Location: Arthropod-borne Animal Diseases Research
2021 Annual Report
Objectives
1. Ascertain the viral ecology of disease and factors mediating the emergence of VSV.
1.A. Characterize epidemiological, biotic and abiotic factors associated with the emergence and transmission of VSV in endemic versus non-endemic settings.
2. Develop intervention strategies to minimize the impact of VSV disease outbreaks.
2.A. Develop means to detect and characterize emergent VSV strains and use these data to generate models that predict future outbreaks.
2.B. Identify vector transmission control strategies based on our understanding of vector-host interactions.
Approach
1. A comprehensive analysis of Vesicular Stomatitis (VS) outbreaks occurring in the U.S. from 2004-2016 will be conducted to determine the relationship between the geographical location of premises reporting VS outbreaks and the spatial and temporal variability in a large suit of ecological variables. Multiple data streams involving disease occurrence and ecological conditions will be obtained from multiple sources and harmonized for integration and analysis. These data sources include; a) outbreak occurrence data inclusive of geo-location, host species, number of animals affected and onset date, b) ecological data analysis c) biotic and abiotic variables inclusive of animal density, hydrological features and streams, elevation and surface water properties, air temperature and precipitation, vegetation ENSO (El Nino Southern Oscilation) data, soil properties and long term trends in environmental variables. Additional data such as water quality monitoring and U.S. census data for human population distribution may be included. These data will be harmonized and univariate and multivariate statistical analysis will be conducted to determine the best set of explanatory variables for temporal and spatial patterns. These analyses will be used to identify ecological variables associated with VS disease occupancy and spread in the western U.S. and to develop predictive models for disease spread.
2. The characterization of VSV transmission in endemic vs non-endemic settings will be conducted in collaboration with Mexico’s SENASICA-EADC laboratory to conduct genomic sequencing and phylogeographic characterization of viral strains collected through VS surveillance activities in Mexico and to identify the ecological and environmental factors associated with the occurrence of VSV in Mexico. A collaboration with USDA-APHIS will established to determine the phylogeopraphic characteristics of VSV strains causing outbreaks in the U.S. This information will be used to create predictive models for VSV occurrence in Northern Mexico and the U.S.
3. To identify intervention strategies agains VSV outbreaks, we will first assess the success of specific lineages to spread over a large geographic range and determine the factors of viral virulence not found in strains remaining within endemic foci. This analysis will be conducted through comparison of the pathogeneis of lineage, identification of phyenotypic differences among strains and mutation of infectious genetic clone derived from virulent strain lineage observed in swine. The endemic and epidemic lineages will be compared to determine transmissibility by insect vectors.
Progress Report
This is the final report for a research project that was conducted by ARS researchers in Manhattan, Kansas and Plum Island, New York. The goal of Objective 1 was to better understand the viral ecology of vesicular stomatitis (VS) virus (VSV) disease and the factors that affect its sporadic emergence into the U.S. from Mexico. Within this Objective, ARS researchers in Manhattan contributed to Sub-Objective 1.1.
Sub-objective 1.1 Research related to biotic factors associated with VSV occurrence in the U.S.: In fiscal year (FY) 18-21, insects were collected on four equine and four bovine farms near Fort Collins, Colorado that are considered likely VSV hotspots from April to September each year. Additional traps were set in the summer of FY 19 due to an ongoing VS outbreak. Collections during FY 20 were interrupted due to COVID. Collected insects were sorted and potential VSV vector species identified. These included biting midges, black flies, and sand flies. Collection efficacy varied month to month and year to year based on weather conditions, especially air temperature and precipitation. Variability was also determined to be due to some user error, some problems with the traps, and general variability in the system. It was determined that black flies were much more prevalent than biting midges early in the season (spring). A general shift towards biting midges occurs in the fall. Despite attempting to collect on VSV positive farms, none of the farms had VSV positive animals during the sampling years as they historically have had during outbreak years.
The goal of Objective 2 was to develop intervention strategies to minimize the impact of VSV disease outbreaks. This included detecting and characterizing emergent VSV strains and using these data to generate models that predict future outbreaks. It also included identifying insect control strategies based on our understanding of vector-host interactions. ARS researchers in Manhattan contributed to Sub-objectives 2.1, 2.2, and 2.3.
Sub-objective 2.1 Research related to virus-vector interactions: In FY 17-18, baseline VSV-Culicoides midge time-course infection studies were conducted with a known archived isolate from a previous VSV outbreak. This provided a baseline with which to compare subsequent infection studies using recombinant VSV lineage isolates constructed by ARS researchers at Plum Island, NY with specific genomic mutations. In 2012, a VSV strain successfully spread northward out of southern Mexico and into the U.S. (epidemic strain) whereas, a genetically related strain from the same area in southern Mexico was not able to spread north (endemic strain). It is not clear why some viral strains escape Mexico and cause outbreaks in the U.S., and some do not. In FY 18-19, the two genetic lineage strains were synthesized by ARS researchers at Plum Island, NY and shipped to ARS researchers in Manhattan, KS for evaluation of their ability to infect and disseminate in the vector, Culicoides midges. In FY 20-21, infection studies were conducted with these two strains to determine whether there were differences in their ability to infect and disseminate in midges. The epidemic strain had much higher infection and dissemination rates compared to the endemic strain. This may help explain why the epidemic strain was able to spread northward from Mexico into the U.S. in 2012. In FY 19-20, a non-conventional VSV-midge transmission mechanism was determined; that of venereal transmission between mating adults. This was the first venereal transmission reported for midges and the first shown for this virus and any of its known vectors. In the summer of 2020, a VSV-Indiana outbreak occurred in Kansas. This was unusually far east for typical VS outbreaks. To determine which suspected vectors may be playing a role in the Kansas outbreak, biting flies were collected near two premises with VSV-infected horses. In FY 20-21, flies were sorted by species and tested for VSV nucleic acid. Two species of biting midges (Culicoides sonorensis and C. stellifer) were positive as well as one species of black fly (Simulium meridionale). This is the first field evidence for C. stellifer as a potential vector for VSV and the first report of S. meridionale black flies as vectors in Kansas. Additionally, some of the positive insects had not yet fed on an animal, indicating these insects were infected by another means, such as through mating (venereal transmission) or they were infected as eggs (transovarial transmission). This is the first field evidence of non-blood feeding infection of any insect species for VSV. During VS outbreaks, biting midges are important in spreading virus during animal quarantines and stop movement measures. In FY 19-20, both Culicoides females and males were shown to be capable of efficient venereal transmission of VSV despite transferring relatively low amounts of virus. In FY 21, to determine if virus propagated in the midge may have increased fitness for subsequent insect infection, we compared infection and dissemination patterns in midges orally infected with insect-derived and mammalian-derived VSV. Our results indicate that midges fed with insect-derived viruses significantly higher infection rates and dissemination rates, suggesting that VSV replication in Culicoides cells results in an increased fitness for efficient midge-to-midge transmission and subsequent replication within the vector. In FY 20-21, we determined the minimum virus dose a midge needs to ingest in a blood meal to become infected, that the ingestion of additional non-infectious blood meals after a VSV infectious meal increases virus infection, and that increased age of midges at the time of infection correlates to increased virus infection rates. Studies are ongoing to determine blood meal temperature preferences and physiological costs in midges. This will inform whether midges are specifically targeting febrile animals for blood meals and whether this preference results in a shorter lifespan.
Sub-objective 2.2 Research related to factors that affect vector abundance and correlate to VSV emergence and spread: From FY 17-20, progress was made toward developing models to predict the emergence and spread of VSV in the U.S. with the goal of determining correlations between biological and physical factors that favor abundant insect vector populations in the vicinity of susceptible livestock. As part of the VSV Grand Challenge project, several new models were developed to identify areas of high risk and to help identify areas for increased insect surveillance. The Animal and Plant Health Inspection Service partnered with ARS to conduct increased surveillance in Western Kansas based on these analyses to determine how accurate the models were and measure which and how many insect vector species were present. Field trapping showed the primary vector species were biting midges and black flies. Population peaks were determined, and Western Kansas farmers were provided information as to how and when to treat their livestock and mitigate insect habitat to minimize risk of VS. In collaboration with the VSV Grand Challenge group, environmental and weather conditions related to vector abundance have been described and several findings relating to VSV vectors (black flies and biting midges) have become clear. Black flies are much more abundant in the spring, whereas biting midges dominate in the fall. Also clear is that based on environmental characteristics that describe habitat characteristics (ex. proximity to flowing water for black flies or dry areas associated with livestock for biting midges) most of Colorado is potential habitat for one or both vectors. Due to highly variable insect collections on the sentinel farms, strong correlations between weather and abundance (predictive models) are not possible at this time. An additional year of sampling may improve the correlations.
