The influence of farm management factors on localized Culicoides species on a lowland farm in South-West England

Abstract

A survey of the localised distribution of Culicoides obsoletus and Culicoides pulicaris was performed on a dairy and sheep farm in south-west England. Culicoides obsoletus and C. pulicaris have both been confirmed as vector species for the transmission of bluetongue virus in Europe. Sampling was done using motorised black-light suction insect traps. Seventeen traps were set around the farmyard and animal housing and five traps were set in varying pasture locations. Sampling was carried out on eight occasions between mid-September and mid-October 2008. The trapped Culicoides were counted, speciated, sexed and the reproductive stages of the females were recorded. Culicoides obsoletus, C. chiopterus, C. scoticus, C. dewulfi and C. pulicaris (group) were identified during the study. The trap sites were selected to examine the Culicoides populations associated with a wide range of microclimates. The selected sites included manure stores; forage feed stores; yard areas and sites surrounding as well as inside the animal housing. Comparisons were made between Culicoides numbers trapped from different directional sides of the animal buildings and the numbers found inside compared to numbers found outside the buildings. Culicoides numbers collected from the animal areas were compared to the non-animal areas and to the manure and forage sites. The field sites consisted of a marsh area; stream; water trough; open field site and a group of trees in a hedge field boundary. Culiccompared catch sizes from the field trap sites were compared to each other and to the farm holding sites. The highest number of Culicoides trapped were at the farm holding sites, apart from one catch on one occasion from a single field site. Weather changes, particularly high wind speeds with direction changes appeared to reduce the catch sizes during some of the trapping occasions. A greater number of C. obsoletus were collected from both the farm and field sites although a higher relative proportion of C. pulicaris was collected from the field sites. Of the C. obsoletus group, C. dewulfi was only found in farm holding catches, not at any of the field sites. The remaining three sibling species were found in both the farm and field catches. Relatively high numbers of Culicoides were found within the animal housing, with external numbers apparently influencing those found within the housing. An increase in numbers of Culicoides trapped inside the buildings may have been associated with a small shed size and possibly with straw bedding. A relative shift in the Culicoides population into the buildings appeared associated with prolonged high wind speeds. Widely varying female life stages found at all of the farm trap sites suggested possible dispersal of the Culicoides populations between these sites. Populations appeared to remain localised around the farm holding, but possibly dispersed over greater distances from the pasture locations. A wide distribution of breeding sites was suspected around the farm holding. A ranking system was used to identify specific areas associated with increased numbers of female Culicoides collected from these sites. Three sites surrounding a straw bedded cow shed were highlighted as higher risk Culicoides exposure sites; two sites adjacent to a cubicle shed; inside the calf housing; the manure store area and the silage store area. A field site with trees in a hedge boundary was the only high-risk field site identified. Multilevel modelling was used to examine for possible factors influencing Culicoides numbers. Factors examined included wind, temperature and humidity variables; distance from manure, forage, water and trees and livestock variables such as: time of contact, time since contact and distance from sheep and cattle. The model suggested wind speed at light trap setting and an increased time since contact with cattle both appeared significantly associated with reduced Culicoides numbers. Culicoides obsoletus numbers also appeared significantly reduced with increasing distance from manure. From an on-farm risk assessment point of view the farm holding area of a dairy farm as a whole should generally be considered a high-risk site for Culicoides exposure and specific pasture sites can periodically become high exposure sites. Copyright