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Opinion of the Scientific Panel AHAW on request from the Commission on bluetongue vectors and vaccines
Last updated: 15 May 2007    
Publication Date: 15 May 2007    

Adopted on 27 April 2007. (Question N° EFSA-Q-2006-311)


The current distribution of bluetongue (BT) in Europe deserves special consideration in regard to i) the spread and epidemiology of BT virus (BTV) into new regions, ii) the surveillance and monitoring activities of MSs, iii) the clinical and laboratory diagnosis of BT, iv) a scientific approach being adopted in the use of vaccines against BTV, and v) possible control of the Culicoides vectors implicated in the spread of all BTVs.
BTV serotype 8 affected five countries in Northern Europe whereas other serotypes (BTV­1, ­2, ­4, ­9 and ­16) had been responsible for the recent and multiple outbreaks of BT in parts of Southern Europe.
In regard to the epidemiological follow-up of the serotype 8 outbreak, in Northern Europe the EFSA epidemiological working group is in the process of conducting a global analysis. In regard to surveillance, monitoring, and the laboratory diagnosis of BT, a harmonised community­based approach has already been introduced with the result that the community Central Reference Laboratory (CRL) has prepared a draft document and which was sent to all CVOs in September 2006.
The Commission required further scientific opinions on the i) the role of vectors in the transmission of BTV and the means to control them; ii) the most recent developments, and experience gained, in the use of different vaccines against BT in sheep, cattle and goats; iii) the scientific assessment of the vaccination campaigns conducted against BTV in Member States, and  iv) the suitability of vaccination as a tool of choice to control BT and, in addition, as tool to facilitate also the safe trade in different livestock species. In order to reply to this mandate a working group was established by the AHAW Panel.
In regard to their role in the transmission of diseases and the means to control them, Culicoides constitute a numerous and widespread group and act as important vectors of many pathogens including BTV. The knowledge of the life cycle of most species of Culicoides in Northern Europe remains incomplete. Nevertheless, it has always been known that Culicoides overwinter in the larval stage, but recent findings in the MSs show that there is now almost continuous emergence of fresh adult midges through the winter at the northern latitudes affected by BTV. Multiple blood feeding events by Culicoides are crucial to the initiation and subsequent spread of BTV. There is strong circumstantial evidence to suggest that though the normal flight range of Culicoides is short, they are also able to travel much longer distances (>100 km) and so may be able to introduce pathogens like BTV into regions remote from the source. The efficiency with which vector species transmit BTV varies according to their susceptibility, biting and survival rates, the virus serotype and the ambient temperature. Although the infection rate in vectors is generally low, transmission from a viraemic host to the vector is much less than the oposite (vectors to animals). However, high biting rates in the field tend to compensate for this. Although Culicoides may be active at temperatures of around 10 °C, virus replication in the insect begins to proceed at 15–18 °C. The optimal temperature for BTV transmission probably lies in the range of 27-30 °C since then most vectors survive long enough to transmit at least once and the virus replication rate is maximal.
The precise levels of protection provided by insecticide treatment and housing of stock is unclear and neither are likely to eliminate the risk of BTV transmission. However, insecticides treatments could provide some measure of protection. Insecticides treatment should thus be included in a set of measures designed to decrease biting midge densities; however, the supposed impact of such measures on the rate of BTV transmission has yet to be quantified in the European context and should therefore be used as a risk mitigation measure in certain circumstances (e.g. breeding animals, trade or in recently infected farms). 
The biological properties of inactivated vaccines and modified live virus vaccines (MLV) are described in detail based upon data received from various manufacturers and upon data obtained during experimental trials and from their in–field use. It can be concluded that all BTV inactivated vaccines, when administered in two separate doses, are able to fully protect animals for a long period. However, a single dose of BTV-4 inactivated vaccine only partially reduced viraemia in cattle when challenged 7 months later. Numerous MLV vaccines have been used under a wide range of conditions in the field. All were found to induce viremia allowing for the potential infection, and possible subsequent transmission, of MLV strains of BTV by insects; furthermore, the magnitude and duration of viremia in animals uninfected and vaccinated with MLVs remains to be determined for most of these vaccines. Viremia in sheep vaccinated with MLV strains of BTV­2, ­4, ­9 and ­16 (including multivalent constructs) persisted for up to 24 days whereas in cattle viraemia persisted for up to 78 days. Statistical analyses of the data available on the levels of viraemia reported in cattle and sheep, after they had been vaccinated with BTV­2 and ­9 MLVs, indicated cattle and sheep could be moved at 32 and 28 days, respectively. However, a low level (<0.01%) of risk of transmission, remains for ruminants, vaccinated in infected areas, when infection with field strains, just prior to or at the moment of vaccination. Thus in infected areas, to minimise the risk of the infection being spread, a waiting period of at least 60 days is required before animals can be moved.
In general, the use of vaccines which prevent viraemia after challenge is recommended. In the case that inactivated vaccines are available such vaccines should be used in preference. The use of MLVs can be considered only after a comprehensive risk/benefit analysis has been made. Ideally, and for the purpose of eradication, a comprehensive vaccination campaign should include all susceptible ruminant species.
Vaccines are suitable for the control of the infection in endemic/epizootic areas. Vaccination, preferably using an inactivated virus, is recommended as a first line of defence but within a set of measures including mainly animal movement control and Culicoides control. To facilitate the availability of authorized Bluetongue vaccines a similar approach could be followed to the one that was recently applied for Avian Influenza vaccines.
To protect southern countries in the EU, preventive vaccination with inactivated vaccines targeted to the serotypes circulating in neighbouring countries outside the EU should be considered to be used in the highest risk areas of free countries on a risk/benefit analysis, when the introduction of BTV by the wind is considered as highly probable.
In general to reduce the risk of introduction of BTV to southern countries in the EU, systematic vaccination in the endemically affected EU neighbouring countries around the Mediterranean basin should be encouraged.
The vaccines are also suitable tools to facilitate the safety of movements of animals in infected areas when several factors are taken into consideration such as: the period of implementation of the vaccination, the vector activity period, the prevalence of the infection, etc. The vaccines used should be authorized by the competent authority (by following, if needed, the emergency procedures) by the competent authorities which, in consequence, will be in a position to check if the manufacturers complies with the GMP and control tests to be carried out as defined in the CVMP guidelines and general E.P. monographs. Wherever possible, a centralised procedure is preferable to allow a better exchange of the data available between MS.
Current pharmaceutical legislation does not cover the assessment and authorization of diagnostic tests, even when such tests are an essential component of the use of a ‘marker’ vaccine in a DIVA (differentiation of vaccination from infection) vaccination campaign. 

The circulation of BTV in wild ruminants can compromise a vaccination campaign and for this reason it is essential to establish what their precise role might be in the epidemiology of the disease.