Sub-objective 2.3 Research related to integrated pest management plans for small farms based on insect and animal behaviors: From FY 17-20, research was conducted toward identifying strategies to control transmission of VSV based on our understanding of vector-host interactions. The goal was to create a customizable integrated pest management plan for small farms based on insect and animal behaviors. A new biting insect mitigation system was designed based on cattle and horse farmer stakeholder feedback. Management methods are typically very limited on dairies due to residual pesticides and horse owners are typically reluctant to apply chemicals to their animals. Therefore, a novel spatial repellent system was designed that protects an area for the animals during moderate to high insect biting times. The new system is solar powered and uses fans to disperse repellent. Each system is fully autonomous, portable, and can be located or relocated to any area within a pasture. This is an important new tool for controlling biting insect helping farmers protect their livestock and companion animals without overexposing them to chemicals. The original plan was to have insect interventions in place during the third sampling year. However, COVID disruptions with the University cooperator in FY 20 and FY 21 caused the past year insect sampling to be disrupted and discontinuous. Therefore, many weeks of data are missing, and the insect abundance is not well correlated to weather variables. Without this baseline data, no insect management interventions were tested for efficacy. Long-lasting insecticidal materials were also supposed to be delivered (one 40 ft container) but the collaborating University declined to accept the container and the materials were not delivered. Therefore, an alternative intervention may be tested within the next year. For additional progress made on this Project Plan by ARS researchers at Plum Island, please refer to project 8064-32000-059-00D.
Accomplishments
1. Viral dose, feeding behavior, and age affect how well insects can spread vesicular stomatitis virus. Vesicular stomatitis virus (VSV) causes a vesicular disease in cattle, horses, and swine. The virus is transmitted by several insects including biting midges, or no-see-ums. ARS researchers in Manhattan, Kansas, found that midges can become infected after ingesting a very low dose of VSV, 10,000 times lower than the amount of virus typically found in lesions on the animal's skin where they feed. Once infected, when midges blood feed again, that second meal helps the virus multiply. Additionally, the older the midge is at the time of infection, the faster the virus multiplies. This research highlights the importance of how insect feeding behavior and age affect the viruses they transmit to livestock and helps us to estimate the risk of VSV transmission by midges more precisely.
Review Publications
Kading, R.C., Cohnstaedt, L.W., Fall, K., Hamer, G.E. 2020. Emergence of arboviruses in the Americas: the boom and bust of funding, innovation, and capacity. Tropical Medicine and Infectious Disease. 5(2):96. https://doi.org/10.3390/tropicalmed5020096.
Rozo-Lopez, P., Londono, B., Drolet, B.S. 2021. Impacts of infectious dose, feeding behavior, and age of Culicoides sonorensis biting midges on infection dynamics of vesicular stomatitis virus. Pathogens. 10(7):816. https://doi.org/10.3390/pathogens10070816.
Morozov, I., Monath, T.P., Meekins, D.A., Trujillo, J.D., Sunwoo, S., Urbaniak, K., Kim, I., Narayanan, S.K., Indran, S.V., Ma, W., Wilson, W.C., O'Connor, C., Dubey, S., Troth, S.P., Coller, B., Nichols, R., Richt, J. 2021. High dose of vesicular stomatitis virus-vectored Ebola vaccine causes vesicular disease in swine without horizontal transmission. Emerging Microbes & Infections. 10(1):651-663. https://doi.org/10.1080/22221751.2021.1903343.
Drolet, B.S., Reeves, W.K., Bennett, K.E., Pauszek, S.J., Bertram, M.R., Rodriguez, L.L. 2021. Identical viral genetic sequence found in black flies (Simulium bivittatum) and the equine index case of the 2006 U.S. vesicular stomatitis outbreak. Pathogens. 10(8):929-938. https://doi.org/10.3390/pathogens10080929.
