Oral susceptibility of South African Culicoides species to live-attenuated serotype-specific vaccine strains of African horse sickness virus (AHSV)

Biting Midges (Diptera: Ceratopogonidae) as Vectors of Viruses

Biting midges of the genus Culicoides occur almost globally and can regionally and seasonally reach high abundances. Most species are hematophagous, feeding on all groups of vertebrates, including humans. In addition to being nuisance pests, they are able to transmit disease agents, with some viruses causing high morbidity and/or mortality in ruminants, horses and humans. Despite their impact on animal husbandry, public health and tourism, knowledge on the biology and ecology of culicoid biting midges and their interactions with ingested pathogens or symbiotic microorganisms is limited. Research is challenging due to unknown larval habitats, the insects' tiny size, the inability to establish and breed most species in the laboratory and the laborious maintenance of colonies of the few species that can be reared in the laboratory. Consequently, the natural transmission of pathogens has experimentally been demonstrated for few species while, for others, only indirect evidence of vector potential exists. Most experimental data are available for Culicoides sonorensis and C. nubeculosus, the only species kept in western-world insectaries. This contribution gives an overview on important biting midge vectors, transmitted viruses, culicoid-borne viral diseases and their epidemiologies and summarizes the little knowledge on interactions between biting midges, their microflora and culicoid-borne arboviruses.

Physicochemical factors affecting the diversity and abundance of Afrotropical Culicoides species in larval habitats in Senegal

Biting midges of the genus Culicoides (Diptera: Ceratopogonidae) are the biological vectors of arboviruses of global importance in animal health. We characterized the physicochemical parameters that determine the density and composition of the main Culicoides species of veterinary interest in larval habitats of the Niayes region of Senegal. For this purpose, we combined larval and substrate sampling in the field in different habitat types with adult emergence and physicochemical analyses in the laboratory. Three major habitat types were identified, conditioning the predominant species of Culicoides and pH and the amount of organic matter were positively correlated with the abundance of larvae and emerging Culicoides, as opposed to salinity. The diversity of emerging Culicoides was positively correlated with pH while it was negatively correlated with salinity. Culicoides distinctipennis was the predominant species in the larval habitat group of freshwater lake edges. In the larval habitat group of pond and puddle edges, C. oxystoma and C. nivosus were predominant; both species were again most abundant in the larval habitat group of saltwater lake edges. These variabilities in physicochemical parameters support the distribution of different Culicoides species in different habitat groups. These results make it possible to implement effective, selective and environmental-friendly control measures but also to improve current models for estimating the abundance of adult vector populations at a local scale.

Culicoides species as potential vectors of African horse sickness virus in the southern regions of South Africa

African horse sickness (AHS), a disease of equids caused by the AHS virus, is of major concern in South Africa. With mortality reaching up to 95% in susceptible horses and the apparent reoccurrence of cases in regions deemed non-endemic, most particularly the Eastern Cape, epidemiological research into factors contributing to the increase in the range of this economically important virus became imperative. The vectors, Culicoides (Diptera: Ceratopogonidae), are considered unable to proliferate during the unfavourable climatic conditions experienced in winter in the province, although the annual occurrence of AHS suggests that the virus has become established and that vector activity continues throughout the year. Surveillance of Culicoides within the province is sparse and little was known of the diversity of vector species or the abundance of known vectors, Culicoides imicola and Culicoides bolitinos. Surveillance was performed using light trapping methods at selected sites with varying equid species over two winter and two outbreak seasons, aiming to determine diversity, abundance and vector epidemiology of Culicoides within the province. The research provided an updated checklist of Culicoides species within the Eastern Cape, contributing to an increase in the knowledge of AHS vector epidemiology, as well as prevention and control in southern Africa.

Bloodmeal analysis in Culicoides midges collected near horses, donkeys and zebras in the Eastern Cape, South Africa

An upsurge in African horse sickness (AHS) in the Eastern Cape, South Africa, from 2006 led to an epidemiological reassessment of the disease there. Light trapping surveys carried out near horses, donkeys and zebras in 2014-2016 collected 39 species of Culicoides midge (Diptera: Ceratopogonidae) that are potential vectors of AHS. To establish if these midges fed on equids, DNA sequences were obtained from the gut contents of 52 female midges (35 freshly blood-fed, 13 gravid and four parous), representing 11 species collected across 11 sites. Culicoides leucostictus fed on all three equids. Culicoides bolitinos, Culicoides imicola and Culicoides magnus fed on both horses and donkeys. Culicoides onderstepoortensis fed on donkeys, and Culicoides similis and Culicoides pycnostictus fed on zebras. Bloodmeals from cows, pigs, warthogs, impalas and a domestic dog were also identified in various species, but none of the midges tested had fed on birds. These results contribute to knowledge of the vectorial capacity of several species of Culicoides with regard to AHS in the Eastern Cape and point to potential reservoir hosts, of which donkeys, zebras and domestic dogs have previously been found to harbour AHS. Blood-fed midges were also obtained throughout winter, indicating the potential for endemic AHS in the province.

Vector Competence of Italian Populations of Culicoides for Some Bluetongue Virus Strains Responsible for Recent Northern African and European Outbreaks

The distribution of Bluetongue virus (BTV) in Europe can be represented by two distinct and interconnected epidemiological systems (episystems), each characterized by different ecological characteristics and vector species. This study investigated the vector competence of Italian populations of Culicoides imicola and Culicoides obsoletus/scoticus to some representative BTV strains after artificial oral infection. The BTV strains were selected according to their ability to spread to one or both episystems and included BTV-4 ITA, responsible of the recent Italian and French BTV-4 outbreaks; the BTV-2 strain which caused the first BTV incursion in Italy, Corsica, and Balearic Islands; BTV-4 MOR, responsible for the epidemic in Morocco; and BTV-8, the strain which spread through Europe between 2006 and 2008. Blood-soaked cotton pledgets and Hemotek membrane feeder using Parafilm® membrane were used to artificially feed midges. For each population/strain, recovery rates (positive/tested heads) were evaluated using serogroup- and serotype-specific RT-PCR. The trial demonstrated that, except for the Abruzzo population of C. obsoletus/C. scoticus, which was refractory to BTV-4 MOR infection, all the investigated Culicoides populations are susceptible to the selected BTV strains and that, if prompt vaccination programs and restriction measures had not been implemented, BTV-2 and BTV-4 MOR could have spread all over Europe.

DNA barcoding and molecular identification of field-collected Culicoides larvae in the Niayes area of Senegal

Background

Biting midge species of the genus Culicoides Latreille (Diptera: Ceratopogonidae) comprise more than 1300 species distributed worldwide. Several species of Culicoides are vectors of various viruses that can affect animals, like the African horse sickness virus (AHSV), known to be endemic in sub-Saharan Africa. The ecological and veterinary interest of Culicoides emphasizes the need for rapid and reliable identification of vector species. However, morphology-based identification has limitations and warrants integration of molecular data. DNA barcoding based on the mitochondrial gene cytochrome c oxidase subunit 1 (cox1) is used as a rapid and authentic tool for species identification in a wide variety of animal taxa across the globe. In this study, our objectives were as follows: (i) establish a reference DNA barcode for Afrotropical Culicoides species; (ii) assess the accuracy of cox1 in identifying Afrotropical Culicoides species; and (iii) test the applicability of DNA barcoding for species identification on a large number of samples of Culicoides larvae from the Niayes area of Senegal, West Africa.

Results

A database of 230 cox1 sequences belonging to 42 Afrotropical Culicoides species was found to be reliable for species-level assignments, which enabled us to identify cox1 sequences of Culicoides larvae from the Niayes area of Senegal. Of the 933 cox1 sequences of Culicoides larvae analyzed, 906 were correctly identified by their barcode sequences corresponding to eight species of Culicoides. A total of 1131 cox1 sequences of adult and larval Culicoides were analyzed, and a hierarchical increase in mean divergence was observed according to two taxonomic levels: within species (mean = 1.92%, SE = 0.00), and within genus (mean = 17.82%, SE = 0.00).

Conclusions

Our study proves the efficiency of DNA barcoding for studying Culicoides larval diversity in field samples. Such a diagnostic tool offers great opportunities for investigating Culicoides immature stages ecology and biology, a prerequisite for the implementation of eco-epidemiological studies to better control AHSV in the Niayes region of Senegal, and more generally in sub-Saharan Africa.

Electronic supplementary material

The online version of this article (10.1186/s13071-018-3176-y) contains supplementary material, which is available to authorized users.

African Horse Sickness Virus: History, Transmission, and Current Status

African horse sickness virus (AHSV) is a lethal arbovirus of equids that is transmitted between hosts primarily by biting midges of the genus Culicoides (Diptera: Ceratopogonidae). AHSV affects draft, thoroughbred, and companion horses and donkeys in Africa, Asia, and Europe. In this review, we examine the impact of AHSV critically and discuss entomological studies that have been conducted to improve understanding of its epidemiology and control. The transmission of AHSV remains a major research focus and we critically review studies that have implicated both Culicoides and other blood-feeding arthropods in this process. We explore AHSV both as an epidemic pathogen and within its endemic range as a barrier to development, an area of interest that has been underrepresented in studies of the virus to date. By discussing AHSV transmission in the African republics of South Africa and Senegal, we provide a more balanced view of the virus as a threat to equids in a diverse range of settings, thus leading to a discussion of key areas in which our knowledge of transmission could be improved. The use of entomological data to detect, predict and control AHSV is also examined, including reference to existing studies carried out during unprecedented outbreaks of bluetongue virus in Europe, an arbovirus of wild and domestic ruminants also transmitted by Culicoides.

Culicoides species composition and environmental factors influencing African horse sickness distribution at three sites in Namibia

African horse sickness (AHS) is one of the most lethal infectious, non-contagious, vector-borne disease of equids. The causative agent, African horse sickness virus (AHSV) is transmitted via Culicoides midges (Diptera: Ceratopogonidae). AHS is endemic to Namibia but detailed studies of Culicoides communities and influencing environmental parameters are limited. This study aims to determine the Culicoides species composition at three different sites and to assess environmental parameters influencing the geographical distribution of AHS in Namibia. Weekly collections of Culicoides were made during the AHS peak season from January to May for 2013 and 2014 using the Onderstepoort 220V UV-light trap. Out of 397 collections made, 124 collections (3287 Culicoides) were analysed for AHSV presence with RT-qPCR. A total of 295 collections were analysed for total Culicoides (all collected Culicoides individuals) and in 75% of these collections the Culicoides were identified to species level. C. imicola was the dominant species with proportional representation of 29.9%. C. subschultzei, C. exspectator and C. ravus each contribute more than 10% to the species composition. The lowest number of Culicoides was collected at Aus 9980, a total of 21819 at Windhoek and the highest number at Okahandja 47343. AHSV was present at all three sites during 2013 but only in Windhoek and Okahandja during 2014. Multivariate analyses of data from the two year survey indicate the environmental parameters in order of importance for the distribution of AHS in Namibia as precipitation>temperature>clay>relative humidity>NDVI. The implication of these findings is that any precipitation event increases Culicoides numbers significantly. Together with these results the high number of species found of which little is known regarding their vector competence, add to the complexity of the distribution of AHS in Namibia.

Insight on the larval habitat of Afrotropical Culicoides Latreille (Diptera: Ceratopogonidae) in the Niayes area of Senegal, West Africa

BACKGROUND

Certain biting midges species of the genus Culicoides (Diptera: Ceratopogonidae) are vectors of virus to livestock worldwide. Culicoides larval ecology has remained overlooked because of difficulties to identify breeding sites, methodological constraints to collect samples and lack of morphological tools to identify field-collected individuals to the species level. After the 2007 unforeseen outbreaks of African horse sickness virus (AHSV) in Senegal (West Africa), there is a need to identify suitable and productive larval habitats in horse farms for the main Culicoides species to evaluate the implementation of vector control measures or preventive actions.

METHODS

We investigate twelve putative larval habitats (habitat types) of Culicoides inside and outside of three horse farms in the Niayes area of Senegal using a combination of flotation and emergence methods during four collection sessions.

RESULTS

Among the three studied horse farms, three habitat types were found positive for Culicoides larvae: pond edge, lake edge and puddle edge. A total of 1420 Culicoides individuals (519♂/901♀) belonging to ten species emerged from the substrate samples. Culicoides oxystoma (40 %), C. similis (25 %) and C. nivosus (24 %) were the most abundant species and emerged from the three habitat types while C. kingi (5 %) was only retrieved from lake edges and one male emerged from puddle edge. Culicoides imicola (1.7 %) was found in low numbers and retrieved only from pond and puddle edges.

CONCLUSIONS

Larval habitats identified were not species-specific. All positive larval habitats were found outside the horse farms. This study provides original baseline information on larval habitats of Culicoides species in Senegal in an area endemic for AHSV, in particular for species of interest in animal health. These data will serve as a point of reference for future investigations on larval ecology and larval control measures.

A web-based survey of horse owners' perceptions and network analysis of horse movements relating to African horse sickness distribution in Namibia and South Africa

Africa horse sickness (AHS) is the most lethal infectious non-contagious horse disease and has accordingly been declared notifiable by the World Organisation for Animal Health. AHS is endemic to sub-Saharan Africa and causes considerable losses to the equestrian industry. The effect of diseases in livestock on socio-economic factors is well researched, but the effect of anthropogenic factors on the distribution of a disease is poorly understood. The purpose of the study was to assess Namibian and South African horse owners' perceptions and the effect of horse movement on AHS distribution. A cross-sectional study was conducted to collect information from horse owners in Namibia and South Africa. To that end 'Fluid survey' was used for survey development. The survey was launched on Facebook and the link shared to horse related focus groups in Namibia and South Africa. A total of 508 responses were collected during the survey period. Of the 417 completed questionnaires received, 22% were from Namibia and 78% from South Africa. The participants comprised of 71% social and 29% professional riders. The most popular precautionary measures used, in addition to vaccination, were chemical repellents (64%) and stabling of horses during dusk and dawn (59%). A network analysis was performed in Gephi 0.8.2.B to illustrate the movement of horses between countries and districts/provinces. Network analysis results indicate that areas with the highest movement of horses corresponded to the areas with a high occurrence of AHS. Although 93% of the participants were aware that AHS is a notifiable and controlled disease, the process and efficiency of reporting is mostly unknown. With this snapshot of horse owners' perceptions and the effect of horse movement on the distribution of AHS, it is clear that a more holistic approach is needed. To that end, all environmental and social factors must be taken into account in effective management strategies.

