Infection of Culicoides brevitarsis and C. wadai (Diptera: Ceratopogonidae) with four Australian serotypes of bluetongue virus

Culicoides-borne Orbivirus epidemiology in a changing climate

Orbiviruses are of significant importance to the health of wildlife and domestic animals worldwide; the major orbiviruses transmitted by multiple biting midge (Culicoides) species include bluetongue virus, epizootic hemorrhagic disease virus, and African horse sickness virus. The viruses, insect vectors, and hosts are anticipated to be impacted by global climate change, altering established Orbivirus epidemiology. Changes in global climate have the potential to alter the vector competence and extrinsic incubation period of certain biting midge species, affect local and long-distance dispersal dynamics, lead to range expansion in the geographic distribution of vector species, and increase transmission period duration (earlier spring onset and later fall transmission). If transmission intensity is associated with weather anomalies such as droughts and wind speeds, there may be changes in the number of outbreaks and periods between outbreaks for some regions. Warmer temperatures and changing climates may impact the viral genome by facilitating reassortment and through the emergence of novel viral mutations. As the climate changes, Orbivirus epidemiology will be inextricably altered as has been seen with recent outbreaks of bluetongue, epizootic hemorrhagic disease, and African horse sickness outside of endemic areas, and requires interdisciplinary teams and approaches to assess and mitigate future outbreak threats.

Endemic and Emerging Arboviruses in Domestic Ruminants in East Asia

Epizootic congenital abnormalities caused by Akabane, Aino, and Chuzan viruses have damaged the reproduction of domestic ruminants in East Asia for many years. In the past, large outbreaks of febrile illness related to bovine ephemeral fever and Ibaraki viruses severely affected the cattle industry in that region. In recent years, vaccines against these viruses have reduced the occurrence of diseases, although the viruses are still circulating and have occasionally caused sporadic and small-scaled epidemics. Over a long-term monitoring period, many arboviruses other than the above-mentioned viruses have been isolated from cattle and Culicoides biting midges in Japan. Several novel arboviruses that may infect ruminants (e.g., mosquito- and tick-borne arboviruses) were recently reported in mainland China based on extensive surveillance. It is noteworthy that some are suspected of being associated with cattle diseases. Malformed calves exposed to an intrauterine infection with orthobunyaviruses (e.g., Peaton and Shamonda viruses) have been observed. Epizootic hemorrhagic disease virus serotype 6 caused a sudden outbreak of hemorrhagic disease in cattle in Japan. Unfortunately, the pathogenicity of many other viruses in ruminants has been uncertain, although these viruses potentially affect livestock production. As global transportation grows, the risk of an accidental incursion of arboviruses is likely to increase in previously non-endemic areas. Global warming will also certainly affect the distribution and active period of vectors, and thus the range of virus spreads will expand to higher-latitude regions. To prevent anticipated damages to the livestock industry, the monitoring system for arboviral circulation and incursion should be strengthened; moreover, the sharing of information and preventive strategies will be essential in East Asia.

Application of an embryonated chicken egg model to assess the vector competence of Australian Culicoides midges for bluetongue viruses

Culicoides biting midges (Diptera: Ceratopogonidae) are vectors of a number of globally important arboviruses that affect livestock, including bluetongue virus (BTV), African horse sickness virus and the recently emerged Schmallenberg virus. In this study, a model using embryonated chicken eggs (ECEs) was utilized to undertake vector competence studies of Australian Culicoides spp. for 13 laboratory-adapted or wild-type virus strains of BTV. A total of 7393 Culicoides brevitarsis were reared from bovine dung, and 3364 Culicoides were induced to feed from ECEs infected with different strains of BTV. Of those, 911 (27%) survived the putative extrinsic incubation period of 9-12 days. In some trials, virus was also transmitted onward to uninfected ECEs, completing the transmission cycle. This model does not rely on the use of colonized midges and has the capacity to assess the vector competence of field-collected insects with strains of virus that have not previously been passaged in laboratory culture systems. There is also potential for this model to be used in investigations of the competence of Culicoides spp. for other arboviruses.

