Colostral transmission of bluetongue virus nucleic acid among newborn dairy calves in California

Vaccination as a Strategy to Prevent Bluetongue Virus Vertical Transmission

Bluetongue virus (BTV) produces an economically important disease in ruminants of compulsory notification to the OIE. BTV is typically transmitted by the bite of Culicoides spp., however, some BTV strains can be transmitted vertically, and this is associated with fetus malformations and abortions. The viral factors associated with the virus potency to cross the placental barrier are not well defined. The potency of vertical transmission is retained and sometimes even increased in live attenuated BTV vaccine strains. Because BTV possesses a segmented genome, the possibility of reassortment of vaccination strains with wild-type virus could even favor the transmission of this phenotype. In the present review, we will describe the non-vector-based BTV infection routes and discuss the experimental vaccination strategies that offer advantages over this drawback of some live attenuated BTV vaccines.

Bluetongue Virus Infections in Cattle Herds of Manabí Province of Ecuador

Bluetongue (BT) is a viral disease transmitted by Culicoides (Diptera: Ceratopogonidae) to domestic and wild ruminants. Infections in cattle are mainly subclinical, but severe necrotic and hemorrhagic illness and death may occur depending on the strain of the virus and other factors; cattle act as a reservoir for the virus. Although the Ecuadorian coast has climatic conditions that favor the presence of the vector, there are few serologic or virologic BTV studies available. Manabí is a coastal province in which livestock farming is mostly implemented in the northern part. We conducted two studies to assess, for the first time, the presence of active BTV infections in Manabí province. We collected 430 serum samples from 38 randomly selected farms between March and July 2019 to perform BTV competitive ELISA. In addition, six seropositive farms were selected to place eight sentinel BTV-naive calves. All these calves were blood sampled and the presence of BTV RNA and antibodies was tested for by RT-PCR and competitive ELISA, respectively, once a week for 6-8 weeks until seroconversion was evidenced. A high individual seroprevalence (99%) was obtained, and all investigated farms had BTV seropositive animals. All sentinel calves became BTV viremic and seroconverted. The first viremia appeared after 2-5 weeks from arrival at the farm; they seroconverted 1-3 weeks later. We demonstrate for the first time that there is a high level of BTV circulation north of Manabí, with active infections on these farms. Integrated control strategies such as hygienic measures on farms to reduce midge populations would be advisable for the owners as mitigation measures.

Bluetongue and Epizootic Hemorrhagic Disease in the United States of America at the Wildlife-Livestock Interface

Bluetongue (BT) and epizootic hemorrhagic disease (EHD) cases have increased worldwide, causing significant economic loss to ruminant livestock production and detrimental effects to susceptible wildlife populations. In recent decades, hemorrhagic disease cases have been reported over expanding geographic areas in the United States. Effective BT and EHD prevention and control strategies for livestock and monitoring of these diseases in wildlife populations depend on an accurate understanding of the distribution of BT and EHD viruses in domestic and wild ruminants and their vectors, the Culicoides biting midges that transmit them. However, national maps showing the distribution of BT and EHD viruses and the presence of Culicoides vectors are incomplete or not available at all. Thus, efforts to accurately describe the potential risk of these viruses on ruminant populations are obstructed by the lack of systematic and routine surveillance of their hosts and vectors. In this review, we: (1) outline animal health impacts of BT and EHD in the USA; (2) describe current knowledge of the distribution and abundance of BT and EHD and their vectors in the USA; and (3) highlight the importance of disease (BT and EHD) and vector surveillance for ruminant populations.

Global emergence and evolutionary dynamics of bluetongue virus

Bluetongue virus (BTV) epidemics are responsible for worldwide economic losses of up to US$ 3 billion. Understanding the global evolutionary epidemiology of BTV is critical in designing intervention programs. Here we employed phylodynamic models to quantify the evolutionary characteristics, spatiotemporal origins, and multi-host transmission dynamics of BTV across the globe. We inferred that goats are the ancestral hosts for BTV but are less likely to be important for cross-species transmission, sheep and cattle continue to be important for the transmission and maintenance of infection between other species. Our models pointed to China and India, countries with the highest population of goats, as the likely ancestral country for BTV emergence and dispersal worldwide over 1000 years ago. However, the increased diversification and dispersal of BTV coincided with the initiation of transcontinental livestock trade after the 1850s. Our analysis uncovered important epidemiological aspects of BTV that may guide future molecular surveillance of BTV.

