Part 1 The importance of blood-sucking insects. Part 2 The evolution of the blood-sucking habit: prolonged close association with vertebrates morphological pre-adaptation for piercing. Part 3 Feeding preferences of blood-sucking insects: host choice host choice and species complexes. Part 4 Location of the host: the behavioural framework of host location appetitive searching activation and orientation attraction movement between hosts. Part 5 Ingestion of the blood meal: vertebrate haemostasis insect anti-haemostatic factors probing stimulants phagostimulants mouthparts blood intake. Part 6 Managing the blood meal: midgut anatomy the blood meal gonotrophic concordance nutrition host hormones in the blood meal partitioning of resources from the blood meal autogeny. Part 7 Host - insect interactions: insect distribution on the surface of the host morphological specializations for life on the host host immune responses to insect salivary secretions behavioural defences of the host density dependent effects on feeding success. Part 8 Transmission of parasites by blood-sucking insects: transmission routes specificity in vector-parasite relationships origin of vector parasite relationships.

Biology of Blood-Sucking Insects

Bluetongue (BT) exists around the world in a broad band covering much of the Americas, Africa, southern Asia, northern Australia and, occasionally, the southern fringe of Europe. It is considered to be one of the most important diseases of domestic livestock. Recently the virus causing this disease has extended its range northwards into areas of Europe never before affected and has persisted in many of these locations causing the greatest epizootic of the disease on record. The reasons for this dramatic change in BT epidemiology are complex but are linked to recent extensions in the distribution of its major vector, Culicoidesimicola, to the involvement of novel Culicoides vector(s) and to an apparent ability of the virus to overwinter in the absence of adult vectors. In addition, the effects of these changes have been exacerbated by problems in control, particularly in relation to vaccination. This paper explores these areas and highlights prospects for the future.

Bluetongue virus in the Mediterranean Basin 1998-2001.

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.

This paper describes the first isolations of African horse sickness virus (AHSV) from insects in Spain. Seven isolations of AHSV serotype 4 were made; four from Culicoides imicola a known vector of the virus elsewhere, two from mixed pools of Culicoides species not including C. imicola and one from blood engorged mosquitoes. Three further isolations of AHSV serotype 4 were also made from horses kept adjacent to the insect collecting sites. This work presents the first definitive identification of the vectors of AHSV in Spain during the 1987, 88 and 89 epizootics. Suggestions are also made concerning the significance of these findings with regard to the epidemiology of African horse sickness in Spain.

/pdf/isolations-of-african-horse-sickness-virus-from-vector-39jyi53g51.pdf

Isolations of African horse sickness virus from vector insects made during the 1988 epizootic in Spain

Bluetongue virus (BTV) and several other Orbivirus species are transmitted between mammalian hosts via bites from adults of certain species of Culicoides midges. However, BTV can survive for 9–12 months (typically during the winter), in the absence of adult vectors, with no detectable cases of viraemia, disease or seroconversion in the host. The survival of the virus from one ‘vector season’ to the next is called ‘overwintering’ but the mechanism involved is not fully understood. It is demonstrated that BTV can persistently infect ovine γδ T-cells in vitro, a process that may also occur during infection and viraemia in mammalian hosts, thus providing a mechanism for virus persistence. Interaction of persistently BTV-infected γδ T-cells with antibody to the γδ T-cell-specific surface molecule WC-1 resulted in conversion to a lytic infection and increased virus release. Skin fibroblasts induce a similar conversion, indicating that they express a counter ligand for WC-1. Feeding of Culicoides midges induces skin inflammation, which is accompanied by recruitment of large numbers of activated γδ T-cells. The interaction of persistently infected γδ T-cells with skin fibroblasts would result in increased virus production at ‘biting sites’, favouring transmission to the insect vector. This suggested mechanism might also involve up-regulation of the WC-1 ligand at inflamed sites. It has been shown previously that cleavage of virus surface proteins by protease enzymes (which may also be associated with inflammation) generates infectious subvirus particles that have enhanced infectivity (100 times) for the insect vector.

A possible overwintering mechanism for bluetongue virus in the absence of the insect vector.

Culicoides nubeculosus under normal experimental conditions was found incapable of supporting multiplication of bluetongue virus (BTV) after infection via the oral route. Simultaneous infection with BTV and the microfilariae of Onchocerca cervicalis, a filarial worm of the horse, transformed this situation. A small proportion of infected midges subsequently developed high titres of virus, which persisted until the termination of the experiment at day 13 post infection. C. nubeculosus intrathoracically inoculated with BTV were found to transmit the virus after 7, 8 and 14 days incubation at 26°C, with no evidence for a ‘salivary gland barrier’. It is therefore suggested that C. nubeculosus can act as a vector of BTV when simultaneously infected with virus and the microfilariae of a filarial worm.

Multiplication of bluetongue virus in culicoides nubeculosus (Meigen) simultaneously infected with the virus and the microfilariae of Onchocerca cervicalis (Railliet & Henry).

J. Boorman

Papers

Multiplication of bluetongue virus in culicoides nubeculosus (Meigen) simultaneously infected with the virus and the microfilariae of Onchocerca cervicalis (Railliet & Henry).