Discussion on a Selection of Key Scientific Articles

By John Atkinson,
Associate Director Intergovernmental Veterinary Health
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Rift Valley fever virus and European mosquitoes: vector competence of Culex pipiens and Stegomyia albopicta (=Aedes albopictus), Brustolin, M., Talavera, S., Nuñez, A., Santamaría, C., Rivas, R., Pujol, N., Valle, M., Verdún, M., Brun, A., Pagès, N., Busquets, N. (2017) Medical and Veterinary Entomology, doi: 10.1111/mve.12254 .

Key Thoughts:
Continued research into the vectors of transboundary diseases such as Rift Valley fever (RVF) is essential to inform disease surveillance and control plans. However, work like that from the paper summarised here has shown that potential vectors are already present in huge numbers in regions currently free of such transboundary diseases. This means that strict controls on animal movements must be continually applied alongside effective disease surveillance programmes. Whilst much focus is understandably given to where disease outbreaks occur, it is just as important to monitor areas where outbreaks are not reported to make sure they are truly free of disease and action taken to keep it that way. This is as true for many other diseases, including lumpy skin disease and foot-and-mouth disease, as it is for RVF.

Article Summary:
Rift Valley fever (RVF) is a zoonotic disease that is caused by Rift Valley fever virus (RVFV) and spread by mosquitos. Not only is the disease seen in a wide range of domestic and wild mammalian hosts, it has been isolated in various species of mosquitoes. Until 2000 RVF was only reported in Africa, but since then significant outbreaks have occurred in the Middle East. Furthermore, the presence of it in North Africa means that it poses a threat to Europe. Whilst restrictions on the import of livestock into Europe reduce the risk of RVF crossing the Mediterranean Sea, it is important to understand whether potential vectors exist in Europe in case the RVFV manages to find a way in. 

Most of the mosquito species that have been found to carry the RVFV belong to the Culex and Stegomyia (Aedes) genera. There have been few studies involving European mosquitoes, but Culex pipiens and Stegomyia albopicta would be the likely vector species because they are present throughout the areas surrounding the Mediterranean Sea. This study used Culex pipiens form molestus, Culex pipiens hybrid form, and Stegomyia albopicta (= Aedes albopictus) collected in Spain, and reared them in environmental conditions that corresponded to the Spanish summer when mosquito density and activity is highest. RT-PCR was used to confirm absence of viral infections, and the virulent RVF 56/74 strain was used.

The infection rate (IR), disseminated infection rate (DIR), and transmission efficiency (TE) were evaluated through three assays involving female mosquitoes that had never blood fed. RT-PCR was performed on viral RNA extracted from saliva samples (taken using FTA™ cards and capillary technique), as well as bodies, legs, and wings. 

RVFV was detected in saliva from two of the mosquito species (Cx. pipiens hybrid form and S. albopicta), indicating that it crossed the salivary gland barriers. The viral dose directly influences infection and dissemination rates for the Cx. pipiens hybrid form, and the viral loads used corresponded to those detected in blood from European lambs from a previous study.

The study shows that two species of European mosquitoes were able to transmit RVFV and support the possibility that an outbreak of RVF may occur if the virus is introduced into Spain. This should help in the development of vector control and RVF surveillance plans. Further work is needed to assess other possible European vectors.

Immunogenicity of plant-produced African horse sickness virus-like particles: implications for a novel vaccine, Dennis, S.J., Meyers, A.E., Guthrie, A.J., Hitzeroth, I.I., Rybicki, E.P. (2017) Plant Biotechnology Journal, pp. 1–9

Key Thoughts:
There are many reasons why vaccines may be available in certain countries or regions but not others. Often this is because a country or region is apparently free of a disease and so there has been no need for a vaccine to be registered. As diseases move across country boundaries into new areas there will be times when a vaccine is needed at short notice in response to a serious disease threat and unregistered products, or those with provisional licenses, may be allowed for a period of time to combat it.

However, in some cases there is a need to develop novel vaccines to overcome concerns, such as reversion to virulence or gene segment reassortment between outbreak and vaccine strains, if they are to be used in areas where a disease is not endemic. Research into new vaccine types and production systems must be encouraged in order to anticipate and rapidly respond to disease challenges of the future. Quite who takes the financial and administrative burden for developing and registering new vaccines against transboundary and emerging diseases is a topic for another discussion.

Article Summary:
African horse sickness (AHS) is one of the most lethal viral diseases of horses and is caused by the African horse sickness virus (AHSV), which is an Orbivirus of the family Reoviridae. There are nine distinct serotypes of AHSV and they are transmitted by biting Culicoides midges. It is endemic to sub-Saharan Africa, but outbreaks have occurred farther northwards including North Africa, Middle East, and the Mediterranean region. The extension of the midge vector’s range and recent outbreaks of the related bluetongue virus in Northern Europe mean that AHS is a concern to many countries where it is not currently endemic.

Whilst live-attenuated vaccines are available in Africa for AHS, there are concerns about reversion to virulence, gene segment reassortment between outbreak and vaccine strains, the absence of DIVA, and the absence of a licensed product outside Africa. A new vaccine that can overcome these concerns could be of great interest for many areas.

Virus-like particles (VLPs) are highly immunogenic, nonreplicating protein assemblies which will not revert to virulence or reassort with wild strains. They also have the advantage that it is possible to discriminate between vaccinated and infected animals because the VLPs do not contain viral RNA or nonstructural proteins.

Recombinant plasmids containing the VP2, VP3, VP5, and VP7 genes were constructed and transient expression of the AHSV serotype 5 capsid proteins in Nictotiana bethamiana was tested by syringe-infiltration of leaves. All four capsid proteins were expressed and self-assembled into complete particles. Density gradient ultracentrifugation was used to purify and concentrate the plant-produced AHSV-5 VLPs. These were tested in guinea pigs to induce an immune response, and the resulting serum samples were sent for serum neutralization tests. A high level of neutralization capability against AHSV-5 was seen, and to a lesser extent against AHSV-8.

This study demonstrated that plant systems may be used to produce VLPs as part of a process that is fast, simple, and scalable. Further work is needed to assess safety and immunogenicity in horses with a view to the development of new vaccines against AHS.


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