A model to test how ticks and louping ill virus can be controlled by treating red grouse with acaricide

Predicting the current and future risk of ticks on livestock farms in Britain using random forest models

The most abundant tick species in northern Europe, Ixodes ricinus, transmits a range of pathogens that cause disease in livestock. As I. ricinus distribution is influenced by climate, tick-borne disease risk is expected to change in the future. The aims of this work were to build a spatial model to predict current and future risk of ticks on livestock farms across Britain. Variables relating both to tick hazard and livestock exposure were included, to capture a niche which may be missed by broader scale models. A random forest machine learning model was used due to its ability to cope with correlated variables and interactions. Data on tick presence and absence on sheep and cattle farms was obtained from a retrospective questionnaire survey of 926 farmers. The ROC of the final model was 0.80. The model outputs matched observed patterns of tick distribution, with areas of highest tick risk in southwest and northwest England, Wales, and west Scotland. Overall, the probability of tick presence on livestock farms was predicted to increase by 5-7 % across Britain under future climate scenarios. The predicted increase is greater at higher altitudes and latitudes, further increasing the risk of tick-borne disease on farms in these areas.

Distribution and prevalence of ticks and tick-borne disease on sheep and cattle farms in Great Britain

INTRODUCTION

The most abundant and widespread tick species in Great Britain, Ixodes ricinus, is responsible for the transmission of a range of pathogens that cause disease in livestock. Empirical data on tick distribution and prevalence are required to inform farm management strategies. However, such data are largely unavailable; previous surveys have been rare and are usually relatively localised.

METHODS

A retrospective questionnaire survey of farmers was used to assess the reported prevalence of ticks on livestock across Great Britain. Spatial scan statistics and kernel density maps were used to assess spatial clustering and identify areas of significantly elevated risk, independent of the underlying distribution of respondents. Logistic regression models were used to identify risk factors for tick presence.

RESULTS

Tick infection risk to livestock is shown to be spatially aggregated, with areas of significantly elevated risk in north Wales, northwest England and western Scotland. Overall, the prevalence of farms reporting tick presence was 13% for sheep farms and 6% for cattle farms, but in "hot spot" clusters prevalence ranged between 48-100%. The prevalence of farms reporting tick-borne disease overall was 6% for sheep and 2% for cattle, but on farms reporting ticks, prevalence was 44% and 33% for sheep and cattle farms, respectively. Upland farming, larger flock sizes, region and the presence of sheep on cattle farms were all significant risk factors for tick presence.

CONCLUSIONS

These data have important implications for assessing both the risk of tick-borne disease in livestock and optimising approaches to disease management. In particular, the study highlights the need for effective livestock tick control in upland regions and the southwest, and provides evidence for the importance of sheep as tick maintenance hosts.

Identifying Environmental Risk Factors for Louping Ill Virus Seroprevalence in Sheep and the Potential to Inform Wildlife Management Policy

Identifying the risk factors for disease is crucial for developing policy and strategies for controlling exposure to pathogens. However, this is often challenging, especially in complex disease systems, such as vector-borne diseases with multiple hosts and other environmental drivers. Here we combine seroprevalence data with GIS-based environmental variables to identify the environmental risk factors associated with an endemic tick-borne pathogen—louping ill virus—in sheep in Scotland. Higher seroprevalences were associated with (i) upland/moorland habitats, in accordance with what we predicted from the habitat preferences of alternative LIV transmission hosts (such as red grouse), (ii) areas of higher deer density, which supports predictions from previous theoretical models, since deer are the key Ixodes ricinus tick reproduction host in this system, and (iii) a warmer climate, concurring with our current knowledge of how temperature affects tick activity and development rates. The implications for policy include adopting increased disease management and awareness in high risk habitats and in the presence of alternative LIV hosts (e.g., grouse) and tick hosts (especially deer). These results can also inform deer management policy, especially where there may be conflict between contrasting upland management objectives, for example, revenue from deer hunting vs. sheep farmers.

Dynamic Modeling of Crimean Congo Hemorrhagic Fever Virus (CCHFV) Spread to Test Control Strategies

Crimean Congo hemorrhagic fever is a zoonotic disease which has emerged or re-emerged recently in Eastern Europe and Turkey. The causative agent is a virus, mainly transmitted by ticks of the species Hyalomma marginatum (Koch, 1844, Ixodida, Amblyommidae). To test potential scenarios for the control of pathogen spread, a dynamic mechanistic model has been developed that takes into account the major processes involved in tick population dynamics and pathogen spread. The tick population dynamics model represents both abiotic (meteorological variables) and biotic (hare and cattle densities) factors in the determination of processes (development, host finding, and mortality). The infection model consists of an SIRS model for the host part whereas a lifelong infectiousness was considered for ticks. The model was first applied to a zone in Central Anatolia (Turkey). Simulated dynamics represent the average reported level of infection in vectors and hosts. A sensitivity analysis to parameter value has been carried out and highlighted the role of transstadial transmission as well as acquisition of the pathogen by immature stages. Applying the model to different sites of Turkey shows different patterns in the dynamics of acarological risk (number of infectious questing adults). This model was thereafter used to test control strategies. Simulation results indicate that acaricide treatments and decrease in hare density could have valuable effects when combined, either on the acarological risk or on the prevalence in cattle. The kind of model we have developed provides insight into the ability of different strategies to prevent and control disease spread.

Can acaricide-impregnated leg bands fitted to female red grouse reduce sheep tick parasitization of chicks and increase chick survival?

In parts of northern England, North Wales and the Scottish Highlands, increasing numbers of sheep ticks Ixodes ricinus (Ixodida: Ixodidae), and the louping ill virus they can carry, are considered to be important factors that reduce red grouse Lagopus lagopus scotica productivity. The present study tested this hypothesis by fitting adult female grouse with leg bands impregnated with the acaricide cypermethrin to experimentally control ticks on their chicks on two managed grouse moors in northeast Scotland. The chicks of females fitted with acaricide leg bands showed reduced tick infestations and improved survival in one of the two study years, relative to chicks of control females. Acaricide leg bands constitute a potential management technique that may be adopted by grouse moor managers in circumstances of high tick infestations on grouse chicks.

Identifying main drivers and testing control strategies for CCHFV spread

Crimean Congo Haemorrhagic Fever (CCHF) is an emerging zoonotic disease. The causative agent is a virus (CCHFV), mainly transmitted by ticks of the species Hyalomma marginatum in Eastern Europe and Turkey. In order to test potential scenarios for the control of pathogen spread, the basic reproduction number (R0) for CCHF was calculated. This calculation was based on a population dynamics model and parameter values from the literature for pathogen transmission. The tick population dynamics model takes into account the major processes involved and gives estimates for tick survival from one stage to the other and number of feeding ticks. It also considers the influence of abiotic (meteorological variables) and biotic factors (host densities) on model outputs, which were compared with data collected in Central Anatolia (Turkey). R0 computation was thereafter used to test control strategies and especially the effect of acaricide treatment. Simulation results indicate that such treatments could have valuable effects provided that the acaricide is applied regularly throughout the spring and summer, and over several years. Furthermore, a sensitivity analysis to abiotic and biotic factors showed that, even though temperature has a strong impact on model outputs, host (mainly hare) densities also play a role. The kind of model we have developed provides insight into the ability of different strategies to prevent and control disease spread and has proved its relevance when associated with field trials.

Louping ill virus in the UK: a review of the hosts, transmission and ecological consequences of control

Louping ill virus (LIV) is a tick-borne flavivirus that is part of the tick-borne encephalitis complex of viruses (TBEV) and has economic and welfare importance by causing illness and death in livestock, especially sheep, Ovies aries, and red grouse, Lagopus lagopus scoticus, an economically valuable gamebird. Unlike Western TBEV which is found primarily in woodlands and is reservoired by small rodents, LIV is not generally transmitted by small rodents but instead by sheep, red grouse and mountain hares and, therefore, is associated with upland heather moorland and rough grazing land. Red grouse are a particularly interesting transmission host because they may acquire most of their LIV infections through eating ticks rather than being bitten by ticks. Furthermore, the main incentive for the application of LIV control methods is not to protect sheep, but to protect red grouse, which is an economically important gamebird. The widespread intensive culling of mountain hares which has been adopted in several areas of Scotland to try to control ticks and LIV has become an important issue in Scotland in recent years. This review outlines the reservoir hosts and transmission cycles of LIV in the UK, then describes the various control methods that have been tried or modelled, with far-reaching implications for conservation and public opinion.

Past and future perspectives on mathematical models of tick-borne pathogens

Ticks are vectors of pathogens which are important both with respect to human health and economically. They have a complex life cycle requiring several blood meals throughout their life. These blood meals take place on different individual hosts and potentially on different host species. Their life cycle is also dependent on environmental conditions such as the temperature and habitat type. Mathematical models have been used for the more than 30 years to help us understand how tick dynamics are dependent on these environmental factors and host availability. In this paper, we review models of tick dynamics and summarize the main results. This summary is split into two parts, one which looks at tick dynamics and one which looks at tick-borne pathogens. In general, the models of tick dynamics are used to determine when the peak in tick densities is likely to occur in the year and how that changes with environmental conditions. The models of tick-borne pathogens focus more on the conditions under which the pathogen can persist and how host population densities might be manipulated to control these pathogens. In the final section of the paper, we identify gaps in the current knowledge and future modelling approaches. These include spatial models linked to environmental information and Geographic Information System maps, and development of new modelling techniques which model tick densities per host more explicitly.

Louping ill virus: an endemic tick-borne disease of Great Britain

In Europe and Asia, Ixodid ticks transmit tick-borne encephalitis virus (TBEV), a flavivirus that causes severe encephalitis in humans but appears to show no virulence for livestock and wildlife. In the British Isles, where TBEV is absent, a closely related tick-borne flavivirus, named louping ill virus (LIV), is present. However, unlike TBEV, LIV causes a febrile illness in sheep, cattle, grouse and some other species, that can progress to fatal encephalitis. The disease is detected predominantly in animals from upland areas of the UK and Ireland. This distribution is closely associated with the presence of its arthropod vector, the hard tick Ixodes ricinus. The virus is a positive-strand RNA virus belonging to the genus Flavivirus, exhibiting a high degree of genetic homology to TBEV and other mammalian tick-borne viruses. In addition to causing acute encephalomyelitis in sheep, other mammals and some avian species, the virus is recognized as a zoonotic agent with occasional reports of seropositive individuals, particularly those whose occupation involves contact with sheep. Preventative vaccination in sheep is effective although there is no treatment for disease. Surveillance for LIV in Great Britain is limited despite an increased awareness of emerging arthropod-borne diseases and potential changes in distribution and epidemiology. This review provides an overview of LIV and highlights areas where further effort is needed to control this disease.

De novo Ixodes ricinus salivary gland transcriptome analysis using two next-generation sequencing methodologies

Tick salivary gland (SG) proteins possess powerful pharmacologic properties that facilitate tick feeding and pathogen transmission. For the first time, SG transcriptomes of Ixodes ricinus, an important disease vector for humans and animals, were analyzed using next-generation sequencing. SGs were collected from different tick life stages fed on various animal species, including cofeeding of nymphs and adults on the same host. Four cDNA samples were sequenced, discriminating tick SG transcriptomes of early- and late-feeding nymphs or adults. In total, 441,381,454 pyrosequencing reads and 67,703,183 Illumina reads were assembled into 272,220 contigs, of which 34,560 extensively annotated coding sequences are disclosed; 8686 coding sequences were submitted to GenBank. Overall, 13% of contigs were classified as secreted proteins that showed significant differences in the transcript representation among the 4 SG samples, including high numbers of sample-specific transcripts. Detailed phylogenetic reconstructions of two relatively abundant SG-secreted protein families demonstrated how this study improves our understanding of the molecular evolution of hematophagy in arthropods. Our data significantly increase the available genomic information for I. ricinus and form a solid basis for future tick genome/transcriptome assemblies and the functional analysis of effectors that mediate the feeding physiology and parasite-vector interaction of I. ricinus.