Host and Potential Vector Susceptibility to an Emerging Orbivirus in the United States

Epizootic hemorrhagic disease viruses (EHDVs) are orbiviruses transmitted by Culicoides biting midges to domestic and wild ruminants. EHDV-1 and EHDV-2 are endemic in the United States, where epizootic hemorrhagic disease is the most significant viral disease of white-tailed deer (WTD; Odocoileus virginianus) and reports of epizootic hemorrhagic disease in cattle are increasing. In 2006, a reassortant EHDV-6 was isolated from dead WTD in Indiana and has been detected each subsequent year over a wide geographic region. Since EHDV-6 is not a historically endemic serotype in the United States, it is important to understand infection outcome in potential hosts. Specifically, we aimed to evaluate the pathogenicity of the virus in 2 primary US ruminant hosts (WTD and cattle) and the susceptibility of a confirmed US vector ( Culicoides sonorensis). Five WTD and 4 cattle were inoculated with >106 TCID50 EHDV-6 by intradermal and subcutaneous injection. All 5 WTD exhibited moderate to severe disease, and 3 died. Viremia was first detected 3 to 5 days postinfection (dpi) with surviving animals seroconverting by 10 dpi. Two of 4 inoculated cattle had detectable viremia, 5 to 10 dpi and 7 to 24 dpi, respectively. No clinical, hematologic, or pathologic abnormalities were observed. Antibodies were detected by 10 dpi in 3 of 4 cows. C. sonorensis were fed on WTD blood spiked with EHDV-6 and held for 4 to 14 days postfeeding at 25°C. From 4 to 14 days postfeeding, 19 of 171 midges were virus isolation positive and 6 of 171 had ≥102.7 TCID50 EHDV-6. Although outcomes varied, these studies demonstrate the susceptibility of ruminant and vector hosts in the United States for this recently emerged EHDV serotype.

Arboviruses: variations on an ancient theme

ABSTRACT 

Arboviruses utilize different strategies to complete their transmission cycle between vertebrate and invertebrate hosts. Most possess an RNA genome coupled with an RNA polymerase lacking proofreading activity and generate large populations of genetically distinct variants, permitting rapid adaptation to environmental changes. With mutation rates of between 10- 6 and 10-4 substitutions per nucleotide, arboviral genomes rapidly acquire mutations that can lead to viral emergence. Arboviruses can be described in seven families, four of which have medical importance: Togaviridae, Flaviviridae, Bunyaviridae and Reoviridae. The Togaviridae and Flaviviridae both have ssRNA genomes, while the Bunyaviridae and Reoviridae possess segmented RNA genomes. Recent epidemics caused by these arboviruses have been associated with specific mutations leading to enhanced host ranges, vector shifts and virulence.

Bluetongue virus genetic and phenotypic diversity: towards identifying the molecular determinants that influence virulence and transmission potential

Bluetongue virus (BTV) is the prototype member of the Orbivirus genus in the family Reoviridae and is the aetiological agent of the arthropod transmitted disease bluetongue (BT) that affects both ruminant and camelid species. The disease is of significant global importance due to its economic impact and effect on animal welfare. Bluetongue virus, a dsRNA virus, evolves through a process of quasispecies evolution that is driven by genetic drift and shift as well as intragenic recombination. Quasispecies evolution coupled with founder effect and evolutionary selective pressures has over time led to the establishment of genetically distinct strains of the virus in different epidemiological systems throughout the world. Bluetongue virus field strains may differ substantially from each other with regards to their phenotypic properties (i.e. virulence and/or transmission potential). The intrinsic molecular determinants that influence the phenotype of BTV have not clearly been characterized. It is currently unclear what contribution each of the viral genome segments have in determining the phenotypic properties of the virus and it is also unknown how genetic variability in the individual viral genes and their functional domains relate to differences in phenotype. In order to understand how genetic variation in particular viral genes could potentially influence the phenotypic properties of the virus; a closer understanding of the BTV virion, its encoded proteins and the evolutionary mechanisms that shape the diversity of the virus is required. This review provides a synopsis of these issues and highlights some of the studies that have been conducted on BTV and the closely related African horse sickness virus (AHSV) that have contributed to ongoing attempts to identify the molecular determinants that influence the virus' phenotype. Different strategies that can be used to generate BTV mutants in vitro and methods through which the causality between particular genetic modifications and changes in phenotype may be determined are also described. Finally examples are highlighted where a clear understanding of the molecular determinants that influence the phenotype of the virus may have contributed to risk assessment and mitigation strategies during recent outbreaks of BT in Europe.

