Philip Scott Mellor

Veterinary Research, a journal on Animal Infection

Bluetongue virus (BTV) is transmitted between its mammalian hosts almost exclusively via bites from the adults of certain species of Culicoides biting midges. Theoretically, the spread of BTV into the more northerly areas of Europe should therefore be terminated by the harsh winters experienced in these regions, when adult midges disappear for extended periods of time. However, it has been shown that BTV can survive for periods as long as 9 to 12 months in such locations in the absence of adult insect vectors, with no detectable cases of viraemia, overt disease or seroconversion in the host species. Virus survival in this manner throughout the winter is called 'overwintering' but the mechanism involved has not been satisfactorily explained. With knowledge currently available and results from a series of preliminary experiments, the authors discuss a possible overwintering mechanism.

Bluetongue virus (BTV) has persisted within Europe for the past five years, highlighting a need for rapid and reliable virus detection and identification methods. Various RT-PCR protocols and strategies, which target genome segment 7, were evaluated for their ability to detect all members of the BTV species (serogroup), with the aim of developing a fully validated reverse transcriptase-polymerase chain reaction- (RT-PCR) based diagnostic assay. A nested PCR strategy, using near terminal and internal segment 7 primers, detected all 24 BTV serotypes, but also cross-reacted with some other related Orbivirus species. In an attempt to circumvent these problems, conventional PCR and touch-down PCR methods, using similar primers were also investigated. Both methods were able to amplify cDNA from only 21 of the 24 BTV types. Further sequence analyses of the VP7 gene from the remaining isolates (types 7, 15 and 19) will permit the design of additional and more effective virus-species specific primers and RT-PCR-based assays. This may include the introduction of a multiplex PCR system.

Determination of the temporal relationships between climate and epidemics of Culicoides-borne viral disease may lead to control measures and surveillance being implemented earlier and more efficiently. Although Israel has reported few cases of bluetongue (BT) during the recent Mediterranean epidemic, outbreaks have occurred almost annually since the disease was first confirmed there (1950) with severe episodes occurring periodically. The south Mediterranean location and intensive farming of BT-susceptible European sheep breeds make the area ideal for investigation of the effect or role of climatic factors versus other potential host or virus factors in governing the timing of severe BT episodes. The authors present regression analyses of 20-year time-series of BT outbreaks versus four remotely sensed climatic variables. Low temperatures and high moisture levels (relative to average levels) in the preceding autumn coincident with the seasonal peak of vector abundance and outbreaks had a positive effect on the number of outbreaks the following year. The positive effects of high moisture levels are postulated to increase breeding site availability and refugia for adult C. imicola vectors (from desiccation) in autumn whilst low temperatures may increase fecundity, offspring size and survival through adulthood in winter by increasing initial vector population size the following year. The proportion of variance in the annual BT outbreak time series accounted for by climate factors was relatively low (approximately 20%), probably because most BT virus (BTV) circulation occurs silently, due to the circulation of non-virulent BTV strains, combined with the prevalence of relatively resistant local sheep breeds. Thus, the level of BTV transmission is poorly correlated with the rate of outbreak notification.

Blacklight traps were used to collect Culicoides biting midges weekly between September 1996 and August 1998 at 40 sites distributed equidistantly across South Africa. The seasonal and geographic prevalences of 86 species of Culicoides were elucidated simultaneously, and included C. imicola Kieffer and C. bolitinos Meiswinkel the principal vectors of bluetongue (BT) and African horse sickness (AHS) in the region. These two species were amongst the most prevalent Culicoides to be found and, together, comprised >50% of the more than three million biting midges captured. The data are presented as coloured matrices, and are transformed also into inverse distance weighting (IDW) interpolative maps. The data reveal that the prevalence of each vector is somewhat fractured and it is posited that this is (in part) due to significant differences in their respective breeding habitats. The results illustrate also that the presence of multiple vectors (in any region of the world) will complicate both the epidemiology of the orbiviral diseases they transmit and the formulation of rational livestock movement and disease control strategies. This is especially true for southern Europe where the recent devastating cycle of BT has been shown to involve at least three vectors. Finally, the influence that man has on the development of large foci of vector Culicoides around livestock may be less important than previously suggested but must be investigated further.

Vector (911 light-trap catches from 269 sites) and serological surveillance data were obtained during recent bluetongue (BT) outbreaks in Sicily. The distributions of Culicoides vectors are compared with that of bluetongue virus (BTV) to determine the relative role of different vectors in BTV transmission in Sicily. The 'best' climatic predictors of distribution for each vector species were selected from 40 remotely-sensed variables and altitude at a 1 km spatial resolution using discriminant analysis. These models were used to predict species presence in unsampled pixels across Italy. Although Culicoides imicola, the main European vector, was found in only 12% of sites, there was close correspondence between its spatial distribution and that of the 2000 and 2001 outbreaks. All three candidate vectors C. pulicaris, C. newsteadi and C. obsoletus group were widespread across 2002 outbreak sites but C. newsteadi was significantly less prevalent in outbreak versus non-outbreak sites in Messina and BTV has been isolated from wild-caught adults of both C. pulicaris and C. obsoletus in Italy. The yearly distribution and intensity of outbreaks is attributable to the distribution and abundance of the vectors operating in each year. Outbreaks were few and coastal in 2000 and 2001 due to the low abundance and prevalence of the vector, C. imicola. They were numerous and widespread in 2002, following hand-over of the virus to more prevalent and abundant novel vector species, C. pulicaris and C. obsoletus. Climatic determinants of distribution were species-specific, with those of C. obsoletus group and C. newsteadi predicted by temperature variables, and those of C. pulicaris and C. imicola determined mainly by normalised difference vegetation index (NDVI), a variable correlated with soil moisture, vegetation biomass and productivity. The predicted continuous presence of C. pulicaris along the Appenine mountains, from north to south Italy, suggests BTV transmission may be possible in a large proportion of this region and that seasonal transhumance between C. imicola-free areas should not generally be considered safe. Future distribution models for C. imicola in Sicily should include non-climatic environmental variables that may influence breeding site suitability such as soil type.

Veterinary Research, a journal on Animal Infection

This report constitutes the first study of Culicoides spp. and their seasonal abundance at Al-Ahsa, the largest oasis in the Eastern Province of Saudi Arabia. New Jersey light traps were used to collect the midges at Mastock farm and Al-Mansura village. The mean monthly abundance was determined from October 1993 to October 1994. The mean monthly number per trap reached its minimum value during January 1994, increasing gradually from February to reach its maximum value during September 1994. During the study period, the following species were collected: Culicoides schultzei group (September), non-spotted group of Culicoides (September), Culicoides imicola (May) and Culicoides newstaedi (March). The potential importance of the Culicoides spp. in relation to arboviral activity in Saudi Arabia is discussed.