Following the reports on Bluetongue Virus 8 (BTV-8) which was recently detected in two herds in north west England and south west Scotland, vets and farmers in Northern Ireland are urged to remain vigilant.
The 10 imported animals in Great Britain were imported from the same assembly centre in France in an area where multiple cases of Bluetongue have been confirmed as recently as September this year.
Whilst Bluetongue does not pose a threat to human health or food safety, it can have a severe impact on affected farms.
The last case of the disease in the UK prior to this occurred in the south of England in 2007. The UK has been officially free from the disease since July 2011. There has never been an outbreak of Bluetongue in Northern Ireland.
Bluetongue virus is a notifiable, midge-borne viral disease which affects all ruminants particularly sheep.
How to spot Bluetongue?
The severity of disease varies among different serotypes with symptoms being most severe in sheep. In highly susceptible sheep, morbidity (the percentage of sheep affected) can be as high as 100%. Mortality averages 2-30%, but can be as high as 70%.
In sheep the main signs of Bluetongue are:
l Ocular and nasal discharges
l Drooling as a result of swelling of the tongue and/or ulcers in the mouth
l Swelling of the mouth, head and neck
l Lameness as a result of swelling of the coronary band (where the skin of the leg meets the horn of the hoof)
l Red skin as a result of blood collecting beneath the surface
l Breathing difficulties
As the midges prefer to bite cattle, they are the main carriers of Bluetongue. Infected cattle generally do not show signs of the disease, but occasionally signs can include:
l Swelling and ulcers in the mouth
l Nasal discharge
l Red skin and eyes
l Swollen teats
Other animals such as deer, goats and camelids rarely show signs of the disease. Bluetongue does not cause disease in humans or animals other than ruminants.
How is Bluetongue spread?
Bluetongue virus is spread via certain species of biting midges, which are known as the vector. The Culicoides species of midge which carries the infection is found in Northern Ireland. The disease is spread when midges bite and feed on an animal infected with the virus. Virus can pass to the midge in the blood meal. In the right conditions the virus multiplies in the midge. When the midge bites another susceptible animal, the virus can then be transmitted and infection can occur.
The midge season is normally from March to October with peak season in late summer and autumn. The weather, especially temperature and wind direction, affects how the disease can spread. Midges can travel up to 1.5-2 km a day in a local area. However, if caught in suitable meteorological conditions and especially over water masses, midges can be carried much farther distances, of more than 200km.
The virus has been found in the semen of infected bulls and rams and can be transmitted to susceptible cows and ewes, but this is not a significant route of infection. Transmission of disease via midges is by far the most important route of spread.
Virus can also be transferred through the placenta to the fetus.
Disease Control Measures
Good biosecurity can reduce the risk of disease exposure, but BTV-8 is a challenging disease to address because it is spread by midges and therefore the actual actions will depend on the circumstances of the outbreak and whether it occurred in the active vector season. The most likely route of disease entry to Northern Ireland is currently through the import of infected animals or germplasm (semen or ova). Therefore farmers are urged not to import animals or germplasm from bluetongue affected countries.
Research into the use of insecticides against the Culicoides midge species, found a deltamethrin-based pour on treatment to be an effective vector control method. In one study, even hair clipped from the feet of cattle 35 days after standard treatment contained enough deltamethrin to kill midges.
Fly & Lice Spot On™ contains deltamethrin and a vegetable oil carrier which ensures the active ingredient is spread over the whole body and kill is achieved in just two hours. When midges land on treated cattle or sheep their feet contact the Fly & Lice Spot On and the deltamethrin affects the midge’s nervous system leading to muscular excitement, paralysis and death.
Whilst vector control methods are an important part of a Bluetongue Virus control strategy, and will certainly be useful at a farm level, alone they are unlikely to be successful at controlling disease over a large area, because of the behaviour and movements of the midges.
Vaccination is the most effective and practical measure to minimise losses related to the disease and to potentially interrupt the cycle from infected animal to vector. It is essential to use a vaccine designed to provide protection against the specific serotype of virus of concern in a particular area.
Currently animal keepers in Northern Ireland are not permitted to vaccinate their animals. However, if Bluetongue was confirmed in Northern Ireland, a veterinary risk assessment would be carried out and a licence may be issued to permit vaccination.
Should bluetongue virus be found to be circulating in GB, restrictions zones of 150km would be placed around infected premises to try to limit spread of infection. This can have a significant impact on animal movements and free trade. Vaccination is one way of mitigating this, allowing trade to continue.
Vaccination against one serotype of Bluetongue virus does not give protection against any other serotype. The serotype currently circulating in France and recently identified in Great Britain is BTV-8.