Investigation of diel activity of Culicoides biting midges (Diptera: Ceratopogonidae) in the United Kingdom by using a vehicle-mounted trap

Arthropod-Borne Viruses of Human and Animal Importance: Overwintering in Temperate Regions of Europe during an Era of Climate Change

The past three decades have seen an increasing number of emerging arthropod-borne viruses in temperate regions This process is ongoing, driven by human activities such as inter-continental travel, combined with the parallel emergence of invasive arthropods and an underlying change in climate that can increase the risk of virus transmission and persistence. In addition, natural events such as bird migration can introduce viruses to new regions. Despite the apparent regularity of virus emergence, arthropod-borne viruses circulating in temperate regions face the challenge of the late autumn and winter months where the arthropod vector is inactive. Viruses therefore need mechanisms to overwinter or they will fail to establish in temperate zones. Prolonged survival of arthropod-borne viruses within the environment, outside of both vertebrate host and arthropod vector, is not thought to occur and therefore is unlikely to contribute to overwintering in temperate zones. One potential mechanism is continued infection of a vertebrate host. However, infection is generally acute, with the host either dying or producing an effective immune response that rapidly clears the virus. There are few exceptions to this, although prolonged infection associated with orbiviruses such as bluetongue virus occurs in certain mammals, and viraemic vertebrate hosts therefore can, in certain circumstances, provide a route for long-term viral persistence in the absence of active vectors. Alternatively, a virus can persist in the arthropod vector as a mechanism for overwintering. However, this is entirely dependent on the ecology of the vector itself and can be influenced by changes in the climate during the winter months. This review considers the mechanisms for virus overwintering in several key arthropod vectors in temperate areas. We also consider how this will be influenced in a warming climate.

Seasonal change and influence of environmental variables on host-seeking activity of the biting midge Culicoides sonorensis at a southern California dairy, USA

BACKGROUND

As a primary vector of bluetongue virus (BTV) in the US, seasonal abundance and diel flight activity of Culicoides sonorensis has been documented, but few studies have examined how time of host-seeking activity is impacted by environmental factors. This knowledge is essential for interpreting surveillance data and modeling pathogen transmission risk.

METHODS

The diel host-seeking activity of C. sonorensis was studied on a California dairy over 3 years using a time-segregated trap baited with CO2. The relationship between environmental variables and diel host-seeking activity (start, peak, and duration of activity) of C. sonorensis was evaluated using multiple linear regression. Fisher's exact test and paired-sample z-test were used to evaluate the seasonal difference and parity difference on diel host-seeking activity.

RESULTS

Host-seeking by C. sonorensis began and reached an activity peak before sunset at a higher frequency during colder months relative to warmer months. The time that host-seeking activity occurred was associated low and high daily temperature as well as wind speed at sunset. Colder temperatures and a greater diurnal temperature range were associated with an earlier peak in host-seeking. Higher wind speeds at sunset were associated with a delayed peak in host-seeking and a shortened duration of host-seeking. Parous midges reached peak host-seeking activity slightly later than nulliparous midges, possibly because of the need for oviposition by gravid females before returning to host-seeking.

CONCLUSIONS

This study demonstrates that during colder months C. sonorensis initiates host-seeking and reaches peak host-seeking activity earlier relative to sunset, often even before sunset, compared to warmer months. Therefore, the commonly used UV light-baited traps are ineffective for midge surveillance before sunset. Based on this study, surveillance methods that do not rely on light trapping would provide a more accurate estimate of host-biting risk across seasons. The association of environmental factors to host-seeking shown in this study can be used to improve modeling or prediction of host-seeking activity. This study identified diurnal temperature range as associated with host-seeking activity, suggesting that Culicoides may respond to a rapidly decreasing temperature by shifting to an earlier host-seeking time, though this association needs further study.

Thermal preference of Culicoides biting midges in laboratory and semi-field settings

Biting midges of the genus Culicoides (Diptera: Ceratopogonidae) are hematophagous insects, and some species can transmit a plethora of pathogens, e.g., bluetongue virus and African horse sickness virus, that mainly affect animals. The transmission of vector-borne pathogens is strongly temperature dependent, and recent studies pointed to the importance of including microclimatic data when modelling disease spread. However, little is known about the preferred temperature of biting midges. The present study addressed the thermal selection of field-caught Culicoides with two experiments. In a laboratory setup, sugar-fed or blood-fed midges were video tracked for 15 min while moving inside a 60 × 30 × 4 cm setup with a 15-25 °C temperature gradient. Culicoides spent over double the time in the coldest zone of the setup compared to the warmest one. This cold selection was significantly stronger for sugar-fed individuals. Calculated preferred temperatures were 18.3 °C and 18.9 °C for sugar-fed and blood-fed Culicoides, respectively. The effect of temperature on walking speed was significant but weak, indicating that their skewed distribution results from preference and not cold trapping. A second experiment consisted of a two-way-choice-setup, performed in a 90 × 45 × 45 cm net cage, placed outdoors in a sheltered environment. Two UV LED CDC traps were placed inside the setup, and a mean temperature difference of 2.2 °C was created between the two traps. Hundred-fifty Culicoides were released per experiment. Recapture rates were negatively correlated with ambient temperature and were on average three times higher in the cooled trap. The higher prevalence of biting midges in cooler environments influences fitness and ability to transmit pathogens and should be considered in models that predict Culicoides disease transmission.

Culicoides (Diptera: Ceratopogonidae) Abundance Is Influenced by Livestock Host Species and Distance to Hosts at the Micro Landscape Scale

The vector/host ratio and host preference are important parameters for the modelling of vector-borne livestock diseases. It can be anticipated that Culicoides abundance is not homogeneously distributed in the landscape. We investigated the influence of host species (dairy cow, sheep, and a light-trap (LT) as a surrogate host) and distance of measurement to hosts on Culicoides abundance using a randomized block-design with 12 measuring days and seven 3-min aerial sweep-netting sessions per whole hour at three distances to the host (0, 10, and 25 m), from five hours before to and including one hour after sunset. Dairy cows were found to be a far stronger attractor of Culicoides midges than sheep, while both hosts were far stronger attractors of midges than the LT. Culicoides abundance declined significantly with increasing distance from the livestock hosts; this phenomenon was much stronger for dairy cows than for ewes. In contrast, Culicoides abundance increased with increasing distance from the LT, pin-pointing the apparent shortcomings of the LT as a surrogate host to lure midges. Our data indicate that livestock host species and the distance from these hosts have a profound effect on Culicoides abundance in the landscape.

The impact of strategic ventilation adjustments on stress responses in horses housed full-time in a vector-protected barn during the African horse sickness outbreak in Thailand

The severe outbreak of African horse sickness (AHS) in Thailand has forced horses to reside full-time inside barns that are covered by a small mesh net to prevent minuscule AHS insect vectors from gaining access. However, housing in the net-covered barn induces stress in horses, which compromises their welfare. Implementing strategic airflow adjustment while retaining the vector-protection characteristics has been proposed to help alleviate this problem. The present study aimed to investigate the effect of strategic ventilation adjustment on blood cortisol levels, heart rate and behaviour in horses in a vector-protected barn. Nine horses underwent two sequential stabling conditions: vector-protected barn housing and housing in a barn in which the air ventilation was explicitly adjusted. Heart rate was higher in the afternoon in horses housed in the barn without ventilation adjustment, whereas no change was observed in the barn with ventilation adjustment. The vector-protected housing increased the horses' behavioural scores. Blood cortisol level declined over time, and an earlier decrease was detected at 1400h in the barn with ventilation adjustment. Although airflow adjustment did not appear to statistically alter the stress response in horses during housing in the vector-protected barn, an earlier decline in cortisol level alongside an unchanged heart rate in horses during the day may indicate the positive impact of ventilation adjustment within the vector-protected barn. With limited options to reduce stress or discomfort in horses, this strategic protocol could, at least in part, be applied to managing horses' welfare during the AHS outbreak.

Comparison of surveillance trapping methods to monitor Culicoides biting midge activity in Trinidad, West Indies

Culicoides biting midges (Diptera: Ceratopogonidae) are biting nuisances and arbovirus vectors of both public health and veterinary significance in Trinidad. We compared sampling methods to define the behaviour and bionomics of adult Culicoides populations at a commercial dairy goat farm. Three static trap designs were compared: (a) Centre for Disease Control (CDC) downdraft UV trap; (b) CDC trap with an incandescent bulb and (c) CDC trap with semiochemical lure consisting of R-(-)-1-octen-3-ol and CO₂ (no bulb). Sweep netting was used to define diel periodicity. A total of 30,701 biting midges were collected using static traps, dominated by female Culicoides furens (>70% of trap collections across all three designs). There was no significant difference in the Margalef's index between the three traps; however, trap designs A and C collected a significantly greater number of individuals than trap B, and trap C gained highest species richness. The greatest species richness and abundance of Culicoides collected by sweep net was observed between 6:00 and 6:15 pm and notable differences in the crepuscular activity pattern of several species were identified. Comparative data on Culicoides species richness, abundance, sex and reproductive status is discussed and can be used to improve surveillance strategies, research designs and risk management.

Stimuli Followed by Avian Malaria Vectors in Host-Seeking Behaviour

Vector-borne infectious diseases (e.g., malaria, dengue fever, and yellow fever) result from a parasite transmitted to humans and other animals by blood-feeding arthropods. They are major contributors to the global disease burden, as they account for nearly a fifth of all infectious diseases worldwide. The interaction between vectors and their hosts plays a key role driving vector-borne disease transmission. Therefore, identifying factors governing host selection by blood-feeding insects is essential to understand the transmission dynamics of vector-borne diseases. Here, we review published information on the physical and chemical stimuli (acoustic, visual, olfactory, moisture and thermal cues) used by mosquitoes and other haemosporidian vectors to detect their vertebrate hosts. We mainly focus on studies on avian malaria and related haemosporidian parasites since this animal model has historically provided important advances in our understanding on ecological and evolutionary process ruling vector-borne disease dynamics and transmission. We also present relevant studies analysing the capacity of feather and skin symbiotic bacteria in the production of volatile compounds with vector attractant properties. Furthermore, we review the role of uropygial secretions and symbiotic bacteria in bird-insect vector interactions. In addition, we present investigations examining the alterations induced by haemosporidian parasites on their arthropod vector and vertebrate host to enhance parasite transmission. Finally, we propose future lines of research for designing successful vector control strategies and for infectious disease management.

The flying activity of biting midges (Ceratopogonidae: Culicoides) in Verkiai Regional Park, southeastern Lithuania

Culicoides biting midges are small dipterous insects (Diptera: Ceratopogonidae) which are known to be vectors of arboviruses, bacteria, protozoan and helminth parasites that can cause disease and mortality in livestock and poultry globally. Detailed knowledge of the Culicoides species composition and biology is essential to assess the risk of the introduction and transmission of pathogens. The aim of this study was to obtain data on Culicoides species composition and flying activity in southeastern Lithuania and to determine the meteorological variables related to the abundance of Culicoides biting midges. Biting midges were collected in Verkiai Regional Park, southeastern Lithuania, using an Onderstepoort trap once a week from April to October 2016 and 2018, and from April to July 2019; 7332 Culicoides females belonging to 22 species were identified. Both morphology and DNA barcoding were used for identification. The number of specimens trapped was highest for the Obsoletus Group, followed by Culicoides kibunensis and Culicoides impunctatus. The highest relative abundance and diversity of biting midges were found in May and June. The number of trapped biting midges correlated positively with the mean air temperature. The first biting midges in spring were caught when the mean daily temperature rose higher than 10 °C. No Culicoides were detected when the air temperature dropped below 5 °C in autumn. Wind speed and air humidity had no statistically significant effect on Culicoides abundance.

Thermal limits for flight activity of field-collected Culicoides in the United Kingdom defined under laboratory conditions

BACKGROUND

Culicoides biting midges (Diptera: Ceratopogonidae) are biological vectors of internationally important arboviruses and inflict biting nuisance on humans, companion animals and livestock. In temperate regions, transmission of arboviruses is limited by temperature thresholds, in both replication and dissemination of arboviruses within the vector and in the flight activity of adult Culicoides. This study aims to determine the cold-temperature thresholds for flight activity of Culicoides from the UK under laboratory conditions.

METHODS

Over 18,000 Culicoides adults were collected from the field using 4 W down-draught miniature ultraviolet Centers for Disease Control traps. Populations of Culicoides were sampled at three different geographical locations within the UK during the summer months and again in the autumn at one geographical location. Activity at constant temperatures was assessed using a bioassay that detected movement of adult Culicoides towards an ultraviolet light source over a 24-h period.

RESULTS

The proportion of active adult Culicoides increased with temperature but cold temperature thresholds for activity varied significantly according to collection season and location. Populations dominated by the subgenus Avaritia collected in South East England had a lower activity threshold temperature in the autumn (4 °C) compared with populations collected in the summer (10 °C). Within the subgenus Avaritia, Culicoides scoticus was significantly more active across all temperatures tested than Culicoides obsoletus within the experimental setup. Populations of Culicoides impunctatus collected in the North East of England were only active once temperatures reached 14 °C. Preliminary data suggested flight activity of the subgenus Avaritia does not differ between populations in South East England and those in the Scottish Borders.

CONCLUSIONS

These findings demonstrate seasonal changes in temperature thresholds for flight and across different populations of Culicoides. These data, alongside that defining thresholds for virus replication within Culicoides, provide a primary tool for risk assessment of arbovirus transmission in temperate regions. In addition, the study also provides a comparison with thermal limits derived directly from light-suction trapping data, which is currently used as the main method to define adult Culicoides activity during surveillance.

Sampling Considerations for Adult and Immature Culicoides (Diptera: Ceratopogonidae)

Developing sampling programs for Culicoides can be challenging due to variation in ecology and behavior of the numerous species as well as their broad distributions and habitats. In this paper, we emphasize the need to clearly define research goals to select appropriate sampling methods. This includes not just the choice of sampling device, but also choice of attractant, site, number of traps per site, the duration and frequency of sampling, and the number of traps per unit area. Animal-baited trapping using enclosure traps and direct animal aspiration is more labor-intensive but yields information on species attracted to specific hosts as well as their biting rates. Sampling immatures is discussed with respect to choosing collection sites in semiaquatic mud, soil, and rich organic habitats. Sorting and extracting larvae using emergence traps, flotation, and Berlese funnels is also discussed.

Influence of light and kairomone baiting systems on trap collections of biting midges in southern Sweden

Effective surveillance is essential for protecting livestock from Culicoides biting midges and the viruses they transmit. The objective of this study was to determine how the baiting system used in traps (UV, incandescent light, incandescent light with CO₂ , and incandescent light with CO₂ and 1-octen-3-ol) influences estimates of midge population abundance, parity, and diel activity. This was achieved through a standardized trapping protocol conducted in three habitats in Sweden. UV light traps caught the most Culicoides species and more C. obsoletus complex females than incandescent light traps. Traps baited with CO₂ plus 1-octen-3-ol caught more female C. impunctatus than incandescent light traps. No consistent effect of bait type was found on C. obsoletus parity rate, as estimated from the proportion of midges with presence or absence of pigmentation. Midge activity, as reflected by trap catches, peaked between -3 h and +3 h relative to sunset, with UV traps catching significantly more female C. obsoletus complex and C. impunctatus at and after sunset than before sunset. We conclude that baiting system can influence biting midge collections, even using identical traps. Effective surveillance may require more than one bait type and kairomones to attract species that do not feed exclusively on cattle.

Evaluating the Addition of CO2 to Black Light Traps for Sampling Culicoides (Diptera: Ceratopogonidae) in Alberta

We examined the effect of augmenting black light trap with CO2 at two release rates for monitoring and surveillance of Culicoides at northern latitudes. Species composition was highly correlated among black light (BL) traps with no CO2 (BL0 traps) and traps augmented with 300 and 1000 ml/min CO2 (BL300 and BL1000 traps, respectively); however, traps augmented with CO2 captured one to three more rare species than BL0 traps and can detect rare species that might be missed if BL0 traps were used. Addition of CO2 increased the number of individuals captured from 1.4 to 38.5-fold for BL300 traps and from 1.5 to 153.5-fold for BL1000 traps. This advantage may be lost in areas with low abundance as might be expected at the limits of species. The daily probability of detecting a species was greater for traps augmented with CO2, but this can be compensated for by operating BL0 traps for sufficient time. The association between numbers captured by BL0 and BL300 and BL1000 traps also suggests that BL0 traps may provide useful measures of relative abundance for Culicoides sonorensis, Culicoides davisi, and Culicoides riethi, again providing the BL0 traps are left to operate for some period. Measurements of diel activity were influenced by trap type only for a single species and were primarily determined by solar radiation, temperature, and windspeed. Overall, the use of BL0 only traps for widescale surveys is supported, especially given their convenience.

The influence of temperature and humidity on the flight activity of Culicoides imicola both under laboratory and field conditions

BACKGROUND

Insight into the factors that regulate the circadian host-seeking flight activity of Culicoides vectors (Diptera: Ceratopogonidae) will be of importance to assess the risk of transmission of Culicoides-borne pathogens. This study aimed to determine the impact of temperature and humidity on the flight activity of Culicoides imicola Kieffer, and other livestock associated Culicoides species, under both laboratory and field conditions.

METHODS

Batches of 500 field-collected C. imicola females were acclimatized at a predetermined range of temperatures (10-29 °C) and relative humidity (34-85%). After acclimatization, these females, prompted by a light source, were allowed to escape through a transparent plastic funnel into a paper cup, where they were counted after an hour. Flight activity under field conditions was determined seasonally by hourly light trap collections done overnight at four sites near cattle.

RESULTS

Experiments conducted at various test conditions in the laboratory indicated that flight activity started at 13 °C. Peak in activity was observed between 16 °C to 18 °C, and temperatures above 20 °C seemingly inhibit flight. Under field conditions, a peak in numbers collected was observed immediately after sunset. With mean nocturnal temperatures below 19 °C, more than 74% of the Culicoides were collected within two to three hours after sunset. With mean nocturnal temperature above 19 °C, the peak in numbers at sunset was sustained until after midnight, with somewhat higher numbers collected after midnight once temperatures dropped below 20 °C. No peak in numbers was observed at dawn. Although very low numbers were collected during the day, which partly may have been a result of the collecting method, Culicoides were present throughout periods of 24 hours. Humidity seemed to play a minor role in the regulation of flight activity.

CONCLUSIONS

Abundance and species diversity results as obtained in this study indicated a high level of risk of virus transmission in the first hours following sunset. A strong relationship was found between host-seeking activity, and hence trap efficiency, and within limits, temperature. Light traps primarily measure flight activity and may as such underestimate adult abundance of C. imicola if deployed at temperatures outside thresholds of 16-20 °C.

The Dark Side of Light Traps

Light-baited suction traps are one of the most widely used tools for vector surveillance. Their popularity stems from ease of use even in remote locations, range and abundance of species caught, and low cost. The availability of smaller, portable models, like the CDC miniature light trap, have further increased their ubiquity in entomological field studies. However, when researchers have looked, light trap collections are usually biased in ways that may affect data interpretation for epidemiological studies. If used alone, light traps may fail to collect important or infected vectors, and light traps are inefficient or ineffective when competing ambient light is present. In this article, we discuss these biases and limitations in terms of their effect on collection efficiency, population data, and pathogen detection. While light trap data certainly have a purpose, an over-reliance on light trapping risks drawing false conclusions about vector populations and vector-borne disease epidemiology. These concerns are especially troubling when light trap data are used to inform policy decisions meant to protect human and animal health. Particularly when a species' response to light is unknown or poorly characterized, light traps should be used in conjunction with supplemental sampling methods. Researchers conducting vector surveillance field studies should carefully consider their study design and objectives when deciding on a trapping method or methods, and specifically endeavor to understand the limitations of their data. Only then can researchers take advantage of the best attributes of light traps while avoiding their dark side.

Challenges and priorities for modelling livestock health and pathogens in the context of climate change

Climate change has the potential to impair livestock health, with consequences for animal welfare, productivity, greenhouse gas emissions, and human livelihoods and health. Modelling has an important role in assessing the impacts of climate change on livestock systems and the efficacy of potential adaptation strategies, to support decision making for more efficient, resilient and sustainable production. However, a coherent set of challenges and research priorities for modelling livestock health and pathogens under climate change has not previously been available. To identify such challenges and priorities, researchers from across Europe were engaged in a horizon-scanning study, involving workshop and questionnaire based exercises and focussed literature reviews. Eighteen key challenges were identified and grouped into six categories based on subject-specific and capacity building requirements. Across a number of challenges, the need for inventories relating model types to different applications (e.g. the pathogen species, region, scale of focus and purpose to which they can be applied) was identified, in order to identify gaps in capability in relation to the impacts of climate change on animal health. The need for collaboration and learning across disciplines was highlighted in several challenges, e.g. to better understand and model complex ecological interactions between pathogens, vectors, wildlife hosts and livestock in the context of climate change. Collaboration between socio-economic and biophysical disciplines was seen as important for better engagement with stakeholders and for improved modelling of the costs and benefits of poor livestock health. The need for more comprehensive validation of empirical relationships, for harmonising terminology and measurements, and for building capacity for under-researched nations, systems and health problems indicated the importance of joined up approaches across nations. The challenges and priorities identified can help focus the development of modelling capacity and future research structures in this vital field. Well-funded networks capable of managing the long-term development of shared resources are required in order to create a cohesive modelling community equipped to tackle the complex challenges of climate change.

A comparison of four light traps for collecting Culicoides biting midges

Epidemiological analyses of vector-associated diseases such as bluetongue (BT), African horse sickness, or epizootic hemorrhagic disease require substantiated data on the species diversity and activity patterns of vector species. To this end, Spain and Italy implemented extensive Culicoides biting midge monitoring programs since 2000, as several other countries did after the arrival of BT in northern Europe in 2006. The seasonal occurrence, spatial distribution, and abundance of Culicoides species, as the major results of such monitoring programs, are used as parameters for assessing the risk of virus introduction and transmission in a given area. However, the quality of entomological monitoring results fundamentally depends on the collection techniques. In this publication, we describe a Latin Square design trial carried out in Germany under field conditions in 2009/2010 to compare the efficacy of four commonly used light baited/suction traps in collecting Culicoides. A total of 2651 Culicoides were caught over 18 nights. In both years, the Onderstepoort and BG-Sentinel traps caught significantly more Culicoides than the Rieb and the CDC trap. Most specimens were caught by the Onderstepoort trap (1246, i.e., 76 % in 2009 and 819, i.e., 82 % in 2010). Most were classified as midges of the Culicoides obsoletus group.

A comparison of commercial light-emitting diode baited suction traps for surveillance of Culicoides in northern Europe

Background

The response of Culicoides biting midges (Diptera: Ceratopogonidae) to artificial light sources has led to the use of light-suction traps in surveillance programmes. Recent integration of light emitting diodes (LED) in traps improves flexibility in trapping through reduced power requirements and also allows the wavelength of light used for trapping to be customized. This study investigates the responses of Culicoides to LED light-suction traps emitting different wavelengths of light to make recommendations for use in surveillance.

Methods

The abundance and diversity of Culicoides collected using commercially available traps fitted with Light Emitting Diode (LED) platforms emitting ultraviolet (UV) (390 nm wavelength), blue (430 nm), green (570 nm), yellow (590 nm), red (660 nm) or white light (425 nm – 750 nm with peaks at 450 nm and 580 nm) were compared. A Centre for Disease Control (CDC) UV light-suction trap was also included within the experimental design which was fitted with a 4 watt UV tube (320-420 nm). Generalised linear models with negative binomial error structure and log-link function were used to compare trap abundance according to LED colour, meteorological conditions and seasonality.

Results

The experiment was conducted over 49 nights with 42,766 Culicoides caught in 329 collections. Culicoides obsoletus Meigen and Culicoides scoticus Downes and Kettle responded indiscriminately to all wavelengths of LED used with the exception of red which was significantly less attractive. In contrast, Culicoides dewulfi Goetghebuer and Culicoides pulicaris Linnaeus were found in significantly greater numbers in the green LED trap than in the UV LED trap. The LED traps collected significantly fewer Culicoides than the standard CDC UV light-suction trap.

Conclusions

Catches of Culicoides were reduced in LED traps when compared to the standard CDC UV trap, however, their reduced power requirement and small size fulfils a requirement for trapping in logistically challenging areas or where many traps are deployed at a single site. Future work should combine light wavelengths to improve trapping sensitivity and potentially enable direct comparisons with collections from hosts, although this may ultimately require different forms of baits to be developed.

Electronic supplementary material

The online version of this article (doi:10.1186/s13071-015-0846-x) contains supplementary material, which is available to authorized users.

Bionomics of temperate and tropical Culicoides midges: knowledge gaps and consequences for transmission of Culicoides-borne viruses

Culicoides midges are abundant hematophagous flies that vector arboviruses of veterinary and medical importance. Dramatic changes in the epidemiology of Culicoides-borne arboviruses have occurred since 1998, including the emergence of exotic viruses in northern temperate regions, increases in global disease incidence, and enhanced virus diversity in tropical zones. Drivers may include changes in climate, land use, trade, and animal husbandry. New Culicoides species and new wild reservoir hosts have been implicated in transmission, highlighting the dynamic nature of pathogen-vector-host interactions. Focusing on potential vector species worldwide and key elements of vectorial capacity, we review the sensitivity of Culicoides life cycles to abiotic and biotic factors. We consider implications for designing control measures and understanding impacts of environmental change in different ecological contexts. Critical geographical, biological, and taxonomic knowledge gaps are prioritized. Recent developments in genomics and mathematical modeling may enhance ecological understanding of these complex arbovirus systems.

An unrecognized species of the Culicoides obsoletus complex feeding on livestock in The Netherlands

In studies on Culicoides attacking livestock in the Netherlands, we chanced upon a species of the Obsoletus complex that we do not recognize, but whose dark wing pattern is distinctive. Nine cytochrome c oxidase (CO1) sequences of our so-called 'dark obsoletus' support its status as a separate species, the sequences differing significantly from those representing Culicoides obsoletus (Meigen) (90-91% homology) and Culicoides scoticus Downes & Kettle (87-88% homology). In the last decade, several research groups in Europe have encountered 'mystery species' related to C. obsoletus and in some instances have made their sequences for various genetic loci available in GenBank. These include a CO1 series submitted from Sweden in 2012 (annotated as 'obsoletus 01, 02, or 03 MA-2012') and of which some share a 99% identity with our sequences for 'dark obsoletus'. Without doubt, the series from the Netherlands, along with a portion of the Swedish submissions, together represent a single species ('dark obsoletus'). Whether this species is referable to the Russian Culicoides gornostaevae Mirzaeva recorded recently from Norway, Sweden and Poland, and based solely upon the external morphology of the male, is not clear. The presence in Western Europe of multiple undescribed species related to C. obsoletus means that the taxonomy of this important vector complex is not fully resolved; consequently, we know little about these cryptic species with regard to seasonality, geographic range and host preference. This is undesirable given that Culicoides-borne arboviruses causing disease in livestock are moving more regularly out of the tropics and spreading north into temperate latitudes.

Long-distance aerial dispersal modelling of Culicoides biting midges: case studies of incursions into Australia

BACKGROUND

Previous studies investigating long-distance, wind-borne dispersal of Culicoides have utilised outbreaks of clinical disease (passive surveillance) to assess the relationship between incursion and dispersal event. In this study, species of exotic Culicoides and isolates of novel bluetongue viruses, collected as part of an active arbovirus surveillance program, were used for the first time to assess dispersal into an endemic region.

RESULTS

A plausible dispersal event was determined for five of the six cases examined. These include exotic Culicoides specimens for which a possible dispersal event was identified within the range of two days--three weeks prior to their collection and novel bluetongue viruses for which a dispersal event was identified between one week and two months prior to their detection in cattle. The source location varied, but ranged from Lombok, in eastern Indonesia, to Timor-Leste and southern Papua New Guinea.

CONCLUSIONS

Where bluetongue virus is endemic, the concurrent use of an atmospheric dispersal model alongside existing arbovirus and Culicoides surveillance may help guide the strategic use of limited surveillance resources as well as contribute to continued model validation and refinement. Further, the value of active surveillance systems in evaluating models for long-distance dispersal is highlighted, particularly in endemic regions where knowledge of background virus and vector status is beneficial.

Towards the PCR-based identification of Palaearctic Culicoides biting midges (Diptera: Ceratopogonidae): results from an international ring trial targeting four species of the subgenus Avaritia

Background

Biting midges of the genus Culicoides (Diptera: Ceratopogonidae) are biological vectors of internationally important arboviruses. To understand the role of Culicoides in the transmission of these viruses, it is essential to correctly identify the species involved. Within the western Palaearctic region, the main suspected vector species, C. obsoletus, C. scoticus, C. dewulfi and C. chiopterus, have similar wing patterns, which makes it difficult to separate and identify them correctly.

Methods

In this study, designed as an inter-laboratory ring trial with twelve partners from Europe and North Africa, we assess four PCR-based assays which are used routinely to differentiate the four species of Culicoides listed above. The assays based on mitochondrial or ribosomal DNA or microarray hybridisation were tested using aliquots of Culicoides DNA (extracted using commercial kits), crude lysates of ground specimens and whole Culicoides (265 individuals), and non-Culicoides Ceratopogonidae (13 individuals) collected from across Europe.

Results

A total of 800 molecular assays were implemented. The in-house assays functioned effectively, although specificity and sensitivity varied according to the molecular marker and DNA extraction method used. The Obsoletus group specificity was overall high (95-99%) while the sensitivity varied greatly (59.6-100%). DNA extraction methods impacted the sensitivity of the assays as well as the type of sample used as template for the DNA extraction.

Conclusions

The results are discussed in terms of current use of species diagnostic assays and the future development of molecular tools for the rapid differentiation of cryptic Culicoides species.

Modelling spatio-temporal patterns of long-distance Culicoides dispersal into northern Australia

Novel arboviruses, including new serotypes of bluetongue virus, are isolated intermittently from cattle and insects in northern Australia. These viruses are thought to be introduced via windborne dispersal of Culicoides from neighbouring land masses to the north. We used the HYSPLIT particle dispersal model to simulate the spatio-temporal patterns of Culicoides dispersal into northern Australia from nine putative source sites across Indonesia, Timor-Leste and Papua New Guinea. Simulated dispersal was found to be possible from each site, with the islands of Timor and Sumba highlighted as the likely principal sources and February the predominant month of dispersal. The results of this study define the likely spatial extent of the source and arrival regions, the relative frequency of dispersal from the putative sources and the temporal nature of seasonal winds from source sites into arrival regions. Importantly, the methodology and results may be applicable to other insect and pathogen incursions into northern Australia.

Host-seeking activity of bluetongue virus vectors: endo/exophagy and circadian rhythm of Culicoides in Western Europe

Feeding success of free-living hematophagous insects depends on their ability to be active when hosts are available and to reach places where hosts are accessible. When the hematophagous insect is a vector of pathogens, determining the components of host-seeking behavior is of primary interest for the assessment of transmission risk. Our aim was to describe endo/exophagy and circadian host-seeking activity of Palaearctic Culicoides species, which are major biting pests and arbovirus vectors, using drop traps and suction traps baited with four sheep, as bluetongue virus hosts. Collections were carried out in the field, a largely-open stable and an enclosed stable during six collection periods of 24 hours in April/May, in late June and in September/October 2010 in western France. A total of 986 Culicoides belonging to 13 species, mainly C. brunnicans and C. obsoletus, was collected on animal baits. Culicoides brunnicans was clearly exophagic, whereas C. obsoletus was able to enter stables. Culicoides brunnicans exhibited a bimodal pattern of host-seeking activity with peaks just after sunrise and sunset. Culicoides obsoletus was active before sunset in spring and autumn and after sunset in summer, thus illustrating influence of other parameters than light, especially temperature. Description of host-seeking behaviors allowed us to discuss control strategies for transmission of Culicoides-borne pathogens, such as bluetongue virus. However, practical vector-control recommendations are difficult to provide because of the variation in the degree of endophagy and time of host-seeking activity.

A survey of Culicoides developmental sites on a farm in northern Spain, with a brief review of immature habitats of European species

Culicoides species (Diptera: Ceratopogonidae) belonging to the Obsoletus and Pulicaris groups are considered to be the main vectors of bluetongue virus (BTV) in non Mediterranean Europe. Selected terrestrial microhabitats (n=17) on a farm in northern Spain were sampled repeatedly over a year-long period and characterized for use by Culicoides species for immature development. Concurrent use of CDC light traps showed the presence of 37 species and 66,575 specimens of adult Culicoides. A total of 28 species and 11,396 individuals emerged from laboratory-maintained soil samples. Culicoides obsoletus and Culicoides scoticus (pooled as Obsoletus complex) were particularly abundant (comprising 58.6% and 74.5% of the total collections in light traps and emergence traps respectively). Potential key vectors of animal viruses (such as BTV) were found in two main terrestrial types of microhabitats. In the case of C. obsoletus, different types of manure (old and composted manure, manure mixed with organic matter, and fresh manure) produced most of the specimens. In contrast, larvae of C. scoticus and Culicoides lupicaris were associated with soil substantially comprised of rotting leaf litter that included the parasitic plant Lathraea clandestina. Several species, Culicoides festivipennis, Culicoides punctatus and Culicoides brunnicans, were very common in mud at pond margins. Indeed, pond microhabitats and runoff below barn rooflines supported the greatest species richness. In the pond habitat, 49.4% of Culicoides specimens emerged from mud at the water edge, as opposed to 50 cm above (32.4%) and 1 meter above waterline (18%). Similar species richness, but statistically significant differences in abundance, were observed among the four pond microhabitats. Overall, the majority of the specimens were found in the upper layer (0-3 cm), except in manure, where they preferred deeper layers (>6 cm). Maximum peaks of abundance occurred in both light traps and soil samples in summer months, whereas increased captures in autumn were noticed only in light traps. Both trapping systems failed to collect adult Culicoides midges in the coldest months of December, January and February. The literature on immature habitats of species suspected in BTV transmission in Europe, the Pulicaris group and particularly the Obsoletus group, is briefly reviewed.

Investigating incursions of bluetongue virus using a model of long-distance Culicoides biting midge dispersal

Bluetongue virus (BTV) is an economically important pathogen of ruminants that is the aetiological agent of the haemorrhagic disease bluetongue. Bluetongue virus is biologically transmitted by Culicoides biting midges (Diptera: Ceratopogonidae), and long-range dispersal of infected vector species contributes substantially to the rapid spread of the virus. The range of semi-passive flights of infected Culicoides on prevailing winds has been inferred to reach several hundred kilometres in a single night over water bodies. In this study, an atmospheric dispersion model was parameterized to simulate Culicoides flight activity based on dedicated entomological data sets collected in the UK. Five outbreaks of BTV in Europe were used to evaluate the model for use as an early warning tool and for retrospective analyses of BTV incursions. In each case, the generated predictions were consistent with epidemiological observations confirming its reliability for use in disease outbreak management. Furthermore, the model aided policy makers to predict, contain and eradicate BTV outbreaks in the UK during 2007 and 2008.