Temperature dependence of the extrinsic incubation period of orbiviruses in Culicoides biting midges

Background

The rate at which viruses replicate and disseminate in competent arthropod vectors is limited by the temperature of their environment, and this can be an important determinant of geographical and seasonal limits to their transmission by arthropods in temperate regions.

Methodology/Principal Findings

Here, we present a novel statistical methodology for estimating the relationship between temperature and the extrinsic incubation period (EIP) and apply it to both published and novel data on virus replication for three internationally important orbiviruses (African horse sickness virus (AHSV), bluetongue virus (BTV) and epizootic haemorrhagic disease virus (EHDV)) in their Culicoides vectors. Our analyses show that there can be differences in vector competence for different orbiviruses in the same vector species and for the same orbivirus in different vector species. Both the rate of virus replication (approximately 0.017-0.021 per degree-day) and the minimum temperature required for replication (11-13°C), however, were generally consistent for different orbiviruses and across different Culicoides vector species. The estimates obtained in the present study suggest that previous publications have underestimated the replication rate and threshold temperature because the statistical methods they used included an implicit assumption that all negative vectors were infected.

Conclusions/Significance

Robust estimates of the temperature dependence of arbovirus replication are essential for building accurate models of transmission and for informing policy decisions about seasonal relaxations to movement restrictions. The methodology developed in this study provides the required robustness and is superior to methods used previously. Importantly, the methods are generic and can readily be applied to other arbovirus-vector systems, as long as the assumptions described in the text are valid.

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.

Oral susceptibility of South African stock-associated Culicoides species to bluetongue virus

Field-collected South African Culicoides species (Diptera, Ceratopogonidae) were fed on sheep blood containing bluetongue virus (BTV) represented by 13 low-passage reference serotypes: -1, -2, -4, -6, -7, -8, -9, -10, -11, -12, -13, -16 and -19. After 10 days of extrinsic incubation at 23.5 degrees C, of the 13 serotypes used, seven were recovered from C. (Avaritia) imicola Kieffer and 11 from C. (A.) bolitinos Meiswinkel. Virus recovery rates and the mean titres for most serotypes were significantly higher in C. bolitinos than in C. imicola. In addition, BTV was recovered from three non-Avaritia Culicoides species, namely C. (Remmia) enderleini Cornet & Brunhes (BTV-9), C. (Hoffmania) milnei Austen (BTV-4) and C. (H.) zuluensis de Meillon (BTV-16). No virus could be recovered from 316 individuals representing a further 14 Culicoides species. In Culicoides species fed on blood containing similar or identical virus titres of distinct BTV serotypes, significant differences were found in virus recovery rates. The results of this study confirm the higher vector competence of C. bolitinos compared with C. imicola.

Oral susceptibility to bluetongue virus of Culicoides (Diptera: Ceratopogonidae) from the United Kingdom

Oral susceptibility to infection with bluetongue virus (family Resviridae, genus Orbivirus, BTV) serotype 9 was characterized in three Palaearctic species of Culicoides (Diptera: Ceratopogonidae). Variation in susceptibility to infection by using a recently described feeding technique was shown to occur between populations of Culicoides obsoletus Meigen complex midges from different geographic regions of the United Kingdom with virus infection rates varying from 0.4 to 7.4% of those tested. Susceptibility to infection was consistent on an annual basis at selected sites. Prevalence of infection in the most susceptible populations of both the C. obsoletus and Culicoides pulicaris L. complexes was comparable with that of Culicoides imicola Kieffer, the major vector of BTV in southern Europe and throughout Africa, when using the same feeding method and virus. These results are discussed with reference to the potential threat of the virus to susceptible livestock in northern Europe.

