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Title: Predicting the unpredictable : the changing epidemiology of Dictyocaulus viviparus in Great Britain
Author: McCarthy, C.
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2019
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Disease caused by the bovine lungworm, Dictyocaulus viviparus, is a cause of significant morbidity and mortality in dairy herds across the world. Subclinical infections are associated with reduced milk production in dairy cattle and growth rate deviations in beef calves whereas clinical outbreaks can be unpredictable and expensive. Current estimates for lungworm prevalence across Great Britain are unknown but have risen significantly since the 1970s. This has been associated with more cases in adult cattle and the disease emerging into more northern regions of England and Scotland. The objectives of this thesis were to quantify and understand changes in lungworm epidemiology which have occurred over the past 40 years, understand the relative role that farm management factors and climate (change) have had on the changing prevalence of the disease and to predict future changes which may occur under climate change conditions. Chapter 2 investigates the spatiotemporal distribution of lungworm cases reported to the Veterinary Investigation and Diagnosis Analysis (VIDA) database from 1974 - 2014. There has been a significant increase in the diagnostic rate of lungworm across Great Britain from 0.97 cases per 1,000 submissions in 1980 to 4.04 cases per 1,000 submissions in 2014, with a dramatically increased rate in Scotland. Moreover, the number of cases in adult cattle has increased since the 1980s. Chapter 3 describes the novel use of an existing D.viviparus ELISA in dairy herds, with an improved test sensitivity of 66.7% and specificity of 95.5% under field conditions. Chapter 4 subsequently made use of this test in a cross - sectional survey of UK dairy farmers, which improved our understanding of farm management practices associated with an increased risk of D.viviparus presence. Chapter 5 quantifies the influence of climatic conditions, building a mathematical model, which predicts the development, survival and migration of infective D.viviparus larvae on pasture. It was validated using a longitudinal study and showed that pasture infectivity occurred 46 days prior to a peak in antibody response (95% confidence intervals 38 - 52 days). The model demonstrated that the climate was conducive to increased D.viviparus transmission in Scotland from 1995 onwards. Under future climate change predictions, by 2055, the majority of England is predicted to have a climate not conducive to D.viviparus transmission. Exceptions to this are in Scotland and the southwest of England, which are predicted to remain hotspots for the disease until at least 2095. In the past, researchers have largely attempted to explain the changes observed in lungworm epidemiology from farm management factors. However, the findings presented here suggest that climate (change) can account for most of the changes observed and may have already had a significant impact on the epidemiology of D.viviparus across Great Britain. Future global changes in livestock farming will continue to threaten the stability of the disease landscape. Mathematical models such as the one described here, can forecast heightened disease risk periods and will be useful in the design of sustainable control measures for lungworm disease.
Supervisor: van Dijk, Jan ; Christley, Rob Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral