Analysis of equid herpesvirus 1 strain variation reveals a point mutation of the DNA polymerase strongly associated with neuropathogenic versus nonneuropathogenic disease outbreaks

Divergent strains of EHV-1 in Swedish outbreaks during 2012 to 2021

Equid alphaherpesvirus 1 (EHV-1) is a ubiquitous and significant viral pathogen in horses worldwide, causing a range of conditions, including fever, respiratory disease, abortion in pregnant mares and the severe neurological disease called equine herpes myeloencephalopathy (EHM). Despite that EHV-1 is a notifiable animal disease in Sweden, there is limited knowledge about the circulating strains. This study aimed to analyze the genetic diversity of EHV-1 strains in equine samples from different Swedish outbreaks by partial genome sequencing. Genotyping based on three selected open reading frames ORF11, ORF30, and ORF34 in the viral genome was conducted for 55 outbreaks of EHV-1 spanning from the years 2012 to 2021. The analysis revealed 14 different genovariants, with one prominent genovariant identified in 49% of the outbreaks. Additionally, the study identified seven mutations not previously described. Three new mutations were demonstrated in ORF11, all synonymous, and four new mutations in ORF34, two synonymous, and two non-synonymous. Notably, different EHV-1 genovariants were found in five out of six studied EHM outbreaks, but clonal spreading was shown within the outbreaks. Moreover, the study demonstrated that healthy (recovered) horses that returned from an EHM outbreak at an international meeting in Valencia, Spain (2021), were positive for the virus clone responsible for the severe disease outbreak despite several weeks of quarantine. These findings shed light on the genetic diversity and transmission dynamics of the virus and significantly contribute to better understanding of the epidemiology of EHV-1 in Sweden and globally.

First identification and isolation of equine herpesvirus type 1 in aborted fetal lung tissues of donkeys

BACKGROUND

Equine herpesvirus type 1 (EHV-1) is commonly associated with horse abortion. Currently, there are no reported cases of abortion resulting from EHV-1 infection in donkeys.

RESULTS

This was the first survey-based study of Chinese donkeys. The presence of EHV-1 was identified by PCR. This survey was conducted in Chabuchar County, North Xinjiang, China, in 2020. A donkey EHV-1 strain (Chabuchar/2020) was successfully isolated in MDBK cells. Seventy-two of 100 donkey sera were able to neutralize the isolated EHV-1. Moreover, the ORF33 sequence of the donkey-origin EHV-1 Chabuchar/2020 strain showed high levels of similarity in both its nucleotide (99.7‒100%) and amino acid (99.5‒100%) sequences, with those of horse EHV-1 strains. EHV-1 Chabuchar/2020 showed significant consistency and was classified within cluster 1 of horse EHV-1 strains. Further, analysis of the expected ORF30 nucleotide sequence revealed that donkey EHV-1 strains contained guanine at position 2254, resulting in a change to aspartic acid at position 752 of the viral DNA polymerase. Therefore, these strains were classified as horse neuropathogenic strains. Lastly, a phylogenetic tree was constructed using the partial ORF68 nucleotide sequences, showing that the identified donkey EHV-1 strain and the EHV-1 strain found in aborted Yili horses in China comprised a novel independent VIII group.

CONCLUSION

This study showed the first isolation and identification of EHV-1 as an etiological agent of abortions in donkeys. Further analysis of the ORF33, ORF30, and ORF68 sequences indicated that the donkey EHV-1 contained the neuropathogenic genotype of strains in the VIII group. It is thus important to be aware of EHV-1 infection in the donkey population, even though the virus has only been identified in donkey abortions in China.

Relationship between equine herpesvirus-1 viremia and abortion or equine herpesvirus myeloencephalopathy in domesticated horses: A systematic review

BACKGROUND

Equine herpes virus type 1 (EHV-1) infection in horses is associated with upper respiratory disease, neurological disease, abortions, and neonatal death.

OBJECTIVE

To determine if there is an association between the level and duration of EHV-1 viremia and either abortion or equine herpesvirus myeloencephalopathy (EHM) in domesticated horses?

METHODS

A systematic review was performed searching numerous databases to identify peer reviewed reports that evaluated viremia and EHM, or viremia and abortion published before January 19, 2021. Randomized controlled trials and observational studies were assessed for risk of bias or publication quality.

RESULTS

A total of 189 unique studies were identified, of which 34 met the inclusion criteria. Thirty studies evaluated viremia and neurologic outcomes including 4 observational studies. Eight experimental studies examined viremia and abortion, which used the Ab4 and OH03 virus strains or recombinant Ab4 derivatives. Incidence rates for both EHM and abortion in experimental studies varied among the studies as did the level of evidence. Viremia was generally detectable before the onset of either EHM or abortion. Risk of bias was generally low to moderate, sample sizes were small, and multiple studies reported negative outcome data.

CONCLUSIONS AND CLINICAL IMPORTANCE

The results of this study support that viremia is regularly present before EHM or abortion occurs. However, no inferences could be made about the relationship between the occurrence of either neurological signs or abortion and the magnitude or duration of viremia.

Detection of equine herpesvirus-1 (EHV-1) in urine samples during outbreaks of equine herpesvirus myeloencephalopathy

BACKGROUND

Real-time PCR is the diagnostic technique of choice for the diagnosis and control of equine herpesvirus-1 (EHV-1) in an outbreak setting. The presence of EHV-1 in nasal swabs (NS), whole blood, brain and spinal cord samples has been extensively described; however, there are no reports on the excretion of EHV-1 in urine, its DNA detection patterns, and the role of urine in viral spread during an outbreak.

OBJECTIVES

To determine the presence of EHV-1 DNA in urine during natural infection and to compare the DNA detection patterns of EHV-1 in urine, buffy coat (BC) and NS.

STUDY DESIGN

Descriptive study of natural infection.

METHODS

Urine and whole blood/NS samples were collected at different time points during the hospitalisation of 21 horses involved in two EHV-1 myeloencephalopathy outbreaks in 2021 and 2023 in Spain. Quantitative real-time PCR was performed to compare the viral DNA load between BC-urine samples in 2021 and NS-urine samples in 2023. Sex, age, breed, presence of neurological signs, EHV-1 vaccination status and treatment data were recorded for all horses.

RESULTS

A total of 18 hospitalised horses during the 2021 and 2023 outbreaks were positive for EHV-1, and viral DNA was detected in urine samples from a total of 11 horses in both outbreaks. Compared with BC samples, DNA presence was detected in urine samples for longer duration and with slightly higher concentration; however, compared with NS, detection of EHV-1 in urine was similar in duration with lower DNA concentrations.

MAIN LIMITATIONS

Limited sample size, different sampling times and protocols (BC vs. NS) in two natural infection outbreak settings.

CONCLUSIONS

EHV-1 was detected in the urine from naturally infected horses. Urine should be considered as complimentary to blood and NS in diagnosis of EHV-1 infection.

Updated ACVIM consensus statement on equine herpesvirus-1

Equine herpesvirus-1 (EHV-1) is a highly prevalent and frequently pathogenic infection of equids. The most serious clinical consequences of infection are abortion and equine herpesvirus myeloencephalopathy (EHM). The previous consensus statement was published in 2009 and considered pathogenesis, strain variation, epidemiology, diagnostic testing, vaccination, outbreak prevention and control, and treatment. A recent survey of American College of Veterinary Internal Medicine large animal diplomates identified the need for a revision to this original consensus statement. This updated consensus statement is underpinned by 4 systematic reviews that addressed key questions concerning vaccination, pharmaceutical treatment, pathogenesis, and diagnostic testing. Evidence for successful vaccination against, or effective treatment of EHV-1 infection was limited, and improvements in experimental design and reporting of results are needed in future studies of this important disease. This consensus statement also updates the topics considered previously in 2009.

Modulation of Equid Herpesvirus-1 Replication Dynamics In Vitro Using CRISPR/Cas9-Assisted Genome Editing

(1) Background: equid alphaherpesvirus-1 (EHV-1) is a highly contagious viral pathogen prevalent in most horse populations worldwide. Genome-editing technologies such as CRISPR/Cas9 have become powerful tools for precise RNA-guided genome modifications; (2) Methods: we designed single guide RNAs (sgRNA) to target three essential (ORF30, ORF31, and ORF7) and one non-essential (ORF74) EHV-1 genes and determine their effect on viral replication dynamics in vitro; (3) Results: we demonstrated that sgRNAs targeting essential lytic genes reduced EHV-1 replication, whereas those targeting ORF74 had a negligible effect. The sgRNAs targeting ORF30 showed the strongest effect on the suppression of EHV-1 replication, with a reduction in viral genomic copy numbers and infectious progeny virus output. Next-generation sequencing identified variants with deletions in the specific cleavage site of selective sgRNAs. Moreover, we evaluated the combination between different sgRNAs and found that the dual combination of sgRNAs targeting ORF30 and ORF7 significantly suppressed viral replication to lower levels compared to the use of a single sgRNA, suggesting a synergic effect; (4) Conclusion: data demonstrate that sgRNA-guided CRISPR/Cas9 can be used to inhibit EHV-1 replication in vitro, indicating that this programmable technique can be used to develop a novel, safe, and efficacious therapeutic and prophylactic approach against EHV-1.