Ecological Dynamics Impacting Bluetongue Virus Transmission in North America

Bluetongue virus (BTV) is an arbovirus transmitted to domestic and wild ruminants by certain species of Culicoides midges. The disease resulting from infection with BTV is economically important and can influence international trade and movement of livestock, the economics of livestock production, and animal welfare. Recent changes in the epidemiology of Culicoides-transmitted viruses, notably the emergence of exotic BTV genotypes in Europe, have demonstrated the devastating economic consequences of BTV epizootics and the complex nature of transmission across host-vector landscapes. Incursions of novel BTV serotypes into historically enzootic countries or regions, including the southeastern United States (US), Israel, Australia, and South America, have also occurred, suggesting diverse pathways for the transmission of these viruses. The abundance of BTV strains and multiple reassortant viruses circulating in Europe and the US in recent years demonstrates considerable genetic diversity of BTV strains and implies a history of reassortment events within the respective regions. While a great deal of emphasis is rightly placed on understanding the epidemiology and emergence of BTV beyond its natural ecosystem, the ecological contexts in which BTV maintains an enzootic cycle may also be of great significance. This review focuses on describing our current knowledge of ecological factors driving BTV transmission in North America. Information presented in this review can help inform future studies that may elucidate factors that are relevant to longstanding and emerging challenges associated with prevention of this disease.

Prevalence and risk factors for bluetongue in the State of São Paulo, Brazil

Bluetongue (BT), caused by Bluetongue virus (BTV), is a disease that affects ruminants such as cattle, sheep, goats and deer. BTV is transmitted by female midges of the genus Culicoides. In Brazil, information on the prevalence of BTV in cattle is limited, so the objective of this work was to identify BTV serotypes in cattle. The State of São Paulo was divided into seven cattle‐producing regions, and in each of them, 300 cattle farms were randomly selected. One animal from each farm (out of a total of 1,598 farms) was selected and its sera tested by virus neutralization technique against BTV serotypes (1–24 and 26) for determining antibody titre. Moreover, for each sampled farm, an epidemiological questionnaire was submitted to verify the type of cattle production and the zootechnical and sanitary practices carried out, which could be associated with a higher risk of BTV infection. In this study, antibodies (percentage, [95% confidence interval]) were identified against 11 serotypes: BTV‐1 (22.15%, [15.72–27.92]), BTV‐2 (31.03%, [26.65–37.98]), BTV‐3 (18.96%, [12.42–24.90]), BTV‐4 (24.90% [19.41–29.12]), BTV‐9 (6.82%, [1.45–11.72]), BTV‐12 (7.50%, [2.82–12.51]), BTV‐17 (23.90%, [17.35–29.35]), BTV‐19 (10.20%, [4.62–5.56]), BTV‐21 (30.66%, [25.00–36.00]), BTV‐22 (12.14%, [5.91–18.55]), BTV‐26 (57.00%, [51.41–63.59]). In this study, for the first time in Brazil serological evidence of the presence of serotypes BTV‐2, BTV‐9, BTV‐21 and BTV‐26 is reported. The variable ‘new cattle entering herd’ was considered a risk factor for the occurrence of infection (OR = 2.183, 95% CI = 1.6–2.9).

Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) No 2016/429): bluetongue

A specific concept of strain was developed in order to classify the BTV serotypes ever reported in Europe based on their properties of animal health impact: the genotype, morbidity, mortality, speed of spread, period and geographical area of occurrence were considered as classification parameters. According to this methodology the strain groups identified were (i) the BTV strains belonging to serotypes BTV-1-24, (ii) some strains of serotypes BTV-16 and (iii) small ruminant-adapted strains belonging to serotypes BTV-25, -27, -30. Those strain groups were assessed according to the criteria of the Animal Health Law (AHL), in particular criteria of Article 7, Article 5 on the eligibility of bluetongue to be listed, Article 9 for the categorisation according to disease prevention and control rules as in Annex IV and Article 8 on the list of animal species related to bluetongue. The assessment has been performed following a methodology composed of information collection, expert judgement at individual and collective level. The output is composed of the categorical answer, and for the questions where no consensus was reached, the different supporting views are reported. The strain group BTV (1-24) can be considered eligible to be listed for Union intervention as laid down in Article 5(3) of the AHL, while the strain group BTV-25-30 and BTV-16 cannot. The strain group BTV-1-24 meets the criteria as in Sections 2 and 5 of Annex IV of the AHL, for the application of the disease prevention and control rules referred to in points (b) and (e) of Article 9(1) of the AHL. The animal species that can be considered to be listed for BTV-1-24 according to Article 8(3) are several species of Bovidae, Cervidae and Camelidae as susceptible species; domestic cattle, sheep and red deer as reservoir hosts, midges insect of genus Culicoides spp. as vector species.

A Deterministic Model to Quantify Risk and Guide Mitigation Strategies to Reduce Bluetongue Virus Transmission in California Dairy Cattle

The global distribution of bluetongue virus (BTV) has been changing recently, perhaps as a result of climate change. To evaluate the risk of BTV infection and transmission in a BTV-endemic region of California, sentinel dairy cows were evaluated for BTV infection, and populations of Culicoides vectors were collected at different sites using carbon dioxide. A deterministic model was developed to quantify risk and guide future mitigation strategies to reduce BTV infection in California dairy cattle. The greatest risk of BTV transmission was predicted within the warm Central Valley of California that contains the highest density of dairy cattle in the United States. Temperature and parameters associated with Culicoides vectors (transmission probabilities, carrying capacity, and survivorship) had the greatest effect on BTV's basic reproduction number, R0. Based on these analyses, optimal control strategies for reducing BTV infection risk in dairy cattle will be highly reliant upon early efforts to reduce vector abundance during the months prior to peak transmission.

Seasonal and interseasonal dynamics of bluetongue virus infection of dairy cattle and Culicoides sonorensis midges in northern California--implications for virus overwintering in temperate zones

Bluetongue virus (BTV) is the cause of an economically important arboviral disease of domestic and wild ruminants. The occurrence of BTV infection of livestock is distinctly seasonal in temperate regions of the world, thus we determined the dynamics of BTV infection (using BTV-specific real time reverse transcriptase polymerase chain reaction) among sentinel cattle and vector Culicoides sonorensis (C. sonorensis) midges on a dairy farm in northern California throughout both the seasonal and interseasonal (overwintering) periods of BTV activity from August 2012 until March 2014. The data confirmed widespread infection of both sentinel cattle and vector midges during the August-November period of seasonal BTV transmission, however BTV infection of parous female midges captured in traps set during daylight hours also was detected in February of both 2013 and 2014, during the interseasonal period. The finding of BTV-infected vector midges during mid-winter suggests that BTV may overwinter in northern California by infection of long-lived female C. sonorensis midges that were infected during the prior seasonal period of virus transmission, and reemerged sporadically during the overwintering period; however the data do not definitively preclude other potential mechanisms of BTV overwintering that are also discussed.

Evidence for transmission of bluetongue virus serotype 26 through direct contact

The aim of this study was to assess the mechanisms of transmission of bluetongue virus serotype 26 (BTV-26) in goats. A previous study, which investigated the pathogenicity and infection kinetics of BTV-26 in goats, unexpectedly revealed that one control goat may have been infected through a direct contact transmission route. To investigate the transmission mechanisms of BTV-26 in more detail an experimental infection study was carried out in which three goats were infected with BTV-26, three goats were kept uninfected, but were housed in direct contact with the infected goats, and an additional four goats were kept in indirect contact separated from infected goats by metal gates. This barrier allowed the goats to have occasional face-to-face contact in the same airspace, but feeding, watering, sampling and environmental cleaning was carried out separately. The three experimentally infected goats did not show clinical signs of BTV, however high levels of viral RNA were detected and virus was isolated from their blood. At 21 dpi viral RNA was detected in, and virus was isolated from the blood of the three direct contact goats, which also seroconverted. The four indirect barrier contact goats remained uninfected throughout the duration of the experiment. In order to assess replication in a laboratory model species of Culicoides biting midge, more than 300 Culicoides sonorensis were fed a BTV-26 spiked blood meal and incubated for 7 days. The dissemination of BTV-26 in individual C. sonorensis was inferred from the quantity of virus RNA and indicated that none of the insects processed at day 7 possessed transmissible infections. This study shows that BTV-26 is easily transmitted through direct contact transmission between goats, and the strain does not seem to replicate in C. sonorensis midges using standard incubation conditions.

