Bluetongue virus (BTV) has long been known to be transmitted biologically by certain species of biting midge belonging to the genus Culicoides (Latreille). Du Toit (1944) in South Africa was the first to implicate a Culicoides species in the transmission of this virus when he showed that C. imicola (pallidipennis) was able to transmit bluetongue from infected to susceptible sheep. Since that time numerous authors have confirmed his observations (Walker and Davies 1971; Braverman and Galun 1973; Braverman et al. 1981, 1985; Mellor et al. 1984a). However, although there are well over 1000 species of Culicoides in the world (Boorman 1988), only 17 have been connected with BTV and to date only six, C. variipennis, C. imicola, C. fulvus, C. actoni, C. wadai and C. nubeculosus, have been proven to transmit the virus (Table 1). This number may soon be increased to eight since on the basis of epidemiological evidence and virus isolations it is 


Table 1
Field and laboratory BTV infections of Culicoides



	
Subgenus
	
Species
	
Virus isolation
	
Laboratory infection
	
Transmission


Avaritia
	
C. actoni
	
−
	
+
	
+
	
	
C. brevipalpis
	
−
	
+
	
−
	
	
C. brevitarsis
	
+
	
+
	
−
	
	
C. fulvus
	
+
	
+
	
+
	
	
C. imicola
	
+
	
+
	
+
	
	
C. obsoletus
	
+
	
−
	
−
	
	
C. tororoensis
	
+
	
−
	
−
	
	
C. wadai
	
−
	
+
	
+
	

Culicoides
	
C. peregrinus
	
−
	
±
	
−
	

Diphaeomia
	
C. debilipalpis
	
−
	
+
	
−
	

Hoffmania
	
C. insignis
	
+
	
−
	
−
	
	
C. milnei
	
+
	
-
	
-
	
	
C. venustus
	
−
	
+
	
−
	

Monoculicoides
	
C. variipennis
	
+
	
+
	
+
	
	
C. nubeculosus
	
−
	
+
	
+
	

Oecacta
	
C. oxytoma
	
−
	
+
	
−
	

Pulicaris
	
C. impunctatus
	
−
	
±
	
−
	


likely that C. insignis and C. brevitarsis will also prove to be competent BTV vectors (Greiner et al. 1985; Standfast et al. 1985). Although some of the remaining species of Culicoides may eventually be shown to be fully competent BTV vectors most species will be refractory to infection. Why this should be is not yet entirely clear, but it is known that the mechanism or mechanisms controlling the oral infection of Culicoides with BTV operate chiefly at the level of the mid-gut wall (Jennings and Mellor 1988), a single layer of cells of epithelial origin supported by a basement lamina (Hardy et al. 1983; Megahed 1956).

The replication of bluetongue virus in Culicoides vectors.

The mosquitoes Anopheles stephensi Liston and Culex quinquefasciatus Say (Diptera: Culicidae), the stable fly Stomoxys calcitrans Linnaeus (Diptera: Muscidae) and the biting midge Culicoides nubeculosus Meigen (Diptera: Ceratopogonidae) were allowed to feed on either lumpy skin disease (LSD) infected animals or through a membrane on a bloodmeal containing lumpy skin disease virus (LSDV). These arthropods were then allowed to refeed on susceptible cattle at various intervals after the infective feed. Virus was detected in the insects by polymerase chain reaction immediately after feeding and at sufficiently high titre to enable transmission to occur. However, no transmission of virus from infected to susceptible animals by An. stephensi, S. calcitrans, C. nubeculosus and Cx. quinquefasciatus was observed.

Attempted mechanical transmission of lumpy skin disease virus by biting insects

Transmission of bluetongue virus (BTV) by a vector species of Culicoides was studied using immunohistochemistry, virus titration and in vitro transmission tests. Adult female C. variipennis were used from two colonies that are either "transmission competent" or "transmission refractory" after oral infection with BTV. Intrathoracic (i.t.) injection of BTV into the haemocoel always resulted in a fully disseminated infection and transmission of virus in saliva. However, after ingestion of an infectious blood meal, only 30% (approximately) of midges from either colony became persistently infected. Although none of the orally infected insects from the "refractory" colony were able to transmit virus, 12% of those from the "competent" colony (containing > or = 10(3.0)TCID50 of virus/midge) did transmit BTV in their saliva. The most important barriers to BTV transmission in Culicoides vector species appeared to be a mesenteron infection barrier (MIB), which controls initial establishment of persistent infection, a mesenteron escape barrier (MEB) which can restrict virus to gut cells and a dissemination barrier (DB) which can prevent virus which enters the haemocoel from infecting secondary target organs. Culicoides variipennis do not appear to present either a salivary gland infection barrier (SGIB), or a salivary gland escape barrier (SGEB) to BTV.

The barriers to bluetongue virus infection, dissemination and transmission in the vector, Culicoides variipennis (Diptera: Ceratopogonidae)

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 (11-13uC), 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.

/pdf/temperature-dependence-of-the-extrinsic-incubation-period-of-cj0vbrabc2.pdf

Temperature Dependence of the Extrinsic Incubation Period of Orbiviruses in Culicoides Biting Midges

A duplex RT-PCR assay for detection of genome segment 7 (VP7 gene) from 24 BTV serotypes

The transmission of Onchocerca cervicalis Rail. and Henry in the horse is being investigated at Winches Farm Field Station, St. Albans. Laboratory colonies of Culicoides nubeculosus Mg., the natural vector of O. cervicalis in Europe, and of C. variipennis sonorensis Coq., a North American midge, have been established. A technique has been developed to facilitate the artificial infection of these midges with O. cervicalis. Experimental flies imbibed a meal of O. cervicalis microfilariae suspended in defibrinated ox blood at 38°C. A plucked skin from a one-day-old chick was used as a membrane. 232 C. nubeculosus and 117 C. v. sonorensis were fed and 5·5% of the C. nubeculosus and 7·7% of the C. v. sonorensis subsequently developed infective (3rd stage) larvae. The fact that 3rd stage larvae developed in specimens of C. v. sonorensis is a point of special interest because this midge has not yet been identified as a vector of O. cervicalis.

A membrane feeding technique for the infection of Culicoides nubeculosus mg. and Culicoides variipennis sonorensis coq. with Onchocerca cervicalis Rail. and Henry

P.S. Mellor

Papers

A membrane feeding technique for the infection of Culicoides nubeculosus mg. and Culicoides variipennis sonorensis coq. with Onchocerca cervicalis Rail. and Henry