UK scientists have shown how a better understanding of midge activity can be used to reduce disease risk from viruses in sheep and cattle.
The findings, reported in a new study published this week in the scientific journal PLOS ONE, should lead to better prediction of the dates when livestock can be moved during disease outbreaks. During a midge-borne disease epidemic, farmers need to coincide cattle movements to when the risk of disease transmission by midges is low over the winter months.
Since 2006, five different strains of bluetongue virus (BTV) transmitted by midges have entered Europe, causing disruption and economic losses within the livestock industry.
The current European Union livestock movement regulations rely on a network of traps to monitor the activity of midges. Livestock movement restrictions are relaxed when less than five adult female midges are caught per trap.
The new study examined whether the length of the overwinter period when adult midges are inactive varied between different midge species and from place to place, and whether this variation can be predicted from environmental conditions.
The results show that there is a large variation in the length of the inactive overwinter period for different Avaritia midge species across the UK, and that we might be able to predict in advance how the period will vary from region to region and year to year and organise animal movements and vaccination accordingly.
This conclusion could mean that rather than having to define a single period from midge trapping applied over a broad area, which is relatively expensive and does not take account of different activity periods for each midge species, a cheaper predictive model could be used that would be responsive to natural variation in midge activity caused by environmental factors.
Paper lead author Dr Kate Searle from the Centre for Ecology & Hydrology (CEH) said, “At present the effectiveness of the movement restrictions is limited because they are applied collectively to a group of midge species (comprising the subgenus Avaritia) with no accommodation for seasonal differences in the activity of individual midge species, or their relationship with environmental conditions.”
The study was led by scientists at the Centre for Ecology & Hydrology working with staff from the Institute of Animal Health, Pirbright, the Met Office and Biomathematics and Statistics Scotland. The research team analysed five years of midge trapping data from across the UK, to determine if different midge species display different seasonal activity patterns, and if these seasonal patterns can be predicted from environmental data such as habitat type, livestock density, or climate.
Dr Searle said, “We found that the length of the period for which midges were absent as adults over winter varied substantially between species by up to eight weeks, but also differed between years. These differences seem to be caused by subtle changes in climate as well as differences in the local availability of livestock and land use types such as pasture.”
Co-author Dr Bethan Purse from the Centre for Ecology & Hydrology said, “Differences of several weeks in adult season between years or species may be particularly important for viruses such as the Schmallenberg virus, which has its greatest impact in terms of abortions and foetal abnormalities when infection occurs during a critical window of pregnancy. For sheep, this coincides with the latter part of the adult vector season in the UK which, if prolonged, could lead to greater disease spread and higher costs to the livestock industry.”
The study recommends that current surveillance methods should be adapted to focus on concentrated measurements of the abundance of individual midge species during the start and end of their activity seasons across a range of climatic zones.
Another outcome of the research was a better understanding of how current practices are impeding the development of predictive models for midge abundance and activity.
Dr Searle added, “When we treated the constituent midge species of the Avaritia subgenus as a single group – as is currently done for EU disease policy – we couldn’t identify any links between environmental variables such as habitat type or climate with the seasonal activity of the midge group. This means that the current practice of treating Avaritia midge species as a single group makes the development of predictive models that could be used to forecast the seasonal activity of midge species very difficult.”
Full paper reference: Kate R Searle, James Barber, Francesca Stubbins, Karien Labuschagne, Simon Carpenter, Adam Butler, Eric Denison, Christopher Sanders, Philip S Mellor, Anthony Wilson, Noel Nelson, Simon Gubbins, and Bethan V Purse (2014) Environmental drivers of Culicoides phenology: how important is species-specific variation when determining disease policy? PLOS ONE. Publication date 11 November 2014. doi: 10.1371/journal.pone.0111876
The UK Culicoides national surveillance dataset was funded by Defra. This work was supported by EU grant FP7-261504 EDENext.