Bluetongue in Sheep and Cattle
Thung Song What is bluetongue virus?
Bluetongue virus (BTV) is a double-stranded RNA virus that causes bluetongue. This is an insect-borne, viral disease which mainly affects sheep and, less frequently, cattle. Bluetongue is a notifiable disease in the UK. Notifiable diseases are any diseases required by law to be reported to government authorities. They must be reported immediately to the Divisional Veterinary Manager at the local Animal Health Office. There are 26 serotypes for this virus. Animals that recover from infection with one serotype will be immune to that strain, but not to others. Bluetongue virus poses no threat to human health and does not affect meat, milk or other animal products.
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Culicoides midges transmit the virus, but it does not normally move through direct contact with affected animals. Peak season for midges occurs during late summer and autumn in Europe. Therefore this is the time when bluetongue is a risk. Additionally, the wind can carry midges over large distances, which aids in transmission of the virus. The midges become infected with the virus when they bite an infected animal When an infected midge bites an uninfected animal, it can transmit the virus. Once a midge has picked up BTV, it will become a carrier for the rest of its life. Bluetongue virus does not survive outside the insect vectors or susceptible hosts. Animal carcasses and products such as meat and wool are not a method of spread.
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The most significant case was the BTV-8 outbreak in Northern Europe in 2006-2008. The outbreak spread to infect approximately 2000 animals in the Netherlands, Belgium, Germany and France. Bluetongue is currently absent in the UK, but there is a significant risk of it reoccurring when midges blow over the English Channel from Europe during European outbreaks.
Besni What are the signs of an infection?
Bluetongue causes changes to the mucous membranes of the mouth and nose. Clinical signs are generally more severe in sheep than in cattle.
- High rectal temperature
- Excess tear production
- Discharge from the eyes and nose
- Appear stiff
- Tongue may become swollen
- Lack of oxygen may make the tongue and mucous membranes appear blue
- Swelling of the mouth, head and neck
- Difficulty breathing
- Bluetongue can also cause pregnant sheep to abort
Diagnosis and treatment
Diagnosis is based upon clinical signs, virus detection using a sensitive technique called PCR or blood sampling to test for antibodies to the virus. Antibiotic therapy to control secondary bacterial infections and anti-inflammatories can be used to treat the symptoms.
How to prevent and control bluetongue virus
Control of bluetongue is very difficult because of the large number of potential hosts and different virus strains. Control of the midges can be attempted with insecticides but can be expensive and does not achieve freedom from midges. Restricted movements on affected animals may help to reduce the spread the disease.
The main prevention for BTV is vaccination. The BTV-8 and BTV-4 vaccines are available in the UK. it is important to realise that there is no cross-protection between serotypes; vaccination against BTV-8 will not protect against other serotypes of BTV. Bluetongue vaccines for sheep and cattle are now available across the UK. The vaccination has to be given twice (three weeks apart) in cattle and sheep. In an event of an outbreak in the UK, measures such as movement restriction would be put in place together with other disease control strategies.
The Culicoides midges that carry the virus usually breed on animal dung and moist soils. Identifying breeding grounds and breaking the cycle will significantly reduce the local midge population. Always practice good biosecurity on your premise to prevent any movement of infected animals.
Gubbins, et al. (2007). Journal of The Royal Society Interface, 5(20), pp.363–371.