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Understanding parameters determining transmission of economically important arboviruses in the United Kingdom - SE4115

The UK is currently at risk from the emergence of both known and unknown Culicoides-borne arboviruses (1-3). In addition to Schmallenberg virus (SBV), which has established itself as an endemic pathogen in this country, circulation of strains of bluetongue virus serotype 8 (BTV-8) and serotype 4 (BTV-4) in France represents a major issue for both animal movements and imminent disease risk. There is also the potential for further, unpredictable emergence of other arboviruses known to be transmitted by Culicoides (e.g. African horse sickness virus; epizootic haemorhagic disease virus; Akabane virus) and arboviruses whose impact on livestock and human hosts have not been described (2).

Spread of arboviruses between hosts occurs by biological transmission where replication and dissemination within the vector is required for transmission. Infection and dissemination of BTV in Culicoides is limited by temperature thresholds (3). In temperate regions, including the UK, these thresholds impose a seasonality on transmission. In a previous Defra project we found that virus replication for BTV was restricted below 12˚C in Culicoides (Project SE: 2618). Similarly, adult Culicoides flight activity is also limited when temperatures fall below approximately 6˚C, resulting in periods over winter when no activity is recorded in surveillance traps (4). This phenomenon is exploited in outbreak response by seasonal vector free periods during winter where animal movements during outbreaks are enabled due to a very low probability of transmission, reducing economic and welfare impacts (5).

The first aim of this project is to define the infectious period for BTV in the bovine host using time-series feeding of Culicoides and compare this to standard diagnostic assays used to infer viraemia. The period of transmissible viraemia remains poorly defined for BTV and is not directly linked to frontline diagnostics (e.g. qPCR; cell-based isolation; ELISA). As a consequence of this, the risk posed by imported animals infected with BTV that do not conform to quarantine restrictions is difficult to assess from the level of viral RNA detected in the host. This issue is exacerbated by the low vector competence of Culicoides for many BTV stains, which has resulted in the vast majority of research to date centring upon the peak viraemic period, where infection is certain. A recent preliminary study at Pirbright using a high vector competence BTV-4 strain in cattle suggest that full infection of Culicoides was possible 21 days after infection of cattle. Defining this period in detail, in particular the beginning and end of the infectious period and how host viraemia relates to Culicoides infection, is important in terms of policy in both underpinning legislative requirements for import and export and rapidly assessing onwards transmission risk.

The second aim of this project is to examine the survival rate of Culicoides during winter. Anecdotal evidence from laboratory studies indicates that Culicoides lifespan can be extended to months under cold conditions and warm periods that occur during winter may allow unexpected arbovirus recrudescence with consequences for movement restrictions, vaccination campaigns and overwintering risk (6). This objective builds on existing age grading techniques and additionally develops emerging techniques to age grade adult Culicoides populations using near-infrared spectroscopy and matrix-assisted laser desorption ionization time of flight mass spectrometry (7,8). These techniques will be validated using populations of Culicoides of known age and then seasonal collections of midges will be used to examine shifts in ages of overwintering populations. Knowing the survival rate of these populations is important in policy in defining how long cases of transmission will continue for, when to conduct surveillance to demonstrate transmission free status, and in determining the probability of overwintering which could have a major economic impact on trade (9).

The third aim of this project is to examine potential mechanical and biological vectors of viruses in an integrated approach across a range of farms. The aim is to provide a balanced understanding of transmission risk and additionally data concerning the survival of large biting flies in the autumn/winter period. In addition to biologically transmitted arboviruses, the UK is also under threat from viruses that are transmitted mechanically between hosts on the mouthparts of blood-feeding arthropods. These pathogens, which include equine infectious anaemia virus and lumpy skin disease virus (LSDV), are primarily transmitted by large biting flies of the families Muscidae (e.g. stable flies; face flies; horn flies) and Tabanidae (horse flies) (10, 11), but may also be transmitted by mosquitoes and/or biting midges. LSDV has recently emerged in south eastern Europe, has had a major economic impact on cattle farming in this region and is now considered a risk to northern Europe (12). In the event of incursion of viruses spread by flies, these data will be used to understand the potential risks posed for onwards transmission and also investigate the value, logistics and intrinsic biases in trapping methods that might be deployed to monitor vector populations.In terms of policy response, these studies provide a range of evidence that is directly relevant to decision making in the event of arbovirus incursion and retain capacity for response in this area of research for the UK. Specific areas of capacity supported include specialised entomological expertise in both the field and under laboratory conditions.

Aim 1: To define the infectious period for BTV in the bovine host using time-series feeding of Culicoides and compare this to standard diagnostic assays used to infer viraemia. Objectives are:

1.1 To establish Culicoides trapping at three sites local to Pirbright and ensure populations of >100 individuals/night are collected under conditions conducive to adult flight (Months 12-15; March-May 2020).
1.2 To infect four cattle with a BTV-4 strain across 4 days (one animal infected/day) using Culicoides sonorensis from the colony maintained at Pirbright and infected via membrane-based feeding 7-10 prior to use. (Months 15-18; June-October 2020 – Seasonally Dependent).
1.3 To feed populations of both UK and colony-derived Culicoides on infected cattle at the beginning and end of the viraemic period (monitored by qPCR). Incubate Culicoides that blood feed on viraemic cattle for 7-10 days and subsequently define infection status using qPCR. For each species, a maximum of 200 individuals/day will be processed (Months 15-18; June-October 2020 – Seasonally Dependent).
1.4 To analyse infection rates of Culicoides according to isolation rates on cell culture, qPCR Ct values and other diagnostic assys to provide comparative data between rates of infection and assays used in outbreak scenarios (Months 21-30; December 2020-December 2021).

As a time series experiment these objectives are inter-dependent, however, the study could be run solely with colony Culicoides in the event of weather that is not conducive to adult flight. All objectives are low risk and techniques have been defined in previous studies.

Aim 2: To define the survival rate of Culicoides during winter using age grading techniques. Objectives are:

2.1 Create a standard range of at least 300 Culicoides of known age using spring emergence from larval habitats, repeated in order to examine year-year variation in response, existing age-grading techniques and to provide samples for 2.2-2.4 (Months 0-6: March-June 2019 and Months 12-15: March-June 2020 – Seasonally dependent).
2.2 Carry out a preliminary study of the use of matrix assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF) for age grading using midges of known age from (2.1). (Months 6-9: June-September 2019).
2.3 Carry out a preliminary study of the use of near infra-red spectroscopy (NIRS) for age grading using midges of known age from (2.1). (Months 15-18: June-September 2020).
2.4 Assess techniques developed in 2.2-2.4 to identify the age of field collected Culicoides from at least one UK surveillance site in order to define survival rate. (Months 18-21: September-December 2020).

Objectives 2.1 and 2.2-2.4 are interdependent. Objectives 2.1 and 2.4 are low risk and based on methods that have been used in other studies. Objectives 2.2 and 2.3 are high risk due to the novelty of the techniques but have demonstrated promise in preliminary studies in other laboratories. To address this risk we conduct these experiments at a limited scale and also will have more traditional tools available to take their place if they do fail. There would also be significant benefits to their use if validated in terms of providing an inexpensive and rapid technique that could be applied to surveillance samples.

Aim 3: Carry out integrated vector trapping at UK farms for use in prediction of seasonality in arbovirus transmission. Objectives are:

3.1 Recruit four UK farms for trapping (2 beef/dairy holdings; 1 sheep holding; 1 equine holding) and carry out site characterisation including establishing a weather station and habitat classification (Months 6-9: September 2019-November 2020 – seasonally dependent).
3.2 Carry out trapping and identify flies that could act as mechanical and biological vectors at farms using multiple approaches across the four locations. (Months 6-12: September 2019-November 2020 – seasonally dependent).
3.3 Repeat trapping and identification conducted in (3.2) to provide information regarding inter-year variation (Months 18-24: September 2020 - March 2021 – seasonally dependent).
3.4 Produce recommendations for data collection cost and utility for arbovirus outbreak monitoring and prediction purposes in the field (December 2021-February 2022).

All four objectives are inter-related, but low risk.

Aim 4: Complete analyses and final report for Defra. Objective is: 

4.1 Carry out final analyses and write and submit report (Months 33-36: December 2021-February 2022
Time-Scale and Cost
From: 2019

To: 2022

Cost: £466,715
Contractor / Funded Organisations
IAH - Institute for Animal Health
Fields of Study
Animal Health