Seasonal variation and impact of waste-water lagoons as larval habitat on the population dynamics of Culicoides sonorensis (Diptera:Ceratpogonidae) at two dairy farms in northern California

The Sacramento (northern Central) Valley of California (CA) has a hot Mediterranean climate and a diverse ecological landscape that is impacted extensively by human activities, which include the intensive farming of crops and livestock. Waste-water ponds, marshes, and irrigated fields associated with these agricultural activities provide abundant larval habitats for C. sonorensis midges, in addition to those sites that exist in the natural environment. Within this region, C. sonorensis is an important vector of bluetongue (BTV) and related viruses that adversely affect the international trade and movement of livestock, the economics of livestock production, and animal welfare. To characterize the seasonal dynamics of immature and adult C. sonorensis populations, abundance was monitored intensively on two dairy farms in the Sacramento Valley from August 2012- to July 2013. Adults were sampled every two weeks for 52 weeks by trapping (CDC style traps without light and baited with dry-ice) along N-S and E-W transects on each farm. One farm had large operational waste-water lagoons, whereas the lagoon on the other farm was drained and remained dry during the study. Spring emergence and seasonal abundance of adult C. sonorensis on both farms coincided with rising vernal temperature. Paradoxically, the abundance of midges on the farm without a functioning waste-water lagoon was increased as compared to abundance on the farm with a waste-water lagoon system, indicating that this infrastructure may not serve as the sole, or even the primary larval habitat. Adult midges disappeared from both farms from late November until May; however, low numbers of parous female midges were detected in traps set during daylight in the inter-seasonal winter period. This latter finding is especially critical as it provides a potential mechanism for the "overwintering" of BTV in temperate regions such as northern CA. Precise documentation of temporal changes in the annual abundance and dispersal of Culicoides midges is essential for the creation of models to predict BTV infection of livestock and to develop sound abatement strategies.

The influence of the wind in the Schmallenberg virus outbreak in Europe

A model previously developed for the wind-borne spread by midges of bluetongue virus in NW Europe in 2006 is here modified and applied to the spread of Schmallenberg virus in 2011. The model estimates that pregnant animals were infected 113 days before producing malformed young, the commonest symptom of reported infection, and explains the spatial and temporal pattern of infection in 70% of the 3,487 affected farms, most of which were infected by midges arriving through downwind movement (62% of explained infections), or a mixture of downwind and random movements (38% of explained infections), during the period of day (1600–2100 h, i.e. dusk) when these insects are known to be most active. The main difference with Bluetongue is the higher rate of spread of SBV, which has important implications for disease control.

Transplacental transmission of field and rescued strains of BTV-2 and BTV-8 in experimentally infected sheep

Transplacental transmission of bluetongue virus has been shown previously for the North European strain of serotype 8 (BTV-8) and for tissue culture or chicken egg-adapted vaccine strains but not for field strains of other serotypes. In this study, pregnant ewes (6 per group) were inoculated with either field or rescued strains of BTV-2 and BTV-8 in order to determine the ability of these viruses to cross the placental barrier. The field BTV-2 and BTV-8 strains was passaged once in Culicoides KC cells and once in mammalian cells. All virus inoculated sheep became infected and seroconverted against the different BTV strains used in this study. BTV RNA was detectable in the blood of all but two ewes for over 28 days but infectious virus could only be detected in the blood for a much shorter period. Interestingly, transplacental transmission of BTV-2 (both field and rescued strains) was demonstrated at high efficiency (6 out of 13 lambs born to BTV-2 infected ewes) while only 1 lamb of 12 born to BTV-8 infected ewes showed evidence of in utero infection. In addition, evidence for horizontal transmission of BTV-2 between ewes was observed. As expected, the parental BTV-2 and BTV-8 viruses and the viruses rescued by reverse genetics showed very similar properties to each other. This study showed, for the first time, that transplacental transmission of BTV-2, which had been minimally passaged in cell culture, can occur; hence such transmission might be more frequent than previously thought.