Vector-borne transmission of lumpy skin disease virus

Lumpy skin disease virus (LSDV) is the causative agent of lumpy skin disease (LSD), an endemic disease of cattle in Africa and the Middle East where it produces significant economic loss and acts as a barrier to trade. Since 2012 LSDV has spread into Turkey, Europe, the Caucasus and Russia . The epidemic has resulted in the slaughter of thousands of cattle, mass vaccination campaigns, animal movement restrictions and export bans. One of the key knowledge gaps hampering control efforts is an understanding of how LSD is transmitted from animal to animal and herd to herd. LSD expert panels and numerous LSD-themed workshops in recent years have all identified research into vector-borne transmission of LSD as a high priority. This will be addressed in this research project.

The project hypothesis is:

Viable LSDV persists in insect vectors at suitable sites and for sufficient time to allow onward transmission to cattle hundreds of kilometres distant.

This proposed work involves two objectives running in parallel over 18 months.

Objective 1 To identify the most likely vector group responsible for transmission of LSDV.
This objective will undertake experimental transmission of LSDV from infected cattle by potential insect vectors in the high containment facilities at The Pirbright Institute. These studies will use contemporary virus detection techniques to distinguish between mechanical and biological transmission, and estimate the risk of different insect vectors that are present in Europe.
The vector potential of four insect species, Stomoxysis calcitrans (stable flies), Culicoides nubeculosus midge, and Aedes aegypti and Culex quinquefasciatus mosquitoes, will be investigated. The four species were each selected on the basis of their distribution in affected and / or threatened areas, evidence from the literature of their experimental or epidemiological link to LSDV transmission, and because they represent different feeding mechanisms (solenophagous vs telmophagous).

Objective 2 To assess the risk posed by LSDV and the potential impact of different control measures.
This objective will integrate the experimental results into models of LSDV and, hence, explore the risk of disease and impact of different control measures.

Overall the project will provide scientific evidence on vector-borne transmission of LSDV to enable effective and proportional LSD control programmes to be designed and the current Eurasian LSD epidemic to be contained.
The project utilises the unique and world-class expertise at Pirbright in lumpy skin disease research, multi-scale modelling of viral diseases of livestock, the biology of blood-feeding insects, in vivo transmission studies of viruses between natural ruminant hosts and vectors, and cutting-edge bioimaging of arthropods to study vector-borne transmission of LSDV.

The project is sponsored by an industrial partner MSD Animal Health, producer of the widely used Lumpyvax vaccine against LSD. MSD Animal Health are partnering Pirbright in this project to enable LSD vaccination campaigns to be augmented with scientifically rational control programmes aimed at reducing vector-borne transmission of LSDV. This approach is in accordance with Merck Animal Health's philosophy that their responsibilities extend beyond their primary business goals. By supporting this research project they aim to add value to efforts to control and eradicate LSD and thereby provide benefit to the environment and wider society as a whole.

The project is also supported by DEFRA, the UK government department responsible for protecting the UK from exotic diseases such as lumpy skin disease (LSD). This support emphasises the impact this research will have on reducing the risk of LSD incursion into the UK, and facilitating its rapid eradication if it does occur.

Project hypothesis:
Viable LSDV persists in insect vectors at suitable sites and for sufficient time to allow onward transmission to cattle hundreds of kilometres distant

Objective 1 will undertake experimental transmission of LSDV by insect vectors in high containment facilities at The Pirbright Institute. Cattle will be inoculated with wildtype LSDV and then used as donor animals for insect feeding. Colony bred stable flies (Stomoxys calcitrans), midges (Culicoides nubeculosus) and mosquitoes (Aedes aegypti, Culex quinquefasciatus) will be fed on the cattle and processed for virus detection by multiple methodology at several incubation time points.
Insect will be processed either whole or dissected to process heads and body separately. Viral DNA and live virus will be quantified in the insects by qPCR and titration.
The location of the virus in the insects will be determined using immunofluorescence and electron microscopy.
Viral replication in the insects will be studied by examining insect tissues for viral mRNA transcripts, and looking for intracellular viral factories using electron microscopy.
The likelihood of onward transmission of LSDV by the insects will be estimated using a swab feeding system.
Viraemic donor blood as potential LSDV source will be trialled using a hemotek membrane feeding system as a future replacement for feeding insects on donor animals.

Objective 2 will address the current lack of a suitable mathematical model of LSD. A model will be built and used to calculate the basic reproduction number (R0) of LSD. This is a powerful tool with which to assess the risk posed by a disease. The experimental results from objective 1 will be used to build and optimise the model. The risk posed by LSD to regions of Europe, including the UK, and the impact of different control strategies will be explored.
This project will provide scientific evidence on vector-borne transmission of LSDV to enable LSD control programmes to be designed

What is the benefit of this research?
Better control of lumpy skin disease. Ever since LSD entered Europe and the Caucasus in 2015 it has spread rapidly. Application of all available tools for LSD control (vaccination, quarantine and stamping out) has so far failed to halt the spread of the disease and it now threatens cattle in the rest of Asia and Europe including the UK. The outcomes from the proposed research project will supply information on vector-borne transmission of LSDV which will facilitate development of more effective, evidence-based control plans for LSD in the UK and elsewhere. The research project will answer key questions:
Which insects are capable of spreading LSD, and how long do they remain infective for (obj 1)?
This information is needed to (a) determine how wide restriction zones around an outbreak of LSD need to be, (b) identify which insects should be targeted in vector elimination programmes, and (c) calculate which climatic conditions and geographic regions of Europe are most conducive to rapid LSD spread, and therefore where pre-emptive vaccination programmes should be targeted towards.
Are subclinically infected cattle, which develop LSDV viraemia but no skin nodules, important in the epidemiology of LSD, and should they be targeted in LSD control programmes (obj 1 and 2)?
In other words should stamping out campaigns stipulate killing all cattle on affected farms or only the clinically disease cattle? Total stamping out is currently mandated by EU legislation but has been very unpopular with many rural groups and is very expensive.
Can vaccination campaigns alone prevent the spread of LSD (obj 2)?
Widespread vaccination to prevent LSD is relatively cheap compared to movement restrictions and stamping out policies. However it is unclear if it alone is efficacious at preventing LSD spread. Comparison of R0 under different conditions will allow policy makers to make informed decisions about which control strategies to implement.

Who will benefit?
For each beneficiary the timescale of the impact has been estimated as immediate (during the time frame of the project), medium term (1-5 years after the project has been completed) or long term (>5 years after the project has been completed)

1. Rural communities. This research will result in better LSD control and prevention strategies worldwide. Further spread of the virus into Europe and Asia will be inhibited and the impact of the disease in endemic areas reduced. This will result in direct economic and social benefits to rural communities in Africa, Asia and Europe. Medium to long term.
2. UK economy. The risk of LSDV reaching the UK will be reduced as a result of this research, therefore avoiding the loss of lucrative trade markets. The main trade-related economic consequence of a LSDV outbreak in the UK is loss of semen and embryo markets (there is very little live cattle trade from the UK). Medium to long term.
3. Vaccine companies. Knowledge of vector-borne LSD transmission will enable regions at most risk of LSD to be identified resulting in more effective targeted use of available vaccines. The benefit of this research to vaccine companies is highlighted by the participation of MSD Animal Health as an industrial partner in this project. Immediate, medium and long term.
4. Non-governmental agencies. LSD is a target for eradication by charities such as GalvMed and The Bill and Melinda Gates Foundation who will seek to implement the outputs from this research. Immediate and medium term.
5. International governments. The research proposed will provide information that can be used in programmes to eradicate LSD from low and middle income countries. LSD is endemic in many countries in Africa and has never been eradicated once it has entered a new region. The findings from the proposed research will contribution to the design of effective and economically viable vector control plans.