Bluetongue is transmitted by Culicoides midges. The geographic location of the disease is dependent on the presence of Culicoides midges. The main midge species involved in the transmission of BT in Europe is Culicoides obsoletus.

Lifecycle

The epidemiology of BT is closely related to the lifecycle of the midge. It is therefore a seasonal disease generally observed in late summer and early autumn when the midges are most active. However, the disease can be transmitted from early spring through until the first frosts of winter.

Female midges have biting mouthparts and require blood meals for egg production. If a midge feeds on a BT infected animal, the virus multiplies inside the insect’s gut ready to be transmitted to a susceptible when the midge feeds again. Midges act as a ‘virus factory’ in this way.  

Males do not feed on blood. Midges breed in moist conditions in a variety of habitats, particularly damp, muddy areas and in dung and plant matter. Prime sites include damp, moist or waterlogged soil, rotting leaves, old dung heaps and in silage effluent.

Midges are usually most active during the afternoon, dusk and at night, preferring still, warm conditions. Due to their small size, they generally do not fly in windy conditions however they are easily spread by light winds and across expanses of water. Some stages of the lifecycle are able to overwinter in protected areas which may include livestock housing.