THE veterinary profession is, by now, well aware of the threat bluetongue poses to the UK livestock industry and understands the vital role of vaccination in preventing infection and the spread of the virus. Use of an inactivated virus vaccine is the preferred first line of defence.
More specifically, it is important to draw attention to the significance of viraemia in the transmission of bluetongue virus, and discuss the merits of controlling viraemia if livestock producers and vets are to gain the highest level of confidence in preventing bluetongue from spreading.
The movement of the Culicoides midges, vectors in the spread of bluetongue, cannot be fully monitored or controlled. Therefore, a vaccine which prevents viraemia is important in managing the potential pool of virus infection in the UK, especially considering that infected cattle can remain viraemic for up to 60 days. The virus can also be detected in sheep for up to 41 days.
Host animals become infective to midges after two to four days Cattle act as virus “factories” as they remain viraemic for a long time – offering a large window of opportunity for on average 10,000 midges to bite per day and transmit the virus to other susceptible livestock.
The 2007 Scientific Opinion of the Scientific Panel on Animal Health and Welfare, on request from the European Commission on bluetongue vectors and vaccines, recommends that vaccines which prevent viraemia after challenge should be used. It goes on to suggest that the absence of viraemia could be used as the best predictor for the efficacy of bluetongue virus (BTV) vaccines in sheep and cattle.
Here it is worth considering the transmission of bluetongue virus between vectors and hosts in more detail. The Culicoides midge species form a numerous and widespread group, and there is not yet a complete understanding of the lifecycle of Culicoides in Northern Europe. What is known, however, is that blood feeding by female Culicoides from hosts, including both sheep and cattle, is crucial to the initiation and subsequent spread of BTV.
Culicoides have long been known to overwinter in the larval stage, but more recent findings in Europe show that there is now almost continuous emergence of fresh adult midges throughout the winter at the northern latitudes affected by BTV. This reiterates the fact that, while Culicoides do not typically begin virus replication until temperatures reach 15°C, we cannot be complacent about when the bluetongue transmission risk period might start and finish.
The optimal temperature for BTV transmission is probably between 27 and 30°C because at this temperature the rate of virus replication within the midge is highest. At 15°C, virus replication in the midge can take up to 20 days. This still allows plenty of feeding time for the long lived Obsoletus midges which are one of the UK’s main groups.
During key BTV transmission periods when climatic conditions are favourable to midge activity, the female Culicoides feeds every three to four days. Increased frequency of blood feeding combined with highdensity vector populations during these key periods lead to more rapid spread of virus between hosts.
It is interesting to note that virus transmission from a vector to a host is a much more efficient process than transmission from a viraemic host to the vector. But high vector numbers and increased blood feeding rates tend to compensate for this.
The normal flight range of the midge is 10m to 5km, but midges can be carried for over 100km via wind currents.
The overall efficiency with which the Culicoides populations transmit BTV depends on factors including their susceptibility, biting and survival rates, the virus strain and the ambient temperature. Given that these can vary throughout the year and between locations, calculation of a midge population’s capacity to spread BTV is difficult.
The disease did not spread widely from the first cases in the UK due to the swift detection and introduction of a movement ban, combined with good uptake of vaccination in the surrounding areas. But next time it arrives in the UK it can never be guaranteed to be recognised before it is transmitted elsewhere.
It could, therefore, quickly end up anywhere in the country via midges, or via livestock movements, whilst there are no on-going restrictions.
This illustrates the unpredictability of vector movements and host interactions which, when set against the certainty that host sheep and cattle remain viraemic long enough for blood feeding to occur on multiple occasions, makes it clear that a reliable vaccination insurance policy is needed.
Vaccines which reduce the level or persistence of viraemia will certainly reduce the rate of spread, but a risk still remains of a midge transmitting the virus when it bites an animal which has been challenged. However, if viraemia is completely prevented, the disease cannot be spread.
The main purposes of BTV vaccination are to prevent clinical disease and its related production losses in livestock and, at the same time, to prevent the spread of the virus across the UK. Therefore, as recommended in the 2007 Scientific Opinion of the Scientific Panel on Animal Health and Welfare, vaccination with a product that prevents viraemia, like Merial’s BTVPUR AlSap 8, should be recommended and used wherever possible.