Disease spread
How does BTV spread?
BTV is mainly spread by adult-infected midges (various Culicoides species) biting (vector) an animal susceptible to the disease. This is classed as ‘vector-borne’ transmission.
Infected midges can spread locally and more widely in certain temperatures and wind conditions.
The second but less common way the disease is spread is through movement of infected animals. That is why movement of animals between the England Restricted Zone (RZ) and Scotland/Wales is controlled.
What are Culicoides?
Culicoides are the biting midges that BTV is transmitted by. These are the smallest blood feeding insects; 2-4mm long and are hugely abundant.
When are these midges most active?
Midges are most active at dusk and dawn. Their activity is strongly influenced by the weather. Midges are not able to fly in high winds or rain. Average daily temperatures need to be above 15 degrees Celsius for BTV to replicate within the midges.
How does BTV transmission occur?
An uninfected adult midge bites an infected ruminant animal taking in blood and therefore taking in the virus. The virus replicates within the insect (4-20 days depending on air temperature) and moves to the salivary glands. The infected adult midge then bites an uninfected ruminant, transmitting the virus. A single bite from a single infected midge is sufficient to reliably transmit the virus. This ruminant animal will be infectious within 2-4 days.
BTV transmission is seasonal because midges are seasonal. Culicoides midges are active in Northern Europe between April and November with peaks in May and September due to temperature.
What factors affect the spread of BTV?
Some significant factors that affect the spread of BTV are:
- The time of year, i.e. whether during the active vector season (normally March-September)
- Meteorological conditions, i.e. temperature and wind direction
- Proximity and density of neighbouring farms
It is however possible to move infected animals at any time of year.
How many infected midges need to bite an animal to transfer the virus and then result in clinical signs?
A single bite from a single infected midge will reliably transmit the virus. Additional bites will not result in greater levels of infection or worse clinical signs.
What is government doing to reduce the risk of an outbreak from imported animals?
We have robust rules and procedures in place to protect our borders from disease. Animals and their germinal products can only be imported if they meet these requirements. It is essential that keeper’s source responsibly.
Over and above this, we carry out compulsory post-import testing of susceptible animals from areas around and known to have BTV infected regions.
Does bluetongue affect food safety?
The disease does not affect meat or animal products, and meat and milk from infected animals are safe to eat and drink. Over and above this, we carry out compulsory post-import testing of susceptible animals from areas around and known to have BTV infected regions.
Can I catch bluetongue virus?
No, BTV viruses do not affect people.
Is any wildlife known to carry the disease?
Ruminant wildlife such as deer could become infected if the disease was circulating in the local midge population.
What is the current disease control strategy for BTV?
As a notifiable disease, the control of bluetongue is legislated for. The control strategy includes good biosecurity and animal care, responsible sourcing of animals, monitoring of the disease situation in Europe and internationally, and having in place appropriate risk-based import conditions and testing.
Vigilance by animal keepers is key. Anyone keeping an animal must notify any suspicion of notifiable disease. This will be investigated by government veterinary inspectors.