Studying longitudinal neutralising antibody levels against Equid herpesvirus 1 in experimentally infected horses using a novel pseudotype based assay

Infection with equid herpesvirus 1 (EHV-1), a DNA virus of the Herpesviridae family represents a significant welfare issue in horses and a great impact on the equine industry. During EHV-1 infection, entry of the virus into different cell types is complex due to the presence of twelve glycoproteins (GPs) on the viral envelope. To investigate virus entry mechanisms, specific combinations of GPs were pseudotyped onto lentiviral vectors. Pseudotyped virus (PV) particles bearing gB, gD, gH and gL were able to transduce several target cell lines (HEK293T/17, RK13, CHO-K1, FHK-Tcl3, MDCK I & II), demonstrating that these four EHV-1 glycoproteins are both essential and sufficient for cell entry. The successful generation of an EHV-1 PV permitted development of a PV neutralisation assay (PVNA). The efficacy of the PVNA was tested by measuring the level of neutralising serum antibodies from EHV-1 experimentally infected horses (n = 52) sampled in a longitudinal manner. The same sera were assessed using a conventional EHV-1 virus neutralisation (VN) assay, exhibiting a strong correlation (r = 0.82) between the two assays. Furthermore, PVs routinely require -80 °C for long term storage and a dry ice cold-chain during transport, which can impede dissemination and utilisation in other stakeholder laboratories. Consequently, lyophilisation of EHV-1 PVs was conducted to address this issue. PVs were lyophilised and pellets either reconstituted immediately or stored under various temperature conditions for different time periods. The recovery and functionality of these lyophilised PVs was compared with standard frozen aliquots in titration and neutralisation tests. Results indicated that lyophilisation could be used to stably preserve such complex herpesvirus pseudotypes, even after weeks of storage at room temperature, and that reconstituted EHV-1 PVs could be successfully employed in antibody neutralisation tests.

Phylogenomic assessment of 23 equid alphaherpesvirus 1 isolates obtained from USA-based equids

BACKGROUND

Equid alphaherpesvirus 1 (EHV-1) is a global viral pathogen of domestic equids which causes reproductive, respiratory and neurological disease. Few isolates acquired from naturally infected USA-based hosts have been fully sequenced and analyzed to date. An ORF 30 (DNA polymerase) variant (A2254G) has previously been associated with neurological disease in host animals. The purpose of this study was to perform phylogenomic analysis of EHV-1 isolates acquired from USA-based hosts and compare these isolates to previously sequenced global isolates.

METHODS

EHV-1 was isolated from 23 naturally infected USA-based equids (6 different states, 15 disease outbreaks) with reproductive (22/23) or neurological disease (1/23). Following virus isolation, EHV-1 DNA was extracted for sequencing using Illumina MiSeq. Following reference-based assembly, whole viral genomes were annotated and assessed. Previously sequenced EHV-1 isolates (n = 114) obtained from global host equids were included in phylogenomic analyses.

RESULTS

The overall average genomic distance was 0.0828% (SE 0.004%) for the 23 newly sequenced USA isolates and 0.0705% (SE 0.003%) when all 137 isolates were included. Clade structure was predominantly based on geographic origin. Numerous nucleotide substitutions (mean [range], 179 [114-297] synonymous and 81 [38-120] non-synonymous substitutions per isolate) were identified throughout the genome of the newly sequenced USA isolates. The previously described ORF 30 A2254G substitution (associated with neurological disease) was found in only one isolate obtained from a host with non-neurological clinical signs (reproductive disease), six additional, unique, non-synonymous ORF 30 substitutions were detected in 22/23 USA isolates. Evidence of recombination was present in most (22/23) of the newly sequenced USA isolates.

CONCLUSIONS

Overall, the genomes of the 23 newly sequenced EHV-1 isolates obtained from USA-based hosts were broadly similar to global isolates. The previously described ORF 30 A2254G neurological substitution was infrequently detected in the newly sequenced USA isolates, most of which were obtained from host animals with reproductive disease. Recombination was likely to be partially responsible for genomic diversity in the newly sequenced USA isolates.

Equine Herpesvirus-1 Outbreak During a Show-Jumping Competition: A Clinical and Epidemiological Study

A total of 752 horses were involved in the CES Valencia Spring Tour 2021. Due to an equine herpesvirus-1 (EHV-1) outbreak, the competition was cancelled and the site was locked down. The objective of this study was to describe epidemiological, clinical, diagnostic, and outcome data of the 160 horses remaining in Valencia. Clinical and quantitative polymerase chain reaction (qPCR) data were analysed for 60 horses in a retrospective case-control observational study. The risk of developing clinical manifestations was explored using a logistic regression approach. EHV-1 was detected by qPCR, genotyped as A2254 (ORF30) and isolated on cell culture. From the 60 horses, 50 (83.3%) showed fever, 30 horses (50%) showed no further signs and 20 (40%) showed neurological signs, with eight horses (16%) hospitalised, of which two died (3%). Stallions and geldings were six times more likely to develop EHV-1 infection compared to mares. Horses older than 9 years, or housed in the middle of the tent were more likely to develop EHV-1 myeloencephalopathy (EHM). These data show that for EHV-1 infection, the risk factor was male sex. For EHM the risk factors were age > 9-year old and location in the middle of the tent. These data highlight the crucial role of stable design, position, and ventilation in EHV-outbreaks. It also showed that PCR testing of the horses was important to manage the quarantine.

Anti-inflammatory compounds reduce equine herpesvirus type 1 replication and cell-to-cell spread

Equine herpesvirus type 1 (EHV-1) is a highly transmissible pathogen that leads to a variety of clinical disease outcomes in infected horses. A major sequela that can occur after an EHV-1 infection is a neurological disease termed equine herpesvirus myeloencephalopathy (EHM). Clinical manifestations of EHM include fever, ataxia, incontinence, and partial to full paralysis, which may ultimately lead to the euthanization of the infected horse. To develop an effective treatment strategy for EHM, it is critical that the specific virus-host interactions that lead to EHM be investigated so that safe and effective therapeutic interventions can be developed and delivered. In this study, we examined the ability of four non-steroidal anti-inflammatory drugs (NSAIDs), a steroidal anti-inflammatory drug (dexamethasone), a Rho-kinase (ROCK) inhibitor, and a JAK/STAT inhibitor (AG490) to reduce EHV-1 virus yields and cell-to-cell spread. We show that the NSAID, flunixin meglumine (FM), and the JAK/STAT inhibitor, AG490, significantly reduced virus yields in endothelial and epithelial cell lines, and this inhibition was similar for two neurologic and two non-neurologic EHV-1 strains. In addition to reducing virus yields, AG490 and FM also significantly reduced the ability of EHV-1 to spread laterally from cell to cell.

Investigation of the EHV-1 Genotype (N752, D752, and H752) in Swabs Collected From Equids With Respiratory and Neurological Disease and Abortion From the United States (2019-2022)

Contemporary data on equine herpesvirus-1 (EHV-1) genotype (non-neuropathogenic or N₇₅₂, neuropathogenic or D₇₅₂ and new variant or H₇₅₂) in clinically diseased equids is important in order to determine the frequency of these genotypes and their association with disease expression. A total of 297 EHV-1 qPCR-positive swabs collected from 2019 to 2022 from horses with respiratory disease (EHV-1), neurological disease (equine herpesvirus-1 myeloencephalopathy [EHM]) and abortion were tested for the three different EHV-1 genotypes (N₇₅₂, D₇₅₂ and H₇₅₂) using qPCR allelic discrimination assays. All submissions originated from the United States and included 257 EHV-1 cases, 35 EHM cases and 5 cases of abortion. EHV-1 qPCR-positive cases were predominantly seen during winter and spring. N₇₅₂ was the predominant genotype detected in EHV-1 cases (87.5%), EHM cases (74.3%) and abortions (80%). D₇₅₂ was detected less frequently in EHV-1 cases (9.3%) and EHM cases (25.7%), while H₇₅₂ was only detected in EHV-1 cases (3.1%). While the N₇₅₂ genotype has remained the predominant genotype affecting horses with respiratory disease and abortion, it has also become a leading genotype in cases of EHM, when compared to historical data. The new H₇₅₂ genotype, first reported in the United States in 2021, has remained confined to a cluster of geographically and temporally related outbreaks and the data showed no emerging spread of H₇₅₂ since it was first reported. While the monitoring of EHV-1 genotypes is important from a diagnostic and epidemiological standpoint, it may also help establish medical interventions and preventive protocols to reduce the risk of severe complications associated with EHV-1 infection.

Eosinophilic Inflammation and Equine Herpesvirus-1 Associated With Haemorrhagic Cystitis in a Horse. Case Report

Equine idiopathic haemorrhagic cystitis (EIHC) is a recently described form of aseptic cystitis in horses in which there is no discernible underlying cause. This case report describes a 9-year-old Thoroughbred gelding that presented with stranguria, pollakiuria, and haematuria. Cystoscopy revealed ulceration and haemorrhage of the bladder mucosa, diffuse mural hyperaemia and marked urine sedimentation. Histopathological evaluation of the bladder revealed chronic active ulcerative neutrophilic, lymphoplasmacytic, and eosinophilic cystitis. There was no bacterial or fungal growth upon culture but polymerase chain reaction (PCR) testing and sequencing for equine herpesvirus-1 (EHV-1) on bladder mucosa was positive. Conservative therapy with broad spectrum antimicrobials and non-steroidal anti-inflammatory therapy yielded complete resolution of clinical signs with significant improvement of macroscopic lesions in 14 days. Although a positive EHV-1 PCR suggests a viral cause, the horse's clinical signs, histology and recovery rate are more consistent with equine idiopathic haemorrhagic cystitis (EIHC). Neutrophilic and lymphoplasmacytic inflammation is a known feature of EIHC but eosinophilic infiltrates have not been previously described. The significance of the eosinophilic involvement is not certain; however, their presence has been associated with fungal, viral, parasitic, and immune-mediated aetiologies in other body systems. This is the first report of a horse with possible EIHC in Australia, as well as the first case with eosinophilic infiltrates and testing positive for EHV-1.

Epidemiological Aspects of Equid Herpesvirus-Associated Myeloencephalopathy (EHM) Outbreaks

Equid Herpesvirus Myeloencephalopathy (EHM) is a multifactorial disease following an EHV-1 infection in Equidae. We investigated a total of 589 horses on 13 premises in Europe in search of risk factors for the development of EHM. We found that fever (p < 0.001), increasing age (p = 0.032), and female sex (p = 0.042) were risk factors for EHM in a logistic mixed model. Some breeds had a decreased risk to develop EHM compared to others (Shetland and Welsh ponies; p = 0.017; p = 0.031), and fewer EHV-1-vaccinated horses were affected by EHM compared to unvaccinated horses (p = 0.02). Data evaluation was complex due to high variability between outbreaks with regards to construction and environment; viral characteristics and the virus's transmissibility were affected by operational management. This study confirms earlier suspected host-specific risk factors, and our data support the benefit of high vaccine coverage at high-traffic boarding facilities.

Infection with a Recently Discovered Gammaherpesvirus Variant in European Badgers, Meles meles, is Associated with Higher Relative Viral Loads in Blood

Herpesviruses are ubiquitous pathogens infecting most animals. Although host immunity continually coevolves to combat virulence, viral variants with enhanced transmissibility or virulence occasionally emerge, resulting in disease burdens in host populations. Mustelid gammaherpesvirus 1 (MusGHV-1) is the only herpesvirus species identified thus far in European badgers, Meles meles. No MusGHV-1 associated pathomorbidity has been reported, but reactivation of MusGHV-1 in genital tracts is linked to impaired female reproductive success. An analysis of a short sequence from the highly conserved DNA polymerase (DNApol) gene previously identified two variants in a single host population. Here we compared genetic variance in blood samples from 66 known individuals of this same free-ranging badger population using a partial sequence comprising 2874 nucleotides of the DNApol gene, among which we identified 15 nucleotide differences resulting in 5 amino acid differences. Prevalence was 86% (59/66) for the common and 17% (11/66) for the novel variant, with 6% (4/66) of badgers presenting with coinfection. MusGHV-1 variants were distributed unevenly across the population, with individuals infected with the novel genotype clustered in 3 of 25 contiguous social groups. Individuals infected with the novel variant had significantly higher MusGHV-1 viral loads in their blood (p = 0.002) after adjusting for age (juveniles > adults, p < 0.001) and season (summer > spring and autumn, p = 0.005; mixed-effect linear regression), likely indicating higher virulence of the novel variant. Further genome-wide analyses of MusGHV-1 host resistance genes and host phenotypic variations are required to clarify the drivers and sequelae of this new MusGHV-1 variant.

Equine Herpesvirus-1 Myeloencephalopathy

Although equine herpesvirus myeloencephalopathy (EHM) is a relatively uncommon manifestation of equine herpesvirus-1 (EHV-1) infection, it can cause devastating losses during outbreaks. Antemortem diagnosis of EHM relies mainly on the molecular detection of EHV-1 in nasal secretions and blood. Management of horses affected by EHM is aimed at supportive nursing and nutritional care, at reducing central nervous system inflammation and preventing thromboembolic sequelae. Horses exhibiting sudden and severe neurologic signs consistent with a diagnosis of EHM pose a definite risk to the surrounding horse population. Consequently, early intervention to prevent the spread of infection is required.

Molecular characteristics and pathogenicity of an equid alphaherpesvirus 1 strain isolated in China

Equid alphaherpesvirus 1 (EHV-1) is prevalent in China, and causes notable economic damage to the equine industry. However, there is no information regarding the molecular characteristics and pathogenicity of the Chinese strains. Therefore, an EHV-1 strain, named YM2019, was isolated from the lung tissue of an aborted horse fetus in Xinjiang, China, and its genome and pathogenicity were analyzed. The full genome of the isolate was 150,267 base pairs in length, with 56.7% G + C content. Genetic and phylogenetic analysis showed that strain YM2019 (GenBank: MT063054) belonged to the ORF30 N752 genotype but displayed a high level of similarity with strain Ab4 (ORF30 D752, GenBank: AY665713) isolated in Britain. Fourteen unique amino acid mutations were found when comparing strain YM2019 with the reference strains Ab4 and V592 (ORF30 N752, GenBank: AY464052). Syrian hamsters infected with strain YM2019 exhibited severe respiratory and neurological clinical signs and died. Infection with strain YM2019 in Yili horses caused rhinopneumonitis, viremia, and neurological clinical signs such as hind limb lameness, prostration, and reduced movement. Here, we describe the features of an EHV-1 strain discovered in China, together with the complete genome sequence, and reveal that a nonneurovirulent strain (ORF30 N752) can also cause neurological signs in horses. The data will be useful in providing some reference for further research into the relationship between viral genotypes and pathogenicity.

Oral Administration of Valganciclovir Reduces Clinical Signs, Virus Shedding and Cell-Associated Viremia in Ponies Experimentally Infected with the Equid Herpesvirus-1 C2254 Variant

Equid alphaherpesvirus-1 (EHV-1) is one of the main pathogens in horses, responsible for respiratory diseases, ocular diseases, abortions, neonatal foal death and neurological complications such as equine herpesvirus myeloencephalopathy (EHM). Current vaccines reduce the excretion and dissemination of the virus and, therefore, the extent of an epizooty. While their efficacy against EHV-1-induced abortion in pregnant mares and the decreased occurrence of an abortion storm in the field have been reported, their potential efficacy against the neurological form of disease remains undocumented. No antiviral treatment against EHV-1 is marketed and recommended to date. This study aimed to measure the protection induced by valganciclovir (VGCV), the prodrug of ganciclovir, in Welsh mountain ponies experimentally infected with an EHV-1 ORF30-C₂₂₅₄ strain. Four ponies were administered VGCV immediately prior to experimental EHV-1 infection, while another four ponies received a placebo. The treatment consisted in 6.5 mg/kg body weight of valganciclovir administered orally three times the first day and twice daily for 13 days. Clinical signs of disease, virus shedding and viraemia were measured for up to 3 weeks. The severity of the cumulative clinical score was significantly reduced in the treated group when compared with the control group. Shedding of infectious EHV-1 was significantly reduced in the treated group when compared with the control group between Day + 1 (D + 1) and D + 12. Viraemia was significantly reduced in the treated group when compared with the control group. Seroconversion was measured in all the ponies included in the study, irrespective of the treatment received. Oral administration of valganciclovir induced no noticeable side effect but reduced clinical signs of disease, infectious virus shedding and viraemia in ponies experimentally infected with the EHV-1 C₂₂₅₄ variant.

Molecular Characterization of Herpesviral Encephalitis in Cetaceans: Correlation with Histopathological and Immunohistochemical Findings

Simple Summary

In this study we describe the molecular and pathological characteristics of alpha- and gamma-herpesvirus infection of the central nervous system of stranded cetaceans and correlate them with viral load, immunohistochemical findings and biological data such as age, sex, and the presence of co-infections. The viruses (alpha- and gamma-herpesvirus) were detected in twelve out of 103 analysed stranded cetaceans and were associated with a wide range of histopathological lesions, as previously described for these and other species. In five out the twelve animals, lesions were severe enough (malacia, neuronal necrosis and neuronophagia) to cause death. Intranuclear inclusions bodies were present in brain tissue samples from half of the HV-positive animals, indicating that the injury was due to an infective agent belonging to a group of filterable viruses. These results are in accordance with immunohistochemical findings, as all the brain tissue samples with INIBs were immunolabeled with Anti-HSV1. Males, juveniles, and calves were predominantly infected among the analysed cetaceans and a 41.6% (5/12) incidence of co-infections in the brain was detected, with three animals co-infected with Dolphin Morbillivirus (DMV). In this study, we present, to the best of our knowledge, the first histopathological evidence of superinfection between HV and DMV pathogens in brain tissue.

Abstract

Herpesviruses are causative agents of meningitis and encephalitis in cetaceans, which are among the main leading known natural causes of death in these species. Brain samples from 103 stranded cetaceans were retrospectively screened for the presence of herpesvirus DNA in the brain. Molecular detection of Cetacean Morbillivirus was performed in HV positive brain cases. Histopathologic evaluation of brain samples included the presence or absence of the following findings (n = 7): meningitis, perivascular cuffings, microgliosis, intranuclear inclusion bodies, malacia, neuronal necrosis and neurophagic nodules, and haemorrhages. Histological evidence of the involvement of other etiological agents led to complementary analysis. We detected the presence of alpha and gamma-HVs in 12 out of 103 (11.6%) brain samples from stranded cetaceans of five different species: one bottlenose dolphin, six striped dolphins, three Atlantic spotted dolphins, one Cuvier’s beaked whale, and one common dolphin. Pathogenic factors such as viral strain, age, sex, and the presence of co-infections were analysed and correlated with the brain histopathological findings in each case. Herpesvirus was more prevalent in males, juveniles, and calves and a 41.6% incidence of co-infections in the brain was detected in our study: three with Dolphin Morbillivirus, one with Staphilococcus aureus septicaemia and one with Brucella spp.

Clinical impact, diagnosis and control of Equine Herpesvirus-1 infection in Europe

Equine herpesvirus-1 (EHV-1) can affect the entire equine sector in EU, and the large outbreak reported in 2021 in Spain drew attention to the needs of the European Commission for scientific advice for the assessment of EHV-1 infection within the framework of Animal Health Law. EHV-1 is considered endemic in the EU; its main risk is linked to the characteristic of producing latent life-long infections. These can reactivate producing clinical disease, which can include respiratory, abortive and possibly fatal neurological forms. From the epidemiological and genomic viewpoint, there are no specific neuropathogenic EHV-1 strains; the respiratory, reproductive and neurological signs are not found to be strain-specific. This was also the case of the virus that caused the outbreak in Valencia (Spain) in 2021, which was genetically closely related to other viruses circulating before in Europe, and did not present the so-called neuropathogenic genotype. The outbreak reported in Valencia was followed by wide geographic spread of the virus possibly due to a delay in diagnosis and late application of biosecurity measures. The recommended and most sensitive diagnostic test for detecting EHV-1 is PCR performed on swabs collected according to the type of clinical signs. Serological assays on paired blood samples can help to detect a recent infection, while no diagnostic methods are available to detect EHV-1 latent infections. Safe movements of horses can be ensured at premovement phase by testing and issuing health certificates, and by isolating animals upon arrival at new premises with regular health monitoring. In case of suspicion, movements should be forbidden and EHV-1 infection early detected/confirmed by validated diagnostic tools. During outbreaks, no movements should be allowed until 21 days after the detection of the last case. In general, vaccination against EHV-1 should be promoted, although this offers limited protection against the neurological form of the disease.

Outbreak of neuropathogenic equid herpesvirus 1 causing abortions in Yili horses of Zhaosu, North Xinjiang, China

Background

EHV-1 is one of the most serious viral pathogens that frequently cause abortion in horses around the world. However, so far, relatively little information is available on EHV-1 infections as they occur in China. In January 2021, during an abortion storm which occurred in Yili horses at the Chinese State Studs of Zhaosu (North Xinjiang, China), 43 out of 800 pregnant mares aborted.

Results

PCR detection revealed the presence of EHV-1 in all samples as the possible cause of all abortions, although EHV-4, EHV-2 and EHV-5 were also found to circulate in the aborted fetuses. Furthermore, the partial ORF33 sequences of the 43 EHV-1 shared 99.3–100% and 99.0–100% similarity in nucleotide and amino acid sequences respectively. These sequences not only indicated a highly conserved region but also allowed the strains to group into six clusters. In addition, based on the predicted ORF30 nucleotide sequence, it was found that all the strains carried a guanine at the 2254 nucleotide position (aspartic acid at position 752 of the viral DNA polymerase) and were, therefore, identified as neuropathogenic strains.

Conclusion

This study is the first one that establishes EHV-1 as the cause of abortions in Yili horses, of China. Further characterization of the ORF30 sequences revealed that all the EHV-1 strains from the study carried the neuropathogenic genotype. Totally, neuropathogenic EHV-1 infection in China’s horse population should be concerned although the virus only detected in Yili horse abortions.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12917-022-03171-1.

Assessment of listing and categorisation of animal diseases within the framework of the Animal Health Law (Regulation (EU) No 2016/429): infection with Equine Herpesvirus-1

Equine Herpesvirus-1 infection has been assessed according to the criteria of the Animal Health Law (AHL), in particular criteria of: Article 7 on disease profile and impacts, Article 5 on the eligibility of the disease to be listed, Article 9 for the categorisation of the disease according to disease prevention and control measures as in Annex IV and Article 8 on the list of animal species related to Equine Herpesvirus-1 infection. The assessment has been performed following a methodology composed of information collection and compilation, and expert judgement on each criterion at individual and collective level. The outcome is the median of the probability ranges provided by the experts, which indicates whether the criterion is fulfilled (66-100%) or not (0-33%), or whether there is uncertainty about fulfilment (33-66%). For the questions where no consensus was reached, the different supporting views are reported. According to the assessment performed, Equine Herpesvirus-1 infection can be considered eligible to be listed for Union intervention according to Article 5 of the Animal Health Law with 33-90% certainty. According to the criteria as in Annex IV of the AHL related to Article 9 of the AHL for the categorisation of diseases according to the level of prevention and control, it was assessed with less than 1% certainty that EHV-1 fulfils the criteria as in Section 1 (category A), 1-5% for the criteria as in Section 2 (category B), 10-66% for the criteria as in Section 3 (category C), 66-90% for the criteria as in Section 4 (category D) and 33-90% for the criteria as in Section 5 (category E). The animal species to be listed for EHV-1 infection according to Article 8(3) criteria are the species belonging to the families of Equidae, Bovidae, Camelidae, Caviidae, Cervidae, Cricetidae, Felidae, Giraffidae, Leporidae, Muridae, Rhinocerontidae, Tapiridae and Ursidae.

A Diagnostic Survey of Aborted Equine Fetuses and Stillborn Premature Foals in Denmark

Background: Loss of pregnancy in mares can have many different causes, including both infectious and non-infectious conditions. Extrapolation of findings from other studies is often uncertain as the significance of each cause varies across regions. Causes of pregnancy loss in mares have never been thoroughly studied in Denmark, so a prospective cross-sectional cohort study targeting the entire Danish population of pregnant mares was performed over a period of 13 months to obtain knowledge of the significance of individual causes. Fifty aborted or prematurely delivered stillborn fetuses were submitted for necropsy and examined by a panel of diagnostic laboratory methods.

Results: Overall, a cause of fetal loss was established for 72% of the examined cases. Most cases (62%) were lost due to a non-infectious cause, of which obstruction of the feto-placental blood circulation due to severe torsion of the umbilical cord was most prevalent. Pregnancy loss due to a variety of opportunistic bacteria, including bacteria not previously associated with abortion in mares, accounted for 12%, while equid alphaherpesvirus (EHV) type 1 was the cause of pregnancy loss in 8% of the cases. EHV type 4 and Chlamydiaceae species were identified in some cases, but not regarded as the cause of fetal loss.

Conclusion: Umbilical cord torsion was found to be the most prevalent cause of fetal loss in Danish mares, while infectious causes such as EHV type 1 and streptococci only accounted for a minor proportion of the losses. The study highlights the need for defined criteria for establishing an abortion diagnosis in mares, particularly in relation to EHV types 1 and 4.

Pathogenesis of Equid Alphaherpesvirus 1 Infection in the Central Nervous System of Mice

Equid alphaherpesvirus 1 (EHV-1) causes myeloencephalopathy in horses and occasionally in non-equid species. Although mouse models have been developed to understand EHV-1 pathogenesis, few EHV-1 strains have been identified as highly neurovirulent to mice. The aim of this study was to evaluate the pathogenesis of 2 neurovirulent EHV-1 strains in mice, and to characterize the inflammatory cells and expression of chemokines and the apoptosis marker caspase-3 in the brain of infected mice. C57BL/6J mice were inoculated intranasally with EHV-1 strains A4/72 or A9/92 and evaluated on 1, 2, and 3 days post inoculation (DPI). EHV-1-infected mice showed severe neurological signs at 3 DPI. Ultrastructural analysis revealed numerous viral nucleocapsids and fewer enveloped virions within degenerated and necrotic neurons and in the surrounding neuropil. Histologically, at 3 DPI, there was severe diffuse neuronal degeneration and liquefactive necrosis, prominent microgliosis, and perivascular cuffing composed of CD3+ cells (T cells) and Iba-1+ cells (macrophages), mainly in the olfactory bulb and ventral portions of the brain. In these areas, moderate numbers of neuroglial cells expressed CCL5 and CCL2 chemokines. Numerous neurons, including those in less affected areas, were immunolabeled for cleaved caspase-3. In conclusion, neurovirulent EHV-1 strains induced a fulminant necrotizing lymphohistiocytic meningoencephalitis in mice, with microgliosis and expression of chemokines and caspase-3. This model will be useful for understanding the mechanisms underlying the extensive neuropathology induced by these viral infections.

Equine Herpesvirus 1 Variant and New Marker for Epidemiologic Surveillance, Europe, 2021

Equine herpesvirus 1 isolates from a 2021 outbreak of neurologic disease in Europe have a mutation, A713G, in open reading frame 11 not detected in 249 other sequences from equine herpesvirus 1 isolates. This single-nucleotide polymorphism could help identify horses infected with the virus strain linked to this outbreak.

Challenges in navigating molecular diagnostics for common equine respiratory viruses

Equine respiratory viruses remain a leading cause of equine morbidity and mortality, with the resurgence of certain infections, an increasing population of elderly, more susceptible horses, the growth of international equine commerce, and an expansion in geographic distribution of pathogens. The focus of rapid diagnosis of infectious diseases has also shifted recently, with the appearance and increasing importance of nucleic acid amplification-based techniques, primarily polymerase chain reaction (PCR), at the expense of traditional methods such as clinical microbiology. While PCR is fast, reliable, cost-effective, and more sensitive than conventional detection methods, careful interpretation of diagnostic test results is required, taking into account the clinical status of the patient, sample type, assay used and biological relevance of the detected viruses. The interpretation of common equine respiratory viruses such as influenza virus (EIV), alpha herpesviruses (EHV-1, EHV-4), arteritis virus (EAV) and rhinoviruses (ERAV, ERBV) is straight forward as causality can generally be established. However, the testing of less-characterized viruses, such as the gamma herpesviruses (EHV-2, EHV-5), may be confusing, considering their well-established host relationship and frequent detection in both diseased and healthy horses. For selected viruses, absolute quantitation (EHV-1 and EHV-4) and genotyping (EIV and EHV-1) has allowed additional information to be gained regarding viral state and virulence, respectively. This information is relevant when managing outbreaks so that adequate biosecurity measures can be instituted and medical interventions can be considered. The goal of this review is to help the equine practitioner navigate through the rapidly expanding field of molecular diagnostics for respiratory viruses and facilitate the interpretation of results.

Proteomic analysis of the secretome of equine herpesvirus-1 infected rabