Transmission of BTV by midges
Bluetongue is an insect-borne viral disease. The disease is transmitted by Culicoides midges. The geographic distribution of the disease is dependent on the distribution of the midges.
About Culicoides Midges
The midges that transmit Bluetongue virus (BTV) belong to the order Diptera (2 winged flies), genus Culicoides. In Western and Northern Europe it has been determined that local Culicoides species are vectors of BTV and not species that have migrated from southern areas.
The African species Culicoides imicola has been determined as the main vector of BTV outbreaks in Africa and Southern Europe. In Western-central Europe the virus has been isolated by Real-Time-PCR technique from Culicoides obsoletus.
Lifecycle
The epidemiology of BTV is closely related to the biology of the vector. It is therefore a seasonal disease generally observed in late summer and early autumn. Virus transmission begins in the early spring with the onset of insect flight activity and continues until the first hard frosts.
In most Culicoides species the lifecycle is poorly understood. The midges breed in moist conditions in a variety of habitats, particularly damp, muddy areas and in faecal and plant matter. During summer the development from egg to adult takes 3-4 weeks. It seems that some larvae and pupae are able to overwinter in protected breeding places.
Scanning electromicrograph of the biting mouthparts of a female Culicoides midge.
The female midges require blood meals for egg production and have biting mouthparts. They 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.
Vectors and the BTV-8 outbreak in Europe
The rapid spread that followed the initial outbreaks of BTV-8 in Northern Europe in 2006 is generally considered a logical consequence of the changes in global climate. These changes have allowed the Northern European midges such as C. obsoletus to become effective transmitters of the virus.
C. obsoletus is an extremely small midge reaching only 0.8 mm in length. It is the most abundant of the Culicoides species in Northern Europe.
Certain peculiarities of the C. obsoletus lifecycle ensure that it is an ideal vector for BTV.
- The species breeds close to cattle and sheep. There is some evidence that the midges may breed in livestock housing, this would cause a year long occurrence of midges even in cold winters.
- Adults fly from March until late December in Europe. The BT virus thus has a longer time to replicate in the female midge and the longer lifespan allows more bites to occur and thus more animals are infected.
- The life span of the adults is 3.5 months, considerably longer than most other Culicoides species.