Detection of African horse sickness virus in Culicoides imicola pools using RT-qPCR

African horse sickness (AHS) is an infectious, non-contagious arthropod-borne disease of equids, caused by the African horse sickness virus (AHSV), an orbivirus of the Reoviridae family. It is endemic in sub-Saharan Africa and thought to be the most lethal viral disease of horses. This study focused on detection of AHSV in Culicoides imicola (Diptera: Ceratopogonidae) pools by the application of a RT-qPCR. Midges were fed on AHSV-infected blood. A single blood-engorged female was allocated to pools of unfed nulliparous female midges. Pool sizes varied from 1 to 200. RNA was extracted and prepared for RT-qPCR. The virus was successfully detected and the optimal pool size for the limit of detection of the virus was determined at a range between 1 to 25. Results from this investigation highlight the need for a standardized protocol for AHSV investigation in Culicoides midges especially for comparison among different studies and for the determination of infection rate.

Foraging range of arthropods with veterinary interest: New insights for Afrotropical Culicoides biting midges (Diptera: Ceratopogonidae) using the ring method

The identification of blood meal source of arthropod vector species contributes to the understanding of host-vector-pathogen interactions. The aim of the current work was to identify blood meal source in Culicoides biting midge species, biological vectors of internationally important arboviruses of livestock and equids, using a new ecological approach. We examined the correlation between blood meal source identified in engorged Culicoides females collected in a suction light trap and the available vertebrate hosts along four rings (200, 500, 1000 and 2000 m) centered at the trap site and described the foraging range of the three main vector species of veterinary interest present in the study area, Culicoides imicola, Culicoides kingi and Culicoides oxystoma. The study was performed in four sites localized in the Niayes region of Senegal (West Africa) where recent outbreaks of African horse sickness occurred. Blood meal source identification was carried out by species-specific multiplex PCRs with genomic DNA extracted from the abdomen of engorged females collected during nine night collections for twenty-six collections. The four most abundant hosts present in the studied area (horse, cattle, goat and sheep) were surveyed in each ring zone. The blood meal source varied according to Culicoides species and host availability in each site. C. oxystoma and C. imicola females mainly fed on horses readily available at 200 m maximum from the trap location whereas females of C. kingi fed mainly on cattle, at variable distances from the traps (200 to 2000 m). C. oxystoma may also feed on other vertebrates. We discuss the results in relation with the transmission of Culicoides-borne arboviruses and the species dispersion capacities.

The occurrence of Culicoides species, the vectors of arboviruses, at selected trap sites in Zimbabwe

A study of the distribution of Culicoides species was conducted by establishing 12 light trap sites over five rainy seasons between 1998 and 2003 covering all the geo-climatic natural regions of Zimbabwe. In total, 279 919 specimens of Culicoides were trapped over a total of 163 trapping nights. The highest median counts of Culicoides per trapping night were recorded in natural region III, which has climatic conditions conducive to the successful development of the larvae. Culicoides imicola, the major vector of bluetongue and African horse sickness viruses in Africa, was found to be the most abundant species (80.4%), followed by Culicoides enderleini (5.9%) and Culicoides milnei (5.2%). This study identified 10 species of Culicoides that had not been previously described in Zimbabwe, including Culicoides loxodontis and Culicoides miombo, which are members of the C. imicola complex. A total of 23 994 Culicoides midges were collected from five trap sites in Harare, Zimbabwe, with the dominant species, C. imicola, representing 91.6% of the total collection. Seventeen arboviruses were isolated from these midges, 15 of which were bluetongue virus. The predominant bluetongue virus serotype was serotype 11, followed by serotypes 1, 8, 12 and 15. Bluetongue virus serotypes 1, 2, 8, 10, 12, 15, 16 and 18, detected in this study, had not been previously reported in Zimbabwe.

The effect of high frequency sound on Culicoides numbers collected with suction light traps

Culicoides midges (Diptera: Ceratopogonidae), are involved in the transmission of various pathogens that cause important diseases of livestock worldwide. The use of insect repellents to reduce the attack rate of these insects on livestock could play an important role as part of an integrated control programme against diseases transmitted by these midges. The objective of this study was to determine whether high frequency sound has any repellent effect on Culicoides midges. The number of midges collected with 220 V Onderstepoort white light traps fitted with electronic mosquito repellents (EMRs), emitting 5-20 KHz multi-frequency sound waves, was compared with that of two untreated traps. Treatments were rotated in two replicates of a 4 x 4 randomised Latin square design. Although fewer midges were collected in the two traps fitted with EMRs, the average number collected over eight consecutive nights was not significantly different. The EMRs also had no influence on any of the physiological groups of Culicoides imicola Kieffer or the species composition of the Culicoides population as determined with light traps. The results indicate that high frequency sound has no repellent effect on Culicoides midges. There is therefore no evidence to support their promotion or use in the protection of animals against pathogens transmitted by Culicoides midges.

The attraction range of the Onderstepoort 220V light trap for Culicoides biting midges as determined under South African field conditions

Despite some limitations suction light traps are the primary tools used for the collection of Culicoides species (Diptera: Ceratopogonidae). The range of attraction of the Onderstepoort light trap is not known but an insight into the range of a trap will determine where the trap must be positioned relative to the hosts present, possible breeding sites and environmental structures in the trapping vicinity. It will therefore contribute to a more meaningful interpretation and comparison of results between trapping events. In the present study the number of Culicoides midges collected in a single trap was compared to those of traps made with an additional trap respectively 1m, 4m and 8.5m away from the first. Treatments between sites were rotated in three replicates of a 4×4 Latin square design. While interactions were found in traps 4m apart no statistically significant interactions were found when they were 8.5m apart. The range of attraction, indicated by the interaction between two traps, will be between 2m and 4m. In interpreting light trap results the limitations of this collection method needs to be taken into consideration.

Risk of introducing African horse sickness virus into the Netherlands by international equine movements

African horse sickness (AHS) is a vector-borne viral disease of equines that is transmitted by Culicoides spp. and can have severe consequences for the horse industry in affected territories. A study was performed to assess the risk of introducing AHS virus (AHSV) into the Netherlands (P_AHS) by international equine movements. The goal of this study was to provide more insight into (a) the regions and equine species that contribute most to this risk, (b) the seasonal variation in this risk, and (c) the effectiveness of measures to prevent introduction of AHSV. Countries worldwide were grouped into three risk regions: (1) high risk, i.e., those countries in which the virus is presumed to circulate, (2) low risk, i.e., those countries that have experienced outbreaks of AHS in the past and/or where the main vector of AHS, Culicoides imicola, is present, and (3) very low risk, i.e., all other countries. A risk model was constructed estimating P_AHS taking into account the probability of release of AHSV in the Netherlands and the probability that local vectors will subsequently transmit the virus to local hosts. Model calculations indicated that P_AHS is very low with a median value of 5.1×10(-4)/year. The risk is highest in July and August, while equine movements in the period October till March pose a negligible risk. High and low risk regions contribute most to P_AHS with 31% and 53%, respectively. Importations of donkeys and zebras constitute the highest risk of AHSV release from high risk regions, while international movements of competition horses constitute the highest risk of AHSV release from low and very low risk regions. Preventive measures currently applied reduce P_AHS by 46% if compared to a situation in which no preventive measures are applied. A prolonged and more effective quarantine period in high risk regions and more stringent import regulations for low risk regions could further reduce P_AHS. Large uncertainty was involved in estimating model input parameters. Sensitivity analysis indicated that uncertainty about the probability of non-notified presence of AHS in low and very low risk regions, the protective effect of quarantine and the vector-host ratio had most impact on the estimated risk. Furthermore, temperature values at the time of release of AHSV largely influenced the probability of onward spread of the virus by local vectors to local hosts.

African horse sickness in The Gambia: circulation of a live-attenuated vaccine-derived strain

African horse sickness virus serotype 9 (AHSV-9) has been known for some time to be circulating amongst equids in West Africa without causing any clinical disease in indigenous horse populations. Whether this is due to local breeds of horses being resistant to disease or whether the AHSV-9 strains circulating are avirulent is currently unknown. This study shows that the majority (96%) of horses and donkeys sampled across The Gambia were seropositive for AHS, despite most being unvaccinated and having no previous history of showing clinical signs of AHS. Most young horses (<3 years) were seropositive with neutralizing antibodies specific to AHSV-9. Eight young equids (<3 years) were positive for AHSV-9 by serotype-specific RT-PCR and live AHSV-9 was isolated from two of these horses. Sequence analysis revealed the presence of an AHSV-9 strain showing 100% identity to Seg-2 of the AHSV-9 reference strain, indicating that the virus circulating in The Gambia was highly likely to have been derived from a live-attenuated AHSV-9 vaccine strain.

The effect of 1-octen-3-ol and 4-methylphenol on Culicoides midge numbers collected with suction light traps in South Africa

Despite some shortcomings, suction light traps are the primary monitoring tool for the collection of Culicoides species (Diptera: Ceratopogonidae). Factors that may increase the efficiency of these traps need to be investigated. In the present study the numbers of Culicoides midges collected with two Onderstepoort black light traps baited with a mixture of 1-octen-3-ol and 4-methylphenol, as a potential olfactory cue, were compared to those of two unbaited traps. Comparisons were done in two and three replicates of a 4 × 4 randomized Latin square design in the presence and absence of cattle. The addition of 1-octen-3-ol and 4-methylphenol, released at 9.1 and 15.5mg/h, respectively, did not influence species richness, numbers collected, sex ratios or age-grading results. Comparisons of Culicoides numbers and especially the abundance of Culicoides imicola Kieffer in collections done in the presence and absence of cattle confirm previous findings that show that host animals will be the primary attraction for Culicoides midges and that light traps mostly sample midges already in the near vicinity of the host.

A comparison of the susceptibility of the biting midge Culicoides imicola to infection with recent and historical isolates of African horse sickness virus

The susceptibility of Culicoides (Avaritia) imicola Kiefer (Diptera: Ceratopogonidae) to 21 isolates representing all nine known serotypes of African horse sickness virus (AHSV), recovered from clinical cases of the disease in South Africa during 1998-2004, was compared with its susceptibility to approximately 40-year-old isolates stored at the Agricultural Research Council-Onderstepoort Veterinary Institute. Field-collected C. imicola were fed through a chicken skin membrane on sheep blood spiked with one of the virus isolates to a concentration in the range of 5.6-7.5 log (10)TCID(50)/mL. After 10 days incubation at 23.5 degrees C, five of the nine historical serotypes (AHSV-1, -2, -3, -7 and -9) could not be isolated from C. imicola. All nine serotypes were recovered for the 21 recent isolates, for 16 of which the virus recovery rates were higher than for the corresponding historical isolates. These results emphasize the need to assess the oral susceptibility of local Culicoides populations to viruses in circulation during outbreaks in order to estimate their vector potential.

The oral susceptibility of South African field populations of Culicoides to African horse sickness virus

Twenty-two isolates of African horse sickness virus (AHSV), representing its distinct serotypes, geographical and historical origins, were fed to three populations of South African livestock-associated Culicoides spp. (Diptera, Ceratopogonidae). Infective blood meals included 12 recent isolates, nine historical reference strains and one live attenuated vaccine strain serotype 7 (AHSV-7) of the virus. Field-collected midges were fed through a chicken-skin membrane on sheep blood spiked with one of the viruses, which concentrations ranged from 5.4 to 8.8 log(10)TCID(50)/mL of blood. After 10 days incubation at 23.5 degrees C, AHSV was isolated from 11 Culicoides species. Standard in vitro passaging of AHSV-7, used for the preparation of live attenuated vaccine, did not reduce its ability to infect Culicoides species. Virus recovery rates in orally infected Culicoides midges differed significantly between species and populations, serotypes, isolates and seasons. Significant variations in oral susceptibility recorded in this study emphasize a complex inter-relationship between virus and vector, which is further influenced by multiple intrinsic and extrinsic factors. As it is not possible to standardize all these factors under laboratory conditions, conclusive assessment of the role of field-collected Culicoides midges in the transmission of orbiviruses remains problematic. Nevertheless, results of this study suggest the potential for multi-vector transmission of AHSV virus in South Africa.

Comparison of the efficiency of five suction light traps under field conditions in South Africa for the collection of Culicoides species

Culicoides biting midges (Diptera: Ceratopogonidae) are involved in the transmission of a variety of pathogens, the economically most important being the orbiviruses that cause bluetongue and African horse sickness; both of which have been shown to be multi-vector diseases. The identification of all potential vectors will be crucial for the implementation of integrated control measures and disease risk analysis. The primary monitoring tools used for the collection of Culicoides midges are various models of suction light traps. In order to facilitate comparison of data between laboratories the efficiency of five traps (Onderstepoort, Rieb, mini-CDC, Pirbright, BG-sentinel), used at present and in the past in Europe, was compared in the field in South Africa. Comparisons were done either in three replicates of a 4x4 or two replicates of a 5x5 randomized Latin square design. The Onderstepoort trap collected significantly more Culicoides midges than the other traps. Relatively small but statistically significant differences were found in the species composition, parous rates, sex ratios as well as the ratio of Culicoides midges to other insects, as determined by the different traps. It will be important to determine the significance and underlying causes for these differences.

An investigation on the Culicoides species composition at seven sites in southern Switzerland

In the past decade, there have been regular outbreaks of bluetongue (BT) in many parts of Europe. Owing to the presence of BT disease and its vectors in countries adjacent to Switzerland, an initial entomological survey was conducted in 2003, which established the presence of several midges of the genus Culicoides (Diptera: Ceratopogonidae). Subsequently, a sentinel herd monitoring system was established with the primary entomological aim being the determination and further study of Culicoides population compositions. Insects were collected in 2005 and 2006 at seven sentinel herd sites in the south of Switzerland (canton of Ticino) near the border of Italy, using Onderstepoort-type light traps. This region is botanically and zoologically similar to the Mediterranean and is one of the warmest and most humid areas of the country, hence it is considered a potential access path for BT disease into Switzerland. Collections were made at four cattle farms, two equestrian centres and one goat farm. Sites were sampled four times per month from June to October. Traps were operated from dusk until dawn and samples were collected monthly for analysis through microscopy as well as a Culicoides imicola-specific PCR. Results confirmed the absence of C. imicola (Kieffer) and demonstrated that the potential BT virus vectors are highly abundant, notably: Culicoides obsoletus (Meigen), Culicoides scoticus (Downes & Kettle) and Culicoides dewulfi (Goetghebuer) subgenus Avaritia and Culicoides pulicaris (Linnaeus) subgenus Culicoides. These findings expand the current knowledge of Culicoides population composition in the southern part of the Switzerland. Culicoides cataneii (Clastrier), Culicoides flavipulicaris (Dzhafarov), Culicoides indistinctus (Khalaf), Culicoides nubeculosus (Meigen) and species of the Grisescens complex were reported for the first time in Switzerland.

Adaptive strategies of African horse sickness virus to facilitate vector transmission

African horse sickness virus (AHSV) is an orbivirus that is usually transmitted between its equid hosts by adult Culicoides midges. In this article, we review the ways in which AHSV may have adapted to this mode of transmission. The AHSV particle can be modified by the pH or proteolytic enzymes of its immediate environment, altering its ability to infect different cell types. The degree of pathogenesis in the host and vector may also represent adaptations maximising the likelihood of successful vectorial transmission. However, speculation upon several adaptations for vectorial transmission is based upon research on related viruses such as bluetongue virus (BTV), and further direct studies of AHSV are required in order to improve our understanding of this important virus.

Virus recovery rates for wild-type and live-attenuated vaccine strains of African horse sickness virus serotype 7 in orally infected South African Culicoides species

Previously reported virus recovery rates from Culicoides (Avaritia) imicola Kieffer and Culicoides (Avaritia) bolitinos Meiswinkel (Diptera, Ceratopogonidae) orally infected with vaccine strain of African horse sickness virus serotype 7 (AHSV-7) were compared with results obtained from concurrently conducted oral infections with five recent AHSV-7 isolates from naturally infected horses from various localities in South Africa. Culicoides were fed sheep bloods spiked with 10(7.6) TCID(50)/mL of a live-attenuated vaccine strain AHSV-7, and with five field isolates in which virus titre in the bloodmeals ranged from 10(7.1) to 10(8.2) TCID(50)/mL). After an extrinsic incubation of 10 days at 23.5 degrees C, virus recovery rates were significantly higher in C. imicola (13.3%) and C. bolitinos (4.2%) infected with the live-attenuated virus than in midges infected with any of the field isolates. The virus recovery rates for the latter groups ranged from 0% to 9.5% for C. imicola and from 0% to 1.5% for C. bolitinos. The C. imicola population at Onderstepoort was significantly more susceptible to infection with AHSV-7 isolated at Onderstepoort than to the virus strains isolated from other localities. Results of this study suggest that tissue culture attenuation of AHSV-7 does not reduce its ability to orally infect competent Culicoides species and may even lead to enhanced replication in the vector. Furthermore, oral susceptibility in a midge population appears to vary for geographically distinct isolates of AHSV-7.

Replication of live-attenuated vaccine strains of bluetongue virus in orally infected South African Culicoides species

Field-collected South African Culicoides (Diptera, Ceratopogonidae) were fed on sheep blood containing 16 live-attenuated vaccine strains of bluetongue virus (BTV) comprising serotypes -1, -2, -3, -4, -5, -6, -7, -8, -9, -10, -11, -12, -13, -14, -16 and -19. After 10 days extrinsic incubation at 23.5 degrees C, 11 and seven of the 16 BTV serotypes used were recovered from Culicoides (Avaritia) imicola Kieffer and Culicoides (A.) bolitinos Meiswinkel, respectively. One serotype was also recovered from Culicoides (Remmia) enderleini Cornet & Brunhes. Bluetongue virus recovery rates and the mean titres for most serotypes were significantly higher in C. bolitinos than in C. imicola. Significant differences were found in virus recovery rates from Culicoides species fed on blood containing similar or identical virus titres of different BTV serotypes. In addition, we demonstrated that a single passage of live-attenuated BTV-1, -2, -4, -9 and -16 through the insect vector, followed by passaging in insect cells, did not alter its infectivity for C. imicola and that the oral susceptibility of C. imicola to the attenuated vaccine strains of BTV-1, -4, -9 and -16 remained similar for at least three consecutive seasons.

An alternative method of blood-feeding Culicoides imicola and other haematophagous Culicoides species for vector competence studies

The use of cotton wool pads saturated with blood/virus mixture for oral infection attempts was compared to membrane feeding for the assessment of vector competence in C. imicola Kieffer and C. bolitinos Meiswinkel (Diptera, Ceratopogonidae). Although lower infection rates were obtained using pad feeding, it was possible to clearly distinguish the levels of competence between species as well as differences in virus infection rates for various serotypes of bluetongue virus. Reduced infection rates with cotton pad feeding was partly due to a smaller volume of blood meals taken up. However, the method described is likely to be useful in situations where membrane feeding is not viable to separate populations with significant differences in vector competence.

A comparison of the susceptibility of Culicoides imicola and C. bolitinos to oral infection with eight serotypes of epizootic haemorrhagic disease virus

The mechanisms involved in introduction, maintenance and perpetuation of epizootic haemorrhagic disease virus (EHDV) in South Africa are not fully understood. This paper reports on the susceptibility of South African livestock associated Culicoides (Diptera: Ceratopogonidae) species to oral infection with eight EHDV serotypes. Virus was recovered from eight of 17 field-collected Culicoides species 10 days after oral feeding on blood/virus mixtures. Six EHDV serotypes were recovered from C. (Avaritia) imicola Kieffer, and seven serotypes were recovered from C. (A.) bolitinos Meiswinkel. Virus recovery rates in C. imicola ranged from 0.4% for EHDV 2 to 14.4% for EHDV 7, and in C. bolitinos from 0.6% for EHDV 6 to 12.3% for EHDV 2. There was a significant difference in virus recovery rates between serotypes in both species. Other Culicoides species that yielded EHDV after 10 days extrinsic incubation included C. (Meijerehelea) leucostictus Kieffer, C. (Culicoides) magnus Colaço, C. (Beltranmyia) nivosus de Meillon, C. (A.) gulbenkiani Caeiro, C. (Hoffmania) zuluensis de Meillon and C. onderstepoortensis Fiedler. Culicoides midges shown in this study to be susceptible to oral infection with EHDV are widely distributed in South Africa but differ considerably in their abundance, host preference and breeding sites.

Active circulation of bluetongue vaccine virus serotype-2 among unvaccinated cattle in central Italy

Several seroconversions occurring in 2002 among sentinel cattle during the bluetongue-vaccination campaign in Lazio and Tuscany (central Italy) led to the suspicion of vaccine-virus circulation. Therefore in 2003, 17 seroconverting sentinel herds were investigated for the characteristics of the virus involved. From these farms, 91 unvaccinated animals and 57 Culicoides pools were tested for the presence of the bluetongue vaccine virus (serotype-2) or other strains. The presence of vaccine virus serotype-2 was confirmed by PCR followed by restriction analysis in the whole blood of 17 unvaccinated sentinel cattle and 12 pools of Culicoides imicola or C. obsoletus. Of the 17 herds, five were positive only for vaccine virus serotype-2, four were positive for other strains and two for both the vaccine and other strains; the remaining premises were virologicaly negative. The vaccine virus serotype-2 also was detected in areas not included in the vaccination campaign.

Preparation of recombinant African horse sickness virus VP7 antigen via a simple method and validation of a VP7-based indirect ELISA for the detection of group-specific IgG antibodies in horse sera

This paper describes the production and purification of a group-specific recombinant protein VP7 of African horse sickness virus serotype 3 (AHSV-3) and validation of an I-ELISA for the detection of IgG-antibodies to VP7 in horse sera. Baculovirus-expressed VP7 crystals were purified from infected insect cells. Analytical accuracy of the I-ELISA was examined using sera (n = 38) from an experimentally infected horse, from foals born to vaccinated mares, from guinea-pigs immunized with nine serotypes of AHSV, and from sera of animals infected with other orbiviruses. Compared to traditional serological assays, the I-ELISA was more sensitive in detection of the earliest immunological response in an infected horse and declining levels of maternal immunity in foals. Antibodies to all nine serotypes of AHSV could be detected. Cross-reactivity to related orbiviruses was not observed. Diagnostic accuracy of the I-ELISA was assessed by testing sera from vaccinated horses (n = 358) residing in AHS-enzootic areas and from unvaccinated horses (n = 481) residing in an AHS-free area. Sera were categorised as positive or negative for antibodies to AHSV using virus neutralisation tests. The TG-ROC analysis was used for the selection of the cut-off value. At a cut-off of 11.9 of the high positive control serum (percentage positivity), the I-ELISA specificity was 100%, sensitivity 99.4%, and the Jouden index was 0.99.

Vector competence of Culicoides species and the seroprevalence of homologous neutralizing antibody in horses for six serotypes of equine encephalosis virus (EEV) in South Africa

Field-collected Culicoides species (Diptera: Ceratopogonidae) were fed on horse blood-virus mixtures containing one of the six serotypes of equine encephalosis virus (EEV1 to EEV6). The virus mean titres in the bloodmeals varied between 6.1 and 7.0 log10TCID50/mL. Of 19 Culicoides species assayed after 10 days extrinsic incubation at 23.5 degrees C, five yielded the challenge virus, namely Culicoides (Avaritia) imicola Kieffer (EEV1-6), C. (A.) bolitinos Meiswinkel (EEV1, 2, 4, 6), C. (Meijerehelea) leucostictus Kiefer (EEV1, 2), C. (Culicoides) magnus Colaço (EEV1) and C. (Hoffmania) zuluensis de Meillon (EEV2). Virus recovery rates ranged from 0.5 to 13%. The mean levels of viral replication differed between serotypes and Culicoides species and ranged from 1.0 to 2.3 log10TCID50/midge. Culicoides midges shown in this study to be susceptible to oral infection with EEV are widely distributed in South Africa but differ considerably in their abundance, host preference and breeding sites. Of 1456 horses tested, 1144 (77%) had antibody to EEV. Homologous virus-neutralizing antibodies to all six serotypes were detected in individual horses from all eight geographical provinces of South Africa. The distribution, prevalence, and the rate of exposure to individual serotypes varied significantly between regions. The potential for vectoring of EEV in the field by several Culicoides species with unique ecologies and lack of cross-protection to re-infection with multiple serotypes highlights some of the mechanisms that are likely to play a role in the virus' natural maintenance cycle and the highly efficient level of countrywide transmission amongst South African horses.