Nematode parasitism of arbovirus vectors: effect of mermithids on Culicoides biting midges and their potential use for disease and vector control

Lead Research Organisation: University of Liverpool
Department Name: Institute of Infection and Global Health

Abstract

Biting midges are a group of tiny flying insects that include a notorious species that plagues parts of Scotland. They are one of the most abundant blood-feeding insects and occur throughout most of the inhabited world. Certain species commonly found on farms are important carriers of livestock viruses in Europe. Methods to control these insects have so far been unsuccessful.
The emergence of the livestock diseases bluetongue (BT) in 2007 and Schmallenberg (SB) in 2011 in the UK highlight that midge transmitted viruses are a potential threat to the UK. They also spread African horse sickness (AHS), a highly lethal disease of horses, considered a risk to Europe. To date, over 50 viruses have been isolated from midges, causing significant economic impact through clinical disease and animal trade restrictions. Outbreaks of BT are ongoing in mainland Europe, so further outbreaks may occur in the UK. Currently we rely on vaccinating animals against some of these diseases, as control of insects transmitting them has been unsuccessful. Such vaccination is not routinely undertaken in UK livestock, and tends to only occur once an outbreak has been confirmed. Vaccines licensed for use in Europe do not exist for many diseases, including AHS and many types of BT. It is therefore paramount to determine other ways to manage such outbreaks to allow informed mitigation procedures in advance of, or in the event of, an outbreak.
My aim is to investigate the effects of a highly prevalent parasitic worm (mermithid nematodes) on the biology of Culicoides midges and to determine whether they affect a midge's ability to be infected by and transmit livestock viruses. Mermithids are a major group of nematode that live inside the larvae of midges, and other insects, often killing them when they emerge; but may be carried into the adult insect, sterilizing them as a result. These mermithids are present in UK midges and the successful use of mermithids to control mosquito populations suggests that there may be potential in their use for midge control, yet no information exists on mermithid parasitism of these midge vectors in Europe.
At the University of Liverpool, using Culicoides from across England and Wales, I aim to study the distribution of these nematodes and assess whether this changes between years. Despite their importance, very few mermithids have been successfully identified so I will use DNA-based techniques to determine the species parasitising UK midges, before establishing reliable identification techniques to enable other researchers to build on such work in the future. Any parasite which may be useful for Culicoides control must have an economical method of mass rearing, so a laboratory rearing method will be established and the possibility of storage and transportation determined, allowing field testing at a later date. Similar rearing methods have been successful in the USA, and this part of the project will be undertaken in collaboration with the University of California, Riverside. Most crucially, the effect of parasitism by mermithids will be assessed on the lifecycle, reproduction and fitness of the midges before being infected with BT and SB viruses and assessed for their ability to transmit virus, the dosage of virus needed to achieve infection, and their survival. Here both wild-caught and colony reared midges, supplied by the Pirbright Institute, will be infected with mermithids and virus at the Liverpool School of Tropical Medicine.
This project will gather information relevant to a wide range of arthropod vectors, such as UK mosquitoes, as well as other humans and livestock vector-borne diseases. In the event of an outbreak the work may provide information for more effective vector control and will arm policy makers with information that can be useful for i) understanding methods to control midge-borne diseases; ii) reducing the chances of an outbreak occurring; iii) more effectively dealing with an outbreak occurrence.

Technical Summary

Culicoides biting midges are the most important biological vectors of livestock arboviruses in Europe. The emergence of bluetongue (BTV) and Schmallenberg viruses (SBV), with their economic impacts through clinical disease and animal trade restrictions, provides evidence of the threat they pose to UK livestock. Despite their importance, methods to control these insect vectors remain unsuccessful and control of such diseases relies on virus-specific vaccines. Mermithids are a major group of parasitic nematode worms that utilise the larvae of midges and other insects as definitive hosts, often killing host larvae on emergence, or being carried to the adult stage, sterilizing host insects as a result. The successful use of mermithids to reduce mosquito populations in the USA suggests their potential use in the biological control of Culicoides in Europe, including the UK. To date, almost no published data exists on mermithid parasitism of midges in Europe. The aim of this project is to determine the effect of mermithid parasitism on the life history characteristics, vector competence and survival of Culicoides in the UK, thereby assessing their potential as agents of biological control.

Culicoides collected from 144 sites in England and Wales will be used to determine the spatial and temporal distribution of the nematode species parasitising UK midges and the levels of parasitism and intersex species. Molecular and/or morphological techniques will be established to facilitate identification of mermithids of Culicoides in the UK. An in-vivo culture method and laboratory model for mermithid infection of midges will be established using C. nubeculosus and C. sonorensis colony larvae. The effect of parasitism on fitness parameters and survival of both adult and larval Culicoides will be determined using the infection model, before mermithid-induced effects on vector competence to both BTV and SBV are determined in colony and field-caught Culicoides.