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.

Effects of altitude, distance and waves of movement on the dispersal in Australia of the arbovirus vector, Culicoides brevitarsis Kieffer (Diptera: Ceratopogonidae)

The dispersal of the biting midge and arbovirus vector Culicoides brevitarsis in the Bellinger, Macleay and Hastings river valleys and up the escarpment of the great dividing range (GDR) of mid-northern coastal New South Wales, Australia, from 1995 to 2003 was studied. The midge moved up these valleys from the endemic coastal plain in at least two waves between October and May, and both waves were modelled. Dispersal time can be explained by direct distance from the coast and the altitude of the sites. Dispersal times due to distance were similar at 18.2 +/- 2.2 (S.D.) and 15.9 +/- 2.6 weeks per 100 km for first- and second-occurrences at fixed altitude. Time of the first wave was extended 0.48 +/- 0.22 weeks for every 100-m rise in altitude and the second by 1.14 +/- 0.24 weeks for every 100-m rise for a set distance. Although C. brevitarsis can move up the escarpment of the GDR (and possibly transmit virus), vector dispersal, survival and establishment at and beyond the top of the range are limited. A third model showed that previously described slower movement of C. brevitarsis up the more-southerly Hunter valley relative to movements down the coastal plain also was related to increasing altitude.

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

The oral susceptibility of livestock-associated South African Culicoides midges (Diptera: Ceratopogonidae) to infection with the tissue culture-attenuated vaccine strains of African horse sickness virus (AHSV) currently in use is reported. Field-collected Culicoides were fed on horse blood-virus mixtures each containing one of the seven serotype-specific vaccine strains of AHSV, namely serotypes 1, 2, 3, 4, 6, 7 and 8. The mean titres of virus in the bloodmeals for the seven vaccine strains were between 6.8 and 7.6 log10TCID50/mL. All females (n = 3262) that survived 10 days extrinsic incubation (10 dEI) at 23.5 degrees C were individually assayed in microplate BHK-21 cell cultures. In midges tested immediately after feeding, AHSV was detected in 96.1% individuals; mean virus titre was 2.0 log10TCID50/midge. After 10 dEI virus recovery rates varied in Culicoides (Avaritia) imicola Kieffer from 1% (AHSV-2) to 11% (AHSV-7) and in Culicoides (A.) bolitinos Meiswinkel from 0% (AHSV-3) to 14.6% (AHSV-2). Although our results indicate that two major field vectors C. imicola and C. bolitinos are susceptible to oral infection with vaccine strains of AHSV, the level of viral replication for most of the vaccine strains tested was below the postulated threshold (=2.5 log10TCID50/midge) for fully disseminated orbivirus infection. In this study, for the first time AHSV has been recovered after 10 dEI from six non-Avaritia livestock-associated Old World species: C. engubandei de Meillon (AHSV-4), C. magnus Colaço (AHSV-3, -4), C. zuluensis de Meillon (AHSV-2, -4), C. pycnostictus Ingram & Macfie (AHSV-2), C. bedfordi Ingram & Macfie (AHSV-7), and C. dutoiti de Meillon (AHSV-7). As little is known about the virogenesis of AHSV in the southern African species of Culicoides, the epidemiological significance of our findings in relation to the potential for transmission of current AHSV vaccine strains by Culicoides requires further assessment.

Determination of the oral susceptibility of South African livestock-associated biting midges, Culicoides species, to bovine ephemeral fever virus

A total of 10 607 Culicoides midges (Diptera: Ceratopogonidae) were fed on either sheep or horse blood containing not less than 6.5 log10 TCID50/ml of bovine ephemeral fever virus (BEFV). Insects were collected during two consecutive summers from two distinct climatic areas. Two seed viruses, originating from either South Africa or Australia, were used separately in the feeding trials. Blood-engorged females were incubated at 23.5 degrees C for 10 days and then individually assayed in microplate BHK-21 cell cultures. Of the 4110 Culicoides that survived, 43% were C. (Avaritia) imicola Kieffer and 27% were C. (A.) bolitinos Meiswinkel. The remainder represented 18 other livestock-associated Culicoides species. Although BEFV was detected in 18.9% of midges assayed immediately after feeding, no virus could be detected after incubation. The absence of evidence of either virus maintenance or measurable replication suggests that most of the abundant livestock-associated Culicoides species found in South Africa are refractory to oral infection with BEFV. Future studies should be carried out using species of mosquitoes that are associated with cattle in the BEF endemic areas.

A comparison of the vector competence of the biting midges, Culicoides (Avaritia) bolitinos and C. (A.) imicola, for the Bryanston serotype of equine encephalosis virus

Equine encephalosis virus (EEV) is widespread and prevalent in southern Africa. In this study, the oral susceptibility of Culicoides (Avaritia) imicola Kieffer (Diptera: Ceratopogonidae) to EEV was confirmed. In addition, C. (A.) bolitinos Meiswinkel, collected in the high-lying eastern Free State, South Africa, was systemically infected with the Bryanston serotype of EEV after feeding through a membrane on artificially infected equine blood containing 4.7 log10 PFU/mL of EEV. The mean infectivity of Bryanston virus in C. bolitinos increased from 1.2 log10 PFU/midge, in midges assayed for virus immediately after feeding on the blood-virus mixture, to 3.1 log10 PFU/midge in midges assayed after 10 days' incubation at 23.5 degrees C. Elevated virus infectivity titres, found in individual infected C. bolitinos, suggested that this Culicoides species is a vector of EEV. This bovine dung-breeding Culicoides species may play an important role in transmitting EEV in the cooler parts of southern Africa, where it can be the most abundant Culicoides species collected near livestock. In the present study the prevalence of infection obtained for C. bolitinos (2.2%) with the Bryanston serotype of EEV was significantly lower than that of C. imicola (18.4%). After incubation, the Bryanston serotype of EEV was also isolated from one of 110 C. onderstepoortensis Fiedler assayed. However, the virus titre in this midge was 1.2 log10 PFU/midge, which is not different from the titre that would be expected immediately after feeding on the blood-virus mixture. Culicoides species that survived the incubation period and that were negative for the presence of Bryanston virus were C. magnus Colaço (96), C. bedfordi Ingram & Macfie (95) and C. pycnostictus Ingram & Macfie (45).

Vector competence of South African Culicoides species for bluetongue virus serotype 1 (BTV-1) with special reference to the effect of temperature on the rate of virus replication in C. imicola and C. bolitinos

The oral susceptibility of 22 South African livestock associated Culicoides species to infection with bluetongue virus serotype 1 (BTV-1) and its replication rate in C. imicola Kieffer and C. bolitinos Meiswinkel (Diptera: Ceratopogonidae) over a range of different incubation periods and temperatures are reported. Field-collected Culicoides were fed on sheep blood containing 7.5 log10TCID50/mL of BTV-1, and then held at constant different temperatures. Virus replication was measured over time by assaying individual flies in BHK-21 cells using a microtitration procedure. Regardless of the incubation temperatures (10, 15, 18, 23.5 and 30 degrees C) the mean virus titre/midge, infection rates (IR) and the proportion of infected females with transmission potential (TP = virus titre/midge > or = 3 log10 TCID50) were found to be significantly higher in C. bolitinos than in C. imicola. Results from days 4-10 post-infection (dpi), at 15-30 degrees C, shows that the mean IR and TP values in C. bolitinos ranged from 36.7 to 87.8%, and from 8.4 to 87.7%, respectively; in C. imicola the respective values were 11.0-13.7% and 0-46.8%. In both species the highest IR was recorded at 25 degrees C and the highest TP at 30 degrees C. The time required for the development of TP in C. bolitinos ranged from 2 dpi at 25 degrees C to 8 dpi at 15 degrees C. In C. imicola it ranged from 4 dpi at 30 degrees C to 10 dpi at 23.5 degrees C; no individuals with TP were detected at 15 degrees C. There was no evidence of virus replication in flies held at 10 degrees C. When, at various points of incubation, individual flies were transferred from 10 degrees C to 23.5 degrees C and then assayed 4-10 days later, virus was recovered from both species. The mean virus titres/midge, and proportion of individuals with TP and IR, were again significantly higher in C. bolitinos than in C. imicola. Also the infection prevalence in C. magnus Colaço was higher than in C. imicola. Low infection prevalences were found in C. bedfordi Ingram & Macfie, C. leucostictus Kieffer, C. pycnostictus Ingram & Macfie, C. gulbenkiani Caeiro and C. milnei Austen. BTV-1 was not detected in 14 other Culicoides species tested; however, some of these were tested in limited numbers. The present study indicates a multivector potential for BTV transmission in South Africa. In C. imicola and C. bolitinos the replication rates are distinct and are significantly influenced by temperature. These findings are discussed in relation to the epidemiology of bluetongue in South Africa.

