Darpel, K. E. | bioRxiv Science

bioRxiv2021

Natural reassortment of a segmented RNA arbovirus illustrates plasticity of phenotype in the arthropod vector and mammalian host in vivo

Segmented RNA viruses are a taxonomically diverse group of 11 families that can infect plant, wildlife, livestock and human hosts. A shared feature of these viruses is the ability to exchange genome segments during co-infection of a host by a process termed reassortment. Reassortment enables rapid evolutionary change, but in the case of segmented RNA viruses utilising an arthropod vector is set against the constraint of purifying selection and genetic bottlenecks imposed by replication in two evolutionarily distant hosts. In this study, we use an in vivo host: arbovirus: vector model to investigate the impact of reassortment on two phenotypic traits: vector competence and virulence in the host. Bluetongue virus (BTV) (Reoviridae) is the causative agent of bluetongue (BT), an economically important disease of domestic and wild ruminants and deer. The genome of BTV is comprised of 10 linear segments of dsRNA and the virus is transmitted between ruminants by Culicoides biting midges (Diptera: Ceratopogonidae). Five strains of BTV representing three serotypes (BTV-1, BTV-4 and BTV-8) were isolated from naturally infected ruminants in Europe and parental/reassortant lineage status assigned through full genome sequencing. Each strain was then assessed in parallel for the ability to infect Culicoides and to cause BT in sheep. Our results demonstrate that two reassortment strains, which themselves became established in the field, had obtained high replication ability in C. sonorensis from one of the parental virus strains which allowed inferences of the genome segments conferring this phenotypic trait. IMPORTANCEReassortment between strains can lead to major shifts in the transmission parameters and virulence of segmented RNA viruses with consequences for spread, persistence and impact. The ability of these pathogens to change their phenotypes rapidly in response to selection pressure in new environments presents a major challenge in understanding factors driving emergence. Utilising a natural mammalian host-insect vector infection and transmission model, we demonstrated for the first time the genetic basis for a phenotypic trait of BTV within strains directly isolated from the field and, hence, selected and relevant for natural transmission.

microbiology↗

bioRxiv2020

Quantifying the acquisition and retention of lumpy skin disease virus by haematophagus insects and the implications for transmission and control.

Lumpy skin disease virus (LSDV), a poxvirus that causes severe disease in cattle, has in the last few years rapidly extended its distribution from Africa and the Middle East into Europe, Russia, and across Asia. LSDV is believed to be primarily spread mechanically by blood-feeding arthropods, however the exact mode of arthropod transmission, the relative ability of different arthropod species to acquire and retain the virus, as well as their comparative importance for LSDV transmission, remain poorly characterised. Since the vector-borne nature of LSDV transmission is believed to have enabled the rapid geographic expansion of this virus, the lack of quantitative evidence on LSDV transmission has impeded effective control of the disease during the current epidemic. Obtaining high quality data on virus transmission by arthropods is challenging, and practical limitations often result in inadequate arthropod numbers or model hosts, limiting the transferability of experimental findings to the natural transmission scenario. We have addressed these limitations in this study. Using a highly representative bovine experimental model of lumpy skin disease we allowed four representative vector species (Aedes aegypti, Culex quinquefasciatus, Stomoxys calcitrans and Culicoides nubeculosus) to blood-feed on LSDV-inoculated cattle in order to examine the acquisition and retention of LSDV by these species in unprecedented detail. We found the probability of LSDV transmission from clinical cattle to vector correlated with disease severity. Subclinical disease was more common than clinical disease in the inoculated cattle, however the probability of vectors acquiring LSDV from subclinical animals was very low. All four potential vector species studied had a similar rate of acquisition of LSDV after feeding on the host, but Aedes aegypti and Stomoxys calcitrans retained the virus for a longer time, up to 8 days. There was no evidence of virus replication in the vector, consistent with mechanical rather than biological transmission. The parameters obtained in the in-vivo transmission experiments subsequently enabled enhanced modelling approaches to determine the basic reproduction number of LSDV in cattle mediated by each of the insect species. This was highest for Stomoxys calcitrans (19.1), C. nubeculosus (7.4), and Ae. aegypti (2.4), surprisingly indicating these three species are all potentially efficient transmitters of LSDV. These results reveal that currently applied LSDV control measures such as stamping out of all cattle on affected premises or insect control measures targeting single species need to be urgently reconsidered. Overall our studies have highlighted that the combination of highly relevant in-vivo experiments and mathematical modelling can be directly applied to devise evidence-based proportionate and targeted control programmes.

microbiology↗