Location: Arthropod-borne Animal Diseases Research
2022 Annual Report
Objectives
Objective 1. Determine biotic and abiotic factors mediating orbivirus transmission in the United States.
• Identify molecular targets for orbivirus cell surface attachment to inform host range susceptibility.
• Determine physiological effects of orbivirus infection of C. sonorensis biting midges on vectorial capacity.
• Assess effects of increasing global temperatures on orbivirus transmission by C. sonorensis biting midges to inform predictive biology and disease ecology.
• Identify and characterize habitats of immature Culicoides to determine species distributions and densities.
Objective 2. Develop intervention strategies to minimize the impact of disease outbreaks caused by Orbiviruses.
• Identify vector transmission control strategies based on our understanding of vector-Orbivirus interactions.
Approach
While there are many critical aspects of orbivirus transmission that are poorly understood for assessing disease risk and developing mitigation strategies, it is clear that the multi-host transmission dynamics and maintenance of orbiviruses in nature requires susceptible, viremic animals, and competent Culicoides biting midge vectors. Identifying biotic and abiotic factors affecting these orbivirus-vector-host interactions from a molecular to organismal scale, and from basic science to applied, is key to understanding orbiviral emergence, spread, adaptation to new environments, and to developing effective control strategies. In Objective 1, we will investigate biotic factors including virus-host molecular interactions, to better understand host range susceptibility (Obj. 1A), and virus-vector interactions, to better understand effects of viral infection on midge physiology (Obj. 1B) and inform how infected vector population densities are affected during outbreaks. We will investigate abiotic factors including the effects of environmental temperature on vector competence, allowing us to determine how rising global temperatures may alter vector-borne transmission dynamics (Obj. 1C), and how ecological conditions alter suitability of midge breeding habitats, affecting population distributions, densities, and transmission potential to nearby animals (Obj. 1D). In Objective 2, we will investigate virus-vector interactions to inform novel methods for disease surveillance, midge management, and identify targets to block virus transmission. We will investigate the effects of virus infection on midge sensory responses and exploit virus-induced physiological changes to improve midge management strategies using light traps (Obj. 2A), specifically targeted to orbivirus-infected Culicoides vectors. Outcomes of this research will 1) address key knowledge gaps in virus-vector-host-environment interactions which underlie orbivirus transmission dynamics and our ability to predict risk to livestock and wildlife, and 2) lead to improved, novel intervention strategies to minimize the impact of these devastating hemorrhagic diseases in the US.
Progress Report
This is the first report for this new project which began during fiscal year (FY) 2022. See the final FY 2021 report for expired project, 3020-32000-010-000D, “Orbivirus Pathogenesis, Epidemiology, and Control Measures” for additional information.
Objective 1: Progress was made toward understanding factors inside Culicoides biting midges that mediate their ability to transmit orbiviruses. Wolbachia is an endosymbiont bacterial species in the gastrointestinal tract of many insects and has been shown to alter infection rates of several viruses for those insects. Wolbachia has previously been identified Culicoides sonorensis colony insects. In collaboration with Texas A&M researchers, ARS researchers determined whether Wolbachia bacteria could interfere with the ability of two orbiviruses [bluetongue virus (BTV) and epizootic hemorrhagic disease virus (EHDV)] to infect midge cells. Wolbachia inhibited the replication of BTV by day 5 and EHDV by day 2 post infection. This suggests Wolbachia could potentially be exploited as an effective strategy for controlling midge transmission of orbiviruses.
Both BTV and EHDV orbiviruses readily infect Culicoides biting midges which multiply the viruses and transmit them to cattle, sheep, and deer, resulting in a severe hemorrhagic disease. Previously we showed that both viruses infect the sensory organs of midges, specifically the eye and the Johnston’s organ of the antennae. Thus, it has been hypothesized that orbivirus infection of midges may lead to changes in gene and protein expression in sensory organs, changes in sensory function, and ultimately, changes in behavior such as photo-attraction, host seeking, and circadian rhythm. In collaboration with Kansas State University, progress was made on assessing the effect of infection on sensory gene expression in female C. sonorensis midges. Initial blood feeding trials with subsequent transcriptome sequencing experiments have been conducted. A high degree of variation in changes in gene expression between midges fed on infected versus uninfected blood were seen. Methodology has been improved to minimize this variation in future experimental trials. A new analysis pipeline has also been developed for examining the transcriptome data which is integral in determining what the effects of orbivirus infection are on gene and protein expression in Culicoides midges.
Progress was also made toward assessing changes in circadian rhythm of orbivirus-infected midges. If orbivirus infection changes when midges are active, it will inform a change to trapping and surveillance strategies and timing. A Drosophila activity monitoring (DAM) system was adapted to Culicoides midges and is being used to establish baseline circadian rhythms to compare with midges infected with BTV. The first replicate trial for BTV has been conducted. Results will be used to assess the effects of BTV infection on daily and seasonal activity of midges and how that affects the risk of orbivirus transmission to cattle, sheep, and deer.
Objective 2: Progress was made toward establishing an embryonated chicken egg (ECE) model to study the transmission of orbiviruses by Culicoides midges. Shell removal was improved to optimize midge access to microvasculature for feeding as well as to preserve embryo longevity. Feeding cages to interface midges with ECE vessels and ECE feeding chambers have been designed, developed, and optimized. BTV inoculation methods and minimum infectious doses for positive ECE controls have been established. Virus can be injected into the egg yolk or white and result in infection. BTV was successfully isolated from embryonic livers 48 hours following needle inoculation. Initial trials are being conducted by feeding BTV-injected, positive control midges on exposed microvasculature of ECEs to demonstrate BTV transmission.
Accomplishments
1. Bacteria that live in biting midge guts may help researchers block virus transmission to livestock. Biting midges become infected with the hemorrhagic orbiviruses, bluetongue virus and epizootic hemorrhagic disease virus, when they feed on infected cattle and sheep. Once virus is in the gut of the insect, it multiplies and spreads throughout the midge, reaching the salivary glands. The next time the midge feeds, the virus is transmitted to another animal. The significant economic impacts of these hemorrhagic diseases result from decreased animal production, livestock mortality, and from national and international trade restrictions imposed on animals and germplasm. As with humans and animals, insects have bacteria in their gut as part of their normal flora to help with metabolism and fend off microbes in a blood meal that could harm them. One type of these helpful insect gut bacteria is called Wolbachia. In collaboration with researchers at Texas A&M, ARS researchers in Manhattan, Kansas, determined that under the right conditions, Wolbachia bacteria could inhibit these orbiviruses from infecting and multiplying in the cells of midges. Thus, exploiting the natural ability of Wolbachia gut bacteria to inhibit orbiviruses in midges may be an effective strategy for preventing initial infection of the insect and the subsequent bite-transmission of these viruses to other livestock.