Vector competence of selected South African Culicoides species for the Bryanston serotype of equine encephalosis virus

Equine encephalosis virus (EEV) was recognized and described in the Republic of South Africa in 1967 and subsequent serological studies have shown this orbivirus to be both widespread and prevalent in southern Africa. In the present study it was shown that wild-caught Culicoides (Avaritia) imicola Kieffer (Diptera: Ceratopogonidae) can become infected with and permit the replication of the Bryanston serotype of EEV following membrane-feeding on infective blood containing 5.0 log10 plaque-forming-units (PFU)/ml. The mean prevalence of Bryanston virus infection in C. imicola after 10 days extrinsic incubation at 23.5 degrees C was 22.3% (23/103). The mean infectivity of Bryanston virus in the infected C. imicola increased from 1.3 log10 PFU/midge, in insects assayed immediately after feeding on the blood-virus mixture, to 2.6 log10 PFU/midge in insects assayed after incubation. The virus concentration in individual C. imicola infected with the Bryanston serotype of EEV ranged from 0.7 to 3.6 log10 PFU/midge. Bryanston virus titres higher than 2.5 log10 TCID50, found in individual C. imicola, suggest that this species may be able to transmit this virus to susceptible hosts. Prevalence of virus infection in C. imicola was determined by PFU and microtitration assays on both BHK and Vero cells and confirmation of the Bryanston serotype of EEV was determined by plaque inhibition. No virus replication could be demonstrated in 102 C. nivosus tested after the incubation period, suggesting that not all Culicoides species are equally susceptible to Bryanston virus infection. Other Culicoides species that survived the incubation period and that were negative for the presence of Bryanston virus were C. pycnostictus (42), C. leucostictus (7), C. magnus (2), C. bolitinos (1) and C. bedfordi (1).

Association between risk of seroconversion of sentinel cattle to bluetongue viruses and Culicoides species (Diptera: Ceratopogonidae) in Queensland, Australia

The association between risk of seroconversion of sentinel cattle to bluetongue viruses and the number of Culicoides brevitarsis Kieffer and C. wadai Kitaoka caught by light traps was investigated using survival analysis. Eight sentinel herds that seroconverted to bluetongue viruses between 1990 and 1994, and for which insect-trapping data were available, were selected for inclusion in the study. These herds were located at six sites along the eastern coast of Queensland, Australia, from approximately latitude 10 degrees South to 25 degrees South. C. brevitarsis was detected at all locations where sentinel herds were maintained, whereas C. wadai was detected at only two locations in northern Queensland where four sentinel herds were maintained during the study period. The mean number of C. brevitarsis and C. wadai caught per month was 230 and 21, respectively. A significant (P = 0.05) positive association was found between the risk of seroconversion of sentinel cattle to bluetongue viruses and the number of C. wadai caught in the same month.