A comparative field study which was carried out on the Veluwe, a forest rich agricultural region in the Netherlands, showed that cows attract nine times more midges than sheep under similar environmental conditions.
Midges are small biting insects that can transmit pathogens, which was the case in 2006-2008 transmitting bluetongue virus and Schmallenberg virus in 2011-2012. This marked difference in preference was shown in a study executed by the Central Veterinary Institute (CVI), part of Wageningen UR. This result is an important factor to be considered in the development of control strategies and in transmission models of vector borne diseases.
Midges play a vital role in the transmission of infectious diseasesOver 1,200 species of biting midges (Culicoides spp.) are described worldwide. They can act as vector for hundreds of different animal pathogens, such as important viruses like African Horse Sickness and Equine encephalosis virus (EEV) to horses, and Akabane virus to cattle and sheep.
In the Netherlands 26 different Culicoides species are described. Approximately 85% the midges in the Netherlands belong to one of the following five species: Culicoides Obsoletus complex (C. obsoletus and C. scoticus), C. dewulfi, C. chiopterus, and C. pulicaris. These five species were also the vectors of bluetongue virus and Schmallenberg virus that have caused epidemics in the past few years in Europe.
Disease transmission models
CVI develops disease transmission models in order to simulate the effects of possible intervention instruments (such as vaccination). Profound knowledge of feeding behaviour of biting midges is central to a proper understanding of how vector-borne viral diseases are transmitted amongst multiple species of livestock hosts, including cattle and sheep. Female midges need host-derived blood for their egg production. While feeding on cattle of sheep the virus, if present in salivary glands of the midge, can be transmitted to the mammal. In the transmission models not only the absolute number of midges is a factor, but also the preference of the midges for different species of livestock. Up to now little information of the preferences of midges for different livestock species was available, reason why in existing models feeding preferences for cattle and sheep were assumed to be equal.
Field study in the VeluweA comparative field study was carried out on a dairy farm during 5 weeks in May and June 2013, on 13 different days. An ewe and a dairy cow were tethered on a pasture, at a distance of 60-65 meters from each other. Culicoides midges were collected from both animals during set times. Different individual animals were used during this collection procedure. In the 21 minutes collecting time each day (every hour during 3 minutes, 5 hours before and 2 hours after sunset) on average 900 midges were collected per cow and 100 midges per sheep. The number of midges collected indicates that adult cattle attract on average nine times more midges than adult sheep under the same circumstances.
The field study also shows that every day enormous amounts of midges land and feed on livestock. Even in the situation where only a low percentage of midges is infected with pathogens (1 or less in every 1,000) there is a reasonable chance that the pathogens are transmitted to the mammal in due time. Cattle appears to play a bigger role than sheep, because they are bitten much more. This is in line with the rapid spread seen during the bluetongue virus outbreak among cattle in 2006-2008. The higher preference of midges for cattle as compared with sheep can be used in the future to develop more accurate transmission models for midge transmissible diseases.