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Monitoring of Putative Vectors of Bluetongue Virus Serotype 8, Germany
review-article
Author(s):
Bernd Hoffmann ,
Burkhard Bauer ,
Christian Bauer ,
Hans-Joachim Bätza ,
Martin Beer ,
Peter-Henning Clausen ,
Martin Geier ,
Jörn M. Gethmann ,
Ellen Kiel ,
Gabriele Liebisch ,
Arndt Liebisch ,
Heinz Mehlhorn ,
Günter A. Schaub ,
Doreen Werner ,
Franz J. Conraths
Publication date (Print):
September 2009
Journal:
Emerging Infectious Diseases
Publisher:
Centers for Disease Control and Prevention
Keywords:
Culicoides,
bluetongue disease,
bluetongue virus,
monitoring,
epidemiology,
RT-PCR,
viruses,
Germany,
dispatch
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Abstract
To identify the vectors of bluetongue virus (BTV) in Germany, we monitored Culicoides spp. biting midges during April 2007–May 2008. Molecular characterization of batches of midges that tested positive for BTV suggests C. obsoletus sensu stricto as a relevant vector of bluetongue disease in central Europe.
Related collections
Most cited references9
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Bluetongue virus detection by two real-time RT-qPCRs targeting two different genomic segments.
J Toussaint, C Sailleau, E. Bréard … (2007)
The detection of the bluetongue virus (BTV) by conventional methods is especially difficult and labour-intensive. Molecular diagnosis is also complex because of the high genetic diversity between and within the 24 serotypes of BTV. In the present study, two laboratories joined forces to develop and validate two new RT-qPCRs detecting and amplifying BTV segments 1 and 5. The 2 assays detect strains from all 24 serotypes. They both have a detection limit of 0.01 ECE50 and all 114 samples from BTV-free goats, sheep and cattle were negative. The two assays resulted in similar C(t) values when testing biological samples collected in sheep infected experimentally with a field strain of BTV from the Mediterranean basin. On average, the C(t) values obtained with the 2 methods applied to the 24 serotypes were not significantly different from each other, but some moderate to high differences were seen with a few strains. Therefore these two methods are complementary and could be used in parallel to confirm the diagnosis of a possible new introduction of BTV. An RT-qPCR amplifying a fragment of the beta-actin mRNA was also developed and validated as internal control for the bluetongue specific assays. The three assays described allow a reliable and rapid detection of BTV.
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Bluetongue Epidemiology in the European Union
Claude Saegerman, Dirk Berkvens, Philip Mellor (2008)
Bluetongue (BT) is an infectious but noncontagious viral disease caused by Bluetongue virus (BTV). The virus belongs to the family Reoviridae, genus Orbivirus; there are 24 serotypes ( 1 ). The viral genome consists of 10 double-stranded RNA segments that encode for 4 nonstructural proteins (NS1, NS2, NS3, and NS3A) and 7 structural (VP1-VP7) proteins ( 2 , 3 ). BTV serotypes 1, 2, 3, 4, 6, and 10 have a high pathogenic index and high epidemic potential ( 4 ). However, a high genetic diversity of BTV exists that is a consequence of both drift (i.e., point mutations) and shift (i.e., reassortment of individual BTV gene segments) so pathogenicity even within a serotype may be highly variable ( 5 ). BT is a World Organization for Animal Health reportable disease and is of considerable socioeconomic concern and of major importance in the international trade of animals and animal products ( 4 ). Before 1998, BT was considered an exotic disease in Europe with just a few sporadic incursions (e.g., Spain and Portugal from 1956 through 1960) ( 6 ). Our aim in this article is to provide a synthesis and some perspectives of BT epidemiology in the European Union (EU) since BTV’s introduction in 1998. To this effect, we provide a short overview of the epidemiologic situation in Europe, followed by a brief description of the susceptible species, a discussion of the vectorial capacity and competence of the Culicoides spp. vectors, and an outline of the modes of introduction and mechanisms of amplification. Epidemiologic Situation in Europe BTV in EU, 1998–2005 During this 8-year period, at least 6 BTV strains belonging to 5 serotypes (BTV-1, BTV-2, BTV-4, BTV-9, and BTV-16) have been continuously present in parts of the Mediterranean Basin, including several member states of the EU (Table, Figure 1) ( 1 , 5 , 7 – 12 ). This emergence of BT into parts of Europe never before affected was attributed mainly to climate change and was linked to the northern expansion of the major Old World vector Culicoides imicola (Kieffer), which is an Afro-Asiatic species of biting midge ( 13 ). Additionally, novel indigenous European vector species of Culicoides within the Obsoletus