Bluetongue (BLU) virus is transmitted from infected to susceptible ruminants by hematophagous vector midges (Culicoides species). Cattle are important reservoir hosts of the virus because infection typically is asymptomatic and characterized by prolonged cell associated viremia, and because at least some species of insect vector preferentially feed on cattle. Interaction of BLU virus with the cell membrane of erythrocytes in infected cattle likely facilitates both prolonged viremia as well as infection of the insect vector. BLU disease is most common in sheep and some wildlife species. A variety of host, agent and environmental factors clearly can influence expression of disease in these species. The pathogenesis of BLU virus infection of cattle and sheep is remarkably similar, thus the basis for expression of disease in sheep but not cattle remains to be firmly established. Some difference in susceptibility of endothelial cells to infection in the two species is one potential explanation.

Ruminants develop a variety of antiviral responses after BLU virus infection. Antibodies to outer capsid protein VP2 are responsible for virus neutralization, and confer resistance to reinfection with the homologous serotype of BLU virus. Antibodies to epitopes on proteins which are common to all viruses of the BLU serogroup form the basis of current diagnostic serologic tests. Cell mediated responses have been incompletely characterized, in part because BLU virus replicates within dividing lymphocytes and virus-mediated cytolysis inhibits in vitro blastogenesis. Immunological competence of ruminants to BLU virus arises prior to midgestation, and suggestions that persistent immune tolerant BLU virus infection occurs after in utero exposure of cattle have not been substantiated and are not consistent with recent findings.

The pathogenesis and immunology of bluetongue virus infection of ruminants

Culicoides spp. biting midges transmit bluetongue virus (BTV), the aetiological agent of bluetongue (BT), an economically important disease of ruminants. In southern India, hyperendemic outbreaks of BT exert high cost to subsistence farmers in the region, impacting on sheep production. Effective Culicoides spp. monitoring methods coupled with accurate species identification can accelerate responses for minimising BT outbreaks. Here, we assessed the utility of sampling methods and DNA barcoding for detection and identification of Culicoides spp. in southern India, in order to provide an informed basis for future monitoring of their populations in the region. Culicoides spp. collected from Tamil Nadu and Karnataka were used to construct a framework for future morphological identification in surveillance, based on sequence comparison of the DNA barcode region of the mitochondrial cytochrome c oxidase I (COI) gene and achieving quality standards defined by the Barcode of Life initiative. Pairwise catches of Culicoides spp. were compared in diversity and abundance between green (570 nm) and ultraviolet (UV) (390 nm) light emitting diode (LED) suction traps at a single site in Chennai, Tamil Nadu over 20 nights of sampling in November 2013. DNA barcode sequences of Culicoides spp. were mostly congruent both with existing DNA barcode data from other countries and with morphological identification of major vector species. However, sequence differences symptomatic of cryptic species diversity were present in some groups which require further investigation. While the diversity of species collected by the UV LED Center for Disease Control (CDC) trap did not significantly vary from that collected by the green LED CDC trap, the UV CDC significantly outperformed the green LED CDC trap with regard to the number of Culicoides individuals collected. Morphological identification of the majority of potential vector species of Culicoides spp. samples within southern India appears relatively robust; however, potential cryptic species diversity was present in some groups requiring further investigation. The UV LED CDC trap is recommended for surveillance of Culicoides in southern India.

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DNA barcoding and surveillance sampling strategies for Culicoides biting midges (Diptera: Ceratopogonidae) in southern India

CHAPTER 4 – The Female Genital System

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.

Field observations during the Bluetongue serotype 8 epidemic in 2006: II. Morbidity and mortality rate, case fatality and clinical recovery in sheep and cattle in the Netherlands

Jones Rh

Papers

Preliminary investigations on insect transmission of bluetongue virus in sheep.

Bluetongue in cattle: effects of Culicoides variipennis-transmitted bluetongue virus on pregnant heifers and their calves.

Transmission of bluetongue between sheep and cattle by Culicoides variipennis.

Bluetongue in cattle: viremia.

Observations on latent bluetongue virus infection in cattle.