Planned Impact

LIVESTOCK INDUSTRIES/ FARMING BODIES: The main aim of the project is to help preserve UK animal and human health by providing information on the ability of mermithids to serve as Culicoides biological control, through effects on survival and vector competence. The National Farmers' Union, committed to the health and welfare of farmed animals, would be interested in on-farm methods of vector control. Entomopathogenic nematode treatment using standard pesticide equipment could be readily employed by farmers/ smallholders, so UK Government could pass disease control responsibilities to industry.
GENERAL PUBLIC/ TOURISM: Certain midges are a biting nuisance impacting on human public health with bites leading, in severe cases, to pruritic wheal-and-flare responses and scarring. They impact on tourism, outdoor pursuits, forestry and agriculture, particularly in Scotland. Research into novel methods of control may lead to benefits for the public and various outdoor industries. While the arboviruses transmitted by midges are largely of veterinary importance, some cause disease in humans. In the event of a future human disease outbreak, as a preventive measure against disease, or in terms of reducing nuisance insects, my work may provide information on effective vector control, leading to direct public benefits.
HORSE OWNERS/ EQUINE INDUSTRIES: Midges cause the hypersensitivity reaction sweet itch in horses, and transmit the highly fatal equine disease African horse sickness. Outbreaks have previously occurred in Spain, but no EU-licensed vaccines are available for this exotic disease. In the UK, organisations such as the Horserace Betting Levy Board, horse charities, and horse owners will be interested in novel methods of vector control which could be applied at yards.
INTEGRATED PEST MANAGEMENT (IPM)/ VECTOR CONTROL AGENCIES: Information on possible biological control agents of midges will be useful to local councils (often responsible for dealing with insect pests), and commercial companies developing insect control tools. This became clear when I visited a company developing low cost IPM programmes for large and small-scale farmers worldwide. Entomopathogenic nematodes fit well in IPM programs because they are non-toxic to humans, relatively specific to their target pest(s), and can be applied with standard pesticide equipment.
UK ECONOMY: The northern European BTV-8 outbreak caused significant costs for national economies, exceeding £1 billion in France in 2007, mainly through lost trade and control costs. The Scottish tourism industry loses around £286 million PA to the biting pests. In this proposal I focus on survival of mermithid-parasitised midges, and vector competence to two midge-borne viruses which can cause high animal production losses to the livestock industry. This study may assist in mitigating outbreaks (of disease or pests) that impact animal health and welfare, as well as tourism, by determining how parasitism can reduce insect numbers plus their ability to transmit disease.
POLICY MAKERS: I will arm policy makers with information for i) more accurate understanding of Culicoides control in the UK; ii) reducing the chances of disease occurrences; iii) more effectively dealing with outbreaks. In 2008, the Science Advisory Council's sub-group on epidemic diseases recommended that Defra should set up a campaign on innovative ways to control midge populations aimed at farmers and also investigate further Culicoides fundamental biology and how this might inform control strategies (Paper: SAC-ED (09) 05; recommendation 7). This project will provide important information for improving contingency plans produced by the devolved administrations (Defra, Welsh Assembly Government, Scottish Executive) and explores an innovative method of controlling midges. As the species to be studied are the vectors throughout northern Europe, the results will be of importance to the European Commission as well as other Members States.

Publications

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Title Culicoides Midge Model for Public Engagement 
Description Production of a detailed, and morphologically accurate, up-scaled model of a Culicoides biting midge parasitised by a nematode in order to be used for public engament activities. The midge model highlights the important morphological changes that take place on a midge as a result of parasitism, allowing my research to be explained visually to a wide variety of audiences. 
Type Of Art Artwork 
Year Produced 2017 
Impact The model has been used in a wide variety of public engagement activities - from scientific conferences, to highlighting the importance of insect science in schools, to explaining to farmers, livestock auctionhouses and abattoirs the science behind my research. 
 
Description BBSRC Research Experience Placement
Amount £2,500 (GBP)
Funding ID BB/M011186/1 
Organisation Biotechnology and Biological Sciences Research Council (BBSRC) 
Sector Public
Country United Kingdom
Start 06/2016 
End 09/2016
 
Description Institute of Infection & Global Health Pump Priming
Amount £1,000 (GBP)
Organisation University of Liverpool 
Sector Academic/University
Country United Kingdom
Start 05/2017 
End 09/2017
 
Description University of Liverpool Pump Priming
Amount £1,000 (GBP)
Organisation University of Liverpool 
Sector Academic/University
Country United Kingdom
Start 08/2018 
End 12/2018
 
Title 3 years of Culicoides Collections from Livestock Auctionhouses and Abattoirs 
Description A database of insect collections taken from abattoirs and livestock auctionhouses in the north-west of the UK, for the last 3 years. Collections have been made on a weekly basis, with samples analysed on collection. Only Culicoides biting midges have been identified and analysed in these collections, however other insects were stored for future use. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? No  
Impact This data provides year-round information on Culicoides biting midge density and temporal changes at important destinations within the food system. Such data is currently lacking in published materials. This method of data collection has also enabled outreach activities and engagement with farmers and livestock owners. 
 
Title Culicoides Collections from Schools - spanning 2 years 
Description A database of insect collections taken from rural schools around the UK for the last 2 years. Collections have been made on a weekly basis, with samples sent back to the University of liverpool for analysis. Only Culicoides biting midges have been identified and analysed in these collections, however other insects were stored for future use. 
Type Of Material Database/Collection of data 
Year Produced 2018 
Provided To Others? No  
Impact This data provides year-round information on Culicoides biting midge density and temporal changes - such data is difficult to access as many insect collections are undertaken using a 'snapshot' approach whereby insects are sampled only at the peak of the insect season. This data will provide me with information in temporal and spatial changes in nematode parasitism throught the UK. The data will continue to be collected for another year before being fully analysed.This method of data collection has also enabled outreach activities and engagement with school-children to encourage an interest in insect science. 
 
Description Culicoides Midge Colonisation 
Organisation The Pirbright Institute
Country United Kingdom 
Sector Academic/University 
PI Contribution Planned colonisation of Culicoides biting midges at the University of Liverpool - currently we have designed a Category 2 Insectary facility which is being put out to tender by the University of Liverpool (in order to house the new colonies).
Collaborator Contribution Colony maintenance; safety guidelines; insectary design infomation; and will be providing us with colonised insects once our insectary is complete.
Impact Design of a Category 2 Insectary facility to be housed at the University of Liverpool's Leahurst Campus, in order to house colonised Culicoides midges from both the UK and the USA.
Start Year 2018
 
Description Culicoides Midge Model for Public Engagement 
Organisation Amalgam Modelmaking Ltd
PI Contribution High resolution photography of both Culicoides midge, and nematode, structure & morphology in order to produce a model of parasitism for public engagement activities.
Collaborator Contribution Production of a detailed, and morphologically accurate, up-scaled model of a Culicoides biting midge parasitised by a nematode in order to be used for public engament activities. The midge model highlighted the important morphological changes that take place on a midge as a result of parasitism, allowing my research to be explained visually to a wide variety of audiences.
Impact The midge model highlights the important morphological changes that take place on a midge as a result of parasitism, allowing my research to be explained visually to a wide variety of audiences. As such, the model has been used in a wide variety of public engagement activities - from scientific conferences, to highlighting the importance of insect science in schools, to explaining to farmers, livestock auctionhouses and abattoirs the science behind my research.
Start Year 2017
 
Description Irish Culicoides Biting Midges 
Organisation Agri-Food and Biosciences Institute
Country United Kingdom 
Sector Public 
PI Contribution Testing of Irish Culicoides for the presence of mermithid nematodes
Collaborator Contribution Supply of Irish Culicoides collected on farms and sent to University of Liverpool for testing for mermithid nematodes
Impact Testing of Irish Culicoides biting midges for nematode parasitism
Start Year 2017
 
Description Farmer Engagement 
Form Of Engagement Activity A talk or presentation
Part Of Official Scheme? No
Geographic Reach Regional
Primary Audience Public/other audiences
Results and Impact Meetings were held at a number of livestock auctionhouses throughout the northwest in order to update farmers on their knowledge of diseases of livestock transmitted by insects, and to recruit them onto a 3 year long study to investigate insects travelling with their animals to both livestock auctionhouses as well as abattoirs. The number of farmers in attendance varied between 20-100 at each location and the talks sparked questions and debate on the importance of various insect-associated livestock diseases. The farmers highlighted that they gained increased knowledge of the diseases discussed and an interested in participating in research with us. This let to the start of a 3-year study to collect insects (primarily Culicoides) from abattoirs and livestock auctionhouses in the north-west of the UK, as well as to trap these insects in livestock trailers/containers both to and from market.
Year(s) Of Engagement Activity 2015,2016,2017,2018
 
Description School Visits 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Geographic Reach National
Primary Audience Schools
Results and Impact Meetings were held at a number of rural schools throughout the UK in order to increase the interest of children in insect-science, as well as to collect weekly samples of (primarily) Culicoides biting midges from these locations. The number of children in attendance varied between 10-20 at each location and the workshops enabled children to investigate a number of insects down a light microscope, understand why certain insects are studied for the diseases they transmit, and how to catch insects in order to study them. This let to the start of a 3-year study to collect insects (primarily Culicoides) from rural schools throughout the UK, and the production of a database of weekly insect collections from schools for the last 2 years.
Year(s) Of Engagement Activity 2016,2017,2018