and Pulicaris complexes were involved. Table Outbreaks of bluetongue in Europe, 1998–2005*† Country Year of first outbreak BTV serotype(s) Main suspected or identified vector(s) Albania 2002 9 Culicoides obsoletus, C. pulicaris Bosnia–Herzegovina 2002 9 ND Bulgaria 1999 9 C. obsoletus, C. pulicaris Croatia 2001 9, 16 C. obsoletus, C. scoticus Cyprus 2003 16 C. imicola, C. obsoletus, Former Yugoslav Republic of Macedonia 2001 9 ND France (Corsica) 2000 2, 4, 16‡ C. imicola, C. pulicaris, C. obsoletus Greece 1998 1, 4, 9, 16 C. imicola, C. obsoletus Italy 2000 1, 2, 4, 9, 16 C. imicola, C. obsoletus, C. pulicaris Kosovo 2001 9 ND Montenegro 2001 9 ND Portugal 2004 2,§ 4 C. imicola, C. obsoletus, C. pulicaris Serbia 2001 9 ND Spain 2000 2 C. imicola,C. obsoletus, C. pulicaris Turkey 1998 4, 9, 16 C. imicola,C. obsoletus, C. pulicaris *BTV, Bluetongue virus; ND, no data recorded.
†Sources ( 5 , 7 – 10 ).
‡This is an insufficiently attenuated vaccine strain ( 11 ).
§This strain is indistinguishable from Onderstepoort BTV-2 live attenuated vaccine strain ( 7 ). Figure 1 The molecular epidemiology of bluetongue virus (BTV) since 1998: routes of introduction of different serotypes and individual virus strains. *Presence of BTV-specific neutralizing antibodies in animals in Bulgaria, but the presence of BTV serotype 8 cannot yet be confirmed. In the Mediterranean Basin 2 epidemiologic systems seem to predominate. The first one is located in the eastern part of the basin, where serotypes 1, 4, 9, and 16 were identified. In this system, the BTV strains originated in the Near, Middle, or Far East. The vectors included other species of Culicoides in addition to C. imicola. This finding was deduced from the fact that the disease penetrated into areas where C. imicola does not occur (the Balkans and beyond) ( 9 ). The involvement of novel vectors was subsequently confirmed when the causative virus was isolated from mixed pools of 2 species, C. obsoletus (Meigen) and C. scoticus (Downes and Kettle), collected in central Italy ( 14 ) and from C. pulicaris (Linnaeus) in Sicily ( 15 ). The second epidemiologic system comprises the western part of the Mediterranean Basin, where serotypes BTV-1, BTV-2, BTV-4, and BTV-16 were identified and the main vector is C. imicola. Although the appearance of BTV serotype 16 in this C. imicola system is the result of the westward spread of the virus across Europe ( 16 ), it is of particular interest because of strong indications that the field virus may represent a reversion to virulence of the attenuated vaccine (e.g., in Corsica and in Sardinia in 2004, strains of BTV-16 isolated from the field were identical to the live attenuated monovalent vaccine strain) ( 1 , 11 ) (Table, Figure 1). BT in Central and Northern Europe, mid-August 2006 to late December 2007 BT was first identified in northern Europe in August 2006 and can be defined as an emergent disease in this zone ( 17 ). Between the date of the first report (August 17, 2006) and February 1, 2007 ( 18 ), 2,122 BT cases were entered into the European Commission’s Animal Disease Notification System (ADNS) (http://ec.europa.eu/food/animal/diseases/adns/index_en.htm) (Figure 2) ( 19 ). In this region, in 2006, a pool of 50 nonengorged, parous C. dewulfi (Goetghebuer) in the Netherlands were positive by PCR for BTV ( 20 ), and several pools of C. obsoletus complex in Germany (i.e., not identified down to species) were also PCR positive for BTV ( 21 ) (Figure 3). Although isolation of live BTV was not attempted in either instance, this research, conducted in an area where C. imicola does not occur, confirms the earlier findings of Mellor and Pitzolis, who isolated infectious BTV from nonengorged parous C. obsoletus in Cyprus, and shows that indigenous European Culicoides species can support a BT epizootic ( 22 ). Because C. obsoletus complex midges and C. dewulfi occur widely across central and northern Europe, this entire area must now be considered to be at risk for BTV ( 23 , 24 ). Figure 2 Monthly distribution of confirmed bluetongue virus 8 (BTV-8) outbreaks in northern and central Europe from August 17, 2006, through February 1, 2007. After January 1, 2007, few BTV cases were reported; those that were probably involved animals that had been infected, but not detected, in 2006. Figure 3 A gravid female Culicoides dewulfi collected from a location near bluetongue outbreaks in Belgium in 2006 (Photograph: Reginald De Deken and Maxime Madder, Institute of Tropical Medicine, Antwerp, Belgium). Moreover, in relation to the demonstrated overwintering ability of the virus in northern Europe, small numbers of adult Culicoides spp. were captured in animal housing during the winter period (November 25, 2006, to March 9, 2007) (i.e., females of C. obsoletus complex, males of C. obsoletus, C. scoticus, and C. dewulfi) ( 25 ). Whether the occurrence of these midges and the possibility of their activity extending over the winter in such climatically protected locations can explain the persistence of virus from 1 vector transmission season to the next ( 13 ) or whether they represent newly emerged midges from nearby breeding sites is not known ( 25 ). Several hypotheses have been formulated to explain the overwintering ability of BTV: by persistence within surviving adult vectors themselves, transovarial transmission through the vector, or prolonged/persistent infection in viremic or aviremic vertebrate hosts ( 13 , 25 , 26 ). The focus of interest now is to see if BTV is able to survive regularly between vector seasons and become endemic to northern Europe. The recrudescence of BTV-8 in northern France, the Netherlands, Belgium, Luxembourg, and Germany in 2007, and also the emergence of BTV-8 in the United Kingdom, Denmark, Switzerland, and the Czech Republic, suggests that this may well be the case ( 27 , 28 ). Unlike farther south, where populations of the traditional vector, C. imicola, peak in the late summer and autumn, when most BT cases occur, populations of the indigenous European vectors peak earlier in the year; whether this will be reflected in a change in the temporal occurrence of BT cases remains to be seen. In the period from January 1, 2006, through December 28, 2007, 12 EU member states and Switzerland reported BT outbreaks on their territories, comprising all of the serotypes reported in Europe since 1998 (Figure 4) ( 29 , 30 ). Figure 4 Number of bluetongue outbreaks in Europe since January 1, 2006 (all serotypes). BE, Belgium; BG, Bulgaria; CH, Switzerland; CS, Czech Republic; DE, Germany; DK, Denmark; ES, Spain; FR, France; IT, Italy; LU, Luxembourg; NL, Netherlands; PT, Portugal; UK, United Kingdom. Susceptible Species BTV is transmitted between its ruminant hosts almost exclusively through the bites of the females of vector species of the Culicoides biting midge ( 31 ). The global distribution of BTV, therefore, is restricted to those regions where these vector species of Culicoides occur, and its transmission period is limited to the times when adult vectors are active. Depending on the species, adult vector activity generally starts some time in spring. Activity is positively correlated with temperature and reaches a maximum between 28°C and 30°C; activity decreases when the temperature drops and, for the traditional Afro-Asiatic vector C. imicola, is probably nonexistent at temperatures <10°C ( 13 , 31 ). BTV can infect a broad spectrum of domestic and wild ruminants. However, serious clinical signs have been observed only in certain breeds of sheep (improved breeds) and a few deer species ( 32 , 33 ). Cattle and goats usually exhibit subclinical infections and therefore may serve as important and covert viral reservoirs for sheep ( 32 ). However, some serotypes such as serotype 8, which recently caused infection in northern Europe, exhibit a more important virulence in cattle ( 34 , 35 ) with serious socioeconomic consequences ( 5 ). Vector Capacity and Competence Risk for BTV infection is linked closely to the presence of adult vector Culicoides spp ( 31 ). Until recently, C. imicola was believed to be the only important vector of BTV in southern Europe, but it is now known that several, newly recognized vector species are also involved. Others may be identified in the future. Vector competence of an insect species and vector capacity of an insect population are important parameters in this respect ( 36 ). Vector competence is the (innate) ability of a vector to acquire a pathogen, maintain it, and successfully transmit it to a susceptible host ( 13 ). Vector competence may be determined in the laboratory by providing groups of insects of a particular species with blood meals of appropriate concentrations of virus and assessing infection and transmission rates. Vector competence is defined as the proportion of feeding insects that support virus replication and transmit virus after a suitable incubation period. In situations where transmission is difficult to demonstrate because of the technical problems in refeeding “difficult” insects such as Culicoides spp., it has become established practice to assume transmission if virus can be recovered from the salivary glands. Vector capacity refers to the potential for virus transmission of an insect population and takes into account a range of insect, host, and environmental variables, including vector abundance, vector survival, biting and transmission rates, host preferences, and host abundances, under a range of external (e.g., bioclimatic) conditions. Vector capacity can be defined as the number of infective bites that an infected vector causes during its lifetime (usually 2–4 weeks in the case of vector species Culicoides) ( 36 , 37 ). Determining the 2 parameters explained above is essential to accurately estimate vector transmission rates and predict whether BTV will become established in an area. Such detailed studies inevitably demand substantial financial and scientific resources and require a multidisciplinary approach. Modes of Introduction and Mechanisms of Amplification Introduction of BTV from 1 area into another can occur in 4 ways: through animal movement (domestic and wild ruminants) or animal product transport (semen, embryos); by infected vector Culicoides spp. carried by various living (plants, animals) or inanimate (airplanes, ships) means; through the active flight of infected vector Culicoides spp. (local propagation); and through passive flight of infected vector Culicoides spp. on the wind (responsible for long-distance dissemination). Whether the virus becomes established in a new area depends upon the number and distribution of susceptible hosts, the duration and titer of the BTV viremia in the hosts, the vector capacity of the local vector population, and the ambient temperature. In essence, establishment depends upon a sufficient number of vector Culicoides spp. becoming infected by feeding upon local viremic hosts, surviving long enough to ensure completion of the intrinsic incubation period (4–20 days, depending on ambient temperature), and transmitting the virus by bite to new hosts ( 13 ). The extrinsic incubation period is the interval between when a vector is infected and when it first becomes capable of transmitting the BTV to a new host ( 38 ). These requirements for BTV establishment have clearly been fulfilled in much of southern Europe, as BTV has survived there in many locations since the late 1990s. Conclusions The widespread recrudescence of BTV-8 infections in northern France, Belgium, the Netherlands, Luxembourg, and Germany in 2007 and the emergence of BTV-8 in the United Kingdom, Denmark, Switzerland, and the Czech Republic in the same year suggest that the requirements for BTV establishment may now also be fulfilled in many more northerly and central parts of Europe (in the absence of C. imicola). In addition, the radial extension of BTV-8 across Europe (including the jump across the English Channel) (Figure 5) ( 39 ) increases the risk for an encounter between this serotype and others, particularly those that occur in the Mediterranean Basin (second epidemiologic system). BTV serotypes 1, 2, 4, and 16 have been identified in this area, and the addition of a further serotype will considerably increase the potential for reassortment between these viruses (Figure 6) ( 27 , 40 ). Indeed, the number of possible reassortments in the case of BTV, which has 10 segments, increases with the number of cocirculating serotypes (e.g., 1,024 for 2 serotypes [210] and 59,049 for 3 serotypes [310]) ( 4 ). Moreover, the phenomenon of reassortment has already been demonstrated during the 1998–2005 BTV outbreaks in Europe ( 5 ). Figure 5 Number of bluetongue virus (BTV) outbreaks caused by BTV-8 (A) and BTV-1 (B) per kilometer (quartile scale) from May 1, 2007, to December 28, 2007 (EU-BTNET system; available from http://eubtnet.izs.it/btnet). Figure 6 Bluetongue virus (BTV) restriction zones in Europe, by serotype. The radial extension of BTV-8 across Europe increases the risk for an encounter between this serotype and other serotypes that occur in the Mediterranean Basin (second epidemiologic system, where serotypes BTV-1, BTV-2, BTV-4, and BTV-16 were identified and the main vector is Culicoides imicola). This situation increases the risk for reassortment of individual BTV gene segments, and, in the more southerly areas, the period of vector activity is also likely to extend, leading to a longer BTV-8 season. In addition, BTV-1, which was first identified in sheep with clinical signs of BT in the south of the Iberian Peninsula in July 2007, has extended its range into northern Spain and southwestern France (Pyrénées-Atlantiques), since November 2007; this ongoing expansion is matter of major concern. Furthermore, in the southern epidemiologic system, C. imicola, the Afro-Asiatic vector of BTV, occurs in addition to the C. obsoletus complex. As the population abundance of C. imicola peaks later in the year than the Obsoletus complex, this means that virus may be transmitted for a much greater portion of the year. With regard to prophylaxis, possibly the best strategic option for control of clinical BT outbreaks in the European endemic areas is vaccination of susceptible animals with inactivated vaccines to protect against disease and to exclude the possibility of reversion to virulence of the vaccine viruses and reassortment between vaccine and field strains of the virus ( 4 , 5 ). Veterinary authorities and legislators throughout northern Europe would do well take note of these recent and considerable changes in the epidemiology of BT.
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First occurrence of Culicoides obsoletus-transmitted Bluetongue virus epidemic in Central Europe.
F Jaeger, B. JAHN, Josef Eschweiler … (2007)
In August 2006, Bluetongue virus disease (BTD) was detected for the first time in the Netherlands, Belgium, Germany and Northern France. Serological tests as well as reverse transcriptase polymerase chain reaction (RT-PCR) proved the occurrence of Bluetongue virus (BTV) in diseased sheep and cattle, and the virus was identified as serotype 8. Therefore, the search for possible vectors was immediately initiated in the outbreak region in Germany. Traps with automatically regulated ultraviolet light lamps were placed at two different farms with sero-positive cattle, and insect monitoring was done from August 2006 until January 2007. The caught arthropods were weekly determined, and it could be observed that midges of the dipteran family Ceratopogonidae occurred in large numbers, sometimes representing up to 40% of all individuals. The microscopical analysis of the wing morphology showed that the species (complex) Culicoides obsoletus was most abundant covering about 97% of the analysed midges. On the second place ranged C. pulicaris, while C. nubeculosus and C. festivipennis were found only as single individuals. Fed and unfed females were separated, sent to the National Reference Laboratory for Bluetongue disease (Friedrich-Loeffler-Institut, Isle of Riems, Germany) and investigated with a BTV-8-specific real-time RT-PCR. It could be demonstrated that at both farms both fed and unfed C. obsoletus were tested positive for BTV-8 genomes, while none of the other species scored positive. This finding strongly supports that the BTD-epidemic, which reached in the meantime wide regions of North Rhine-Westphalia in Germany and of the neighbouring countries with several hundreds of affected farms, is initiated by virus transmission during the blood meal of midges of the C. obsoletus complex. Since they were captured still at the 21st of December close to cattle with clinical signs, it must be feared that BTV-8 is now established in Central Europe, where it had been absent until now.
Author and article information
Journal
Journal ID (nlm-ta): Emerg Infect Dis
Journal ID (publisher-id): EID
Title:
Emerging Infectious Diseases
Publisher:
Centers for Disease Control and Prevention
ISSN
(Print):
1080-6040
ISSN
(Electronic):
1080-6059
Publication date
(Print):
September
2009
Volume: 15
Issue: 9
Pages: 1481-1484
Affiliations
Author notes
Address for correspondence: Franz J. Conraths, Friedrich-Loeffler-Institute, Federal
Research Institute for Animal Health, Institute of Epidemiology, Seestrasse 55, 16868
Wusterhausen, Germany; email:
franz.conraths@
123456fli.bund.de
Article
Publisher ID: 09-0562DOI: 10.3201/eid1509.090562
PMC ID: 2819873
PubMed ID: 19788820
SO-VID: cdf3573f-10f5-4388-9a85-af02bef803fb
History
Categories
Subject:
Dispatch
ScienceOpen disciplines: Infectious disease & Microbiology
Data availability:
ScienceOpen disciplines: Infectious disease & Microbiology
Keywords: rt-pcr, dispatch, germany, monitoring, bluetongue virus, culicoides, epidemiology, viruses, bluetongue disease