and 23 serotypes have been reported from India. Transplacental transmission (TPT) and fetal abnormalities in ruminants had been reported with cell culture adopted live-attenuated vaccine strains of BTV. However, emergence of BTV-8 in Europe during 2006, confirmed TPT of wild-type/field strains of BTV. Diagnosis of BT is more important for control of disease and to ensure BTV-free trade of animals and their products. Reverse transcription polymerase chain reaction, agar gel immunodiffusion assay and competitive enzyme-linked immunosorbent assay are found to be sensitive and OIE recommended tests for diagnosis of BTV for international trade. Control measures include mass vaccination (most effective method), serological and entomological surveillance, forming restriction zones and sentinel programs. Major hindrances with control of BT in India are the presence of multiple BTV serotypes, high density of ruminant and vector populations. A pentavalent inactivated, adjuvanted vaccine is administered currently in India to control BT. Recombinant vaccines with DIVA strategies are urgently needed to combat this disease. This review is the first to summarise the seroprevalence of BTV in India for 40 years, economic impact and pathobiology.

Next-generation tools to control biting midge populations and reduce pathogen transmission

Biting midges of the genus Culicoides transmit disease-causing agents resulting in a significant economic impact on livestock industries in many parts of the world. Localized control efforts, such as removal of larval habitat or pesticide application, can be logistically difficult, expensive and ineffective if not instituted and maintained properly. With these limitations, a population-level approach to the management of Culicoides midges should be investigated as a means to replace or supplement existing control strategies. Next-generation control methods such as Wolbachia- and genetic-based population suppression and replacement are being investigated in several vector species. Here we assess the feasibility and applicability of these approaches for use against biting midges. We also discuss the technical and logistical hurdles needing to be addressed for each method to be successful, as well as emphasize the importance of addressing community engagement and involving stakeholders in the investigation and development of these approaches.

Host gill attachment causes blood-feeding by the salmon louse (Lepeophtheirus salmonis) chalimus larvae and alters parasite development and transcriptome

BACKGROUND

Blood-feeding is a common strategy among parasitizing arthropods, including the ectoparasitic salmon louse (Lepeophtheirus salmonis), feeding off its salmon host's skin and blood. Blood is rich in nutrients, among these iron and heme. These are essential molecules for the louse, yet their oxidative properties render them toxic to cells if not handled appropriately. Blood-feeding might therefore alter parasite gene expression.

METHODS

We infected Atlantic salmon with salmon louse copepodids and sampled the lice in two different experiments at day 10 and 18 post-infestation. Parasite development and presence of host blood in their intestines were determined. Lice of similar instar age sampled from body parts with differential access to blood, namely from gills versus lice from skin epidermis, were analysed for gene expression by RNA-sequencing in samples taken at day 10 for both experiments and at day 18 for one of the experiments.

RESULTS

We found that lice started feeding on blood when becoming mobile preadults if sitting on the fish body; however, they may initiate blood-feeding at the chalimus I stage if attached to gills. Lice attached to gills develop at a slower rate. By differential expression analysis, we found 355 transcripts elevated in lice sampled from gills and 202 transcripts elevated in lice sampled from skin consistent in all samplings. Genes annotated with "peptidase activity" were among the ones elevated in lice sampled from gills, while in the other group genes annotated with "phosphorylation" and "phosphatase" were pervasive. Transcripts elevated in lice sampled from gills were often genes relatively highly expressed in the louse intestine compared with other tissues, while this was not the case for transcripts elevated in lice sampled from skin. In both groups, more than half of the transcripts were from genes more highly expressed after attachment.

CONCLUSIONS

Gill settlement results in an alteration in gene expression and a premature onset of blood-feeding likely causes the parasite to develop at a slower pace.

Transcriptome Response of Female Culicoides sonorensis Biting Midges (Diptera: Ceratopogonidae) to Early Infection with Epizootic Hemorrhagic Disease Virus (EHDV-2)

Female Culicoides sonorensis biting midges are vectors of epizootic hemorrhagic disease virus (EHDV), which causes morbidity and mortality in wild and domesticated ruminants. The aims in this study were to identify key changes in female midge transcriptome profiles occurring during early infection with EHDV-2. Midges were fed either negative control bloodmeals or bloodmeals containing EHDV-2 and transcriptomes were acquired at 36 h through deep sequencing. Reads were de novo assembled into a transcriptome comprised of 18,754 unigenes. Overall, there were 2401 differentially expressed unigenes and ~60% were downregulated in response to the virus (953 up; 1448 down). Downstream Gene Ontology enrichment, KEGG pathway mapping, and manual analyses were used to identify the effect of virus ingestion at both the gene and pathway levels. Downregulated unigenes were predominantly assigned to pathways related to cell/tissue structure and integrity (actin cytoskeleton, adherens junction, focal adhesion, hippo signaling), calcium signaling, eye morphogenesis and axon guidance. Unigenes attributed to sensory functions (especially vision), behavior, learning and memory were largely downregulated. Upregulated unigenes included those coding for innate immune processes, olfaction and photoreceptor pigments. Our results suggest that midges respond to virus infection as soon as 36 h post-ingestion, and that EHDV-2 may have a significant phenotypic effect on sensory and neural tissues.

Bovine Ephemeral Fever in Asia: Recent Status and Research Gaps

Bovine ephemeral fever is an arthropod-borne viral disease affecting mainly domestic cattle and water buffalo. The etiological agent of this disease is bovine ephemeral fever virus, a member of the genus Ephemerovirus within the family Rhabdoviridae. Bovine ephemeral fever causes economic losses by a sudden drop in milk production in dairy cattle and loss of condition in beef cattle. Although mortality resulting from this disease is usually lower than 1%, it can reach 20% or even higher. Bovine ephemeral fever is distributed across many countries in Asia, Australia, the Middle East, and Africa. Prevention and control of the disease mainly relies on regular vaccination. The impact of bovine ephemeral fever on the cattle industry may be underestimated, and the introduction of bovine ephemeral fever into European countries is possible, similar to the spread of bluetongue virus and Schmallenberg virus. Research on bovine ephemeral fever remains limited and priority of investigation should be given to defining the biological vectors of this disease and identifying virulence determinants.

The genome of the biting midge Culicoides sonorensis and gene expression analyses of vector competence for bluetongue virus

BACKGROUND

The new genomic technologies have provided novel insights into the genetics of interactions between vectors, viruses and hosts, which are leading to advances in the control of arboviruses of medical importance. However, the development of tools and resources available for vectors of non-zoonotic arboviruses remains neglected. Biting midges of the genus Culicoides transmit some of the most important arboviruses of wildlife and livestock worldwide, with a global impact on economic productivity, health and welfare. The absence of a suitable reference genome has hindered genomic analyses to date in this important genus of vectors. In the present study, the genome of Culicoides sonorensis, a vector of bluetongue virus (BTV) in the USA, has been sequenced to provide the first reference genome for these vectors. In this study, we also report the use of the reference genome to perform initial transcriptomic analyses of vector competence for BTV.

RESULTS

Our analyses reveal that the genome is 189 Mb, assembled in 7974 scaffolds. Its annotation using the transcriptomic data generated in this study and in a previous study has identified 15,612 genes. Gene expression analyses of C. sonorensis females infected with BTV performed in this study revealed 165 genes that were differentially expressed between vector competent and refractory females. Two candidate genes, glutathione S-transferase (gst) and the antiviral helicase ski2, previously recognized as involved in vector competence for BTV in C. sonorensis (gst) and repressing dsRNA virus propagation (ski2), were confirmed in this study.

CONCLUSIONS

The reference genome of C. sonorensis has enabled preliminary analyses of the gene expression profiles of vector competent and refractory individuals. The genome and transcriptomes generated in this study provide suitable tools for future research on arbovirus transmission. These provide a valuable resource for these vector lineage, which diverged from other major Dipteran vector families over 200 million years ago. The genome will be a valuable source of comparative data for other important Dipteran vector families including mosquitoes (Culicidae) and sandflies (Psychodidae), and together with the transcriptomic data can yield potential targets for transgenic modification in vector control and functional studies.

Culicoides-virus interactions: infection barriers and possible factors underlying vector competence

In the United States, Culicoides midges vector arboviruses of economic importance such as Bluetongue Virus and Epizootic Hemorrhagic Disease Virus. A limited number of studies have demonstrated the complexities of midge-virus interactions, including dynamic changes in virus titer and prevalence over the infection time course. These dynamics are, in part, dictated by mesenteron infection and escape barriers. This review summarizes the overarching trends in viral titer and prevalence throughout the course of infection. Essential barriers to infection and dissemination in the midge are highlighted, along with heritable and extrinsic factors that likely contribute to these barriers. Next generation molecular tools and techniques, now available for Culicoides midges, give researchers the opportunity to test how these factors contribute to vector competence.

Functional Validation of Apoptosis Genes IAP1 and DRONC in Midgut Tissue of the Biting Midge Culicoides sonorensis (Diptera: Ceratopogonidae) by RNAi

Culicoides biting midges transmit multiple ruminant viruses, including bluetongue virus and epizootic hemorrhagic disease virus, causing significant economic burden worldwide. To further enhance current control techniques, understanding vector-virus interactions within the midge is critical. We developed previously a double-stranded RNA (dsRNA) delivery method to induce RNA interference (RNAi) for targeted gene knockdown in adult Culicoides sonorensis Wirth & Jones. Here, we confirm the C. sonorensis inhibitor of apoptosis 1 (CsIAP1) as an anti-apoptotic functional ortholog of IAP1 in Drosophila, identify the ortholog of the Drosophila initiator caspase DRONC (CsDRONC), and demonstrate that injection of dsRNA into the hemocoel can be used for targeted knockdown in the midgut in C. sonorensis. We observed CsIAP1 transcript reduction in whole midges, with highest transcript reduction in midgut tissues. IAP1knockdown (kd) resulted in pro-apoptotic caspase activation in midgut tissues. In IAP1kd midges, midgut tissue integrity and size were severely compromised. This phenotype, as well as reduced longevity, was partially reverted by co-RNAi suppression of CsDRONC and CsIAP1. Therefore, RNAi can be directed to the midgut of C. sonorensis, the initial site of virus infection, using dsRNA injection into the hemocoel. In addition, we provide evidence that the core apoptosis pathway is conserved in C. sonorensis and can be experimentally activated in the midgut to reduce longevity in C. sonorensis. This study thus paves the way for future reverse genetic analyses of midgut-virus interactions in C. sonorensis, including the putative antiviral properties of RNAi and apoptosis pathways.

Bluetongue: control, surveillance and safe movement of animals

The performance of different bluetongue control measures related to both vaccination and protection from bluetongue virus (BTV) vectors was assessed. By means of a mathematical model, it was concluded that when vaccination is applied on 95% of animals even for 3 years, bluetongue cannot be eradicated and is able to re‐emerge. Only after 5 years of vaccination, the infection may be close to the eradication levels. In the absence of vaccination, the disease can persist for several years, reaching an endemic condition with low level of prevalence of infection. Among the mechanisms for bluetongue persistence, the persistence in the wildlife, the transplacental transmission in the host, the duration of viraemia and the possible vertical transmission in vectors were assessed. The criteria of the current surveillance scheme in place in the EU for demonstration of the virus absence need revision, because it was highlighted that under the current surveillance policy bluetongue circulation might occur undetected. For the safe movement of animals, newborn ruminants from vaccinated mothers with neutralising antibodies can be considered protected against infection, although a protective titre threshold cannot be identified. The presence of colostral antibodies interferes with the vaccine immunisation in the newborn for more than 3 months after birth, whereas the minimum time after vaccination of animal to be considered immune can be up to 48 days. The knowledge about vectors ecology, mechanisms of over‐wintering and criteria for the seasonally vector‐free period was updated. Some Culicoides species are active throughout the year and an absolute vector‐free period may not exist at least in some areas in Europe. To date, there is no evidence that the use of insecticides and repellents reduce the transmission of BTV in the field, although this may reduce host/vector contact. By only using pour‐on insecticides, protection of animals is lower than the one provided by vector‐proof establishments.

This publication is linked to the following EFSA Supporting Publications article: http://onlinelibrary.wiley.com/doi/10.2903/sp.efsa.2017.EN-1182/full, http://onlinelibrary.wiley.com/doi/10.2903/sp.efsa.2017.EN-1171/full

Management of North American Culicoides Biting Midges: Current Knowledge and Research Needs

Culicoides biting midges (Diptera: Ceratopogonidae) are biological vectors of two important viruses impacting North American ruminants--bluetongue virus (BTV) and epizootic hemorrhagic disease virus (EHDV). These viruses have been identified for over 60 years in North America, but we still lack an adequate understanding of the basic biology and ecology of the confirmed vector, Culicoides sonorensis, and know even less about other putative Culicoides vector species. The major gaps in our knowledge of the biology of Culicoides midges are broad and include an understanding of the ecology of juveniles, the identity of potential alternate vector species, interactions of midges with both pathogens and vertebrates, and the effectiveness of potential control measures. Due to these broad and numerous fundamental knowledge gaps, vector biologists and livestock producers are left with few options to respond to or understand outbreaks of EHD or BT in North America, or respond to emerging or exotic Culicoides-transmitted pathogens. Here we outline current knowledge of vector ecology and control tactics for North American Culicoides species, and delineate research recommendations aimed to fill knowledge gaps.

Inducing RNA interference in the arbovirus vector, Culicoides sonorensis

Biting midges in the genus Culicoides are important vectors of arboviral diseases, including epizootic haemorrhagic disease, bluetongue and most likely Schmallenberg, which cause significant economic burdens worldwide. Research on these vectors has been hindered by the lack of a sequenced genome, the difficulty of consistent culturing of certain species and the absence of molecular techniques such as RNA interference (RNAi). Here, we report the establishment of RNAi as a research tool for the adult midge, Culicoides sonorensis. Based on previous research and transcriptome analysis, which revealed putative small interfering RNA pathway member orthologues, we hypothesized that adult C. sonorensis midges have the molecular machinery needed to perform RNA silencing. Injection of control double-stranded RNA targeting green fluorescent protein (dsGFP), into the haemocoel of 2-3-day-old adult female midges resulted in survival curves that support virus transmission. dsRNA injection targeting the newly identified C. sonorensis inhibitor of apoptosis protein 1 (CsIAP1) orthologue resulted in a 40% decrease of transcript levels and 73% shorter median survivals as compared with dsGFP-injected controls. These results reveal the conserved function of IAP1. Importantly, they also demonstrate the feasibility of RNAi by dsRNA injection in adult midges, which will greatly facilitate studies of the underlying mechanisms of vector competence in C. sonorensis.

Gene discovery and differential expression analysis of humoral immune response elements in female Culicoides sonorensis (Diptera: Ceratopogonidae)

BACKGROUND

Female Culicoides sonorensis midges (Diptera: Ceratopogonidae) are vectors of pathogens that impact livestock and wildlife in the United States. Little is known about their biology on a molecular-genetic level, including components of their immune system. Because the insect immune response is involved with important processes such as gut microbial homeostasis and vector competence, our aims were to identify components of the midge innate immune system and examine their expression profiles in response to diet across time.

METHODS

In our previous work, we de novo sequenced and analyzed the transcriptional landscape of female midges under several feeding states including teneral (unfed) and early and late time points after blood and sucrose. Here, those transcriptomes were further analyzed to identify insect innate immune orthologs, particularly humoral immune response elements. Additionally, we examined immune gene expression profiles in response to diet over time, on both a transcriptome-wide, whole-midge level and more specifically via qRTPCR analysis of antimicrobial peptide (AMP) expression in the alimentary canal.

RESULTS

We identified functional units comprising the immune deficiency (Imd), Toll and JAK/STAT pathways, including humoral factors, transmembrane receptors, signaling components, transcription factors/regulators and effectors such as AMPs. Feeding altered the expression of receptors, regulators, AMPs, prophenoloxidase and thioester-containing proteins, where blood had a greater effect than sucrose on the expression profiles of most innate immune components. qRTPCR of AMP genes showed that all five were significantly upregulated in the alimentary canal after blood feeding, possibly in response to proliferating populations of gut bacteria.

CONCLUSIONS

Identification and functional insight of humoral/innate immune components in female C. sonorensis updates our knowledge of the molecular biology of this important vector. Because diet alone influenced the expression of immune pathway components, including their effectors, subsequent study of the role of innate immunity in biological processes such as gut homeostasis and life history are being pursued. Furthermore, since the humoral response is a key contributor in gut immunity, manipulating immune gene expression will help in uncovering genetic components of vector competence, including midgut barriers to infection. The results of such studies will serve as a platform for designing novel transmission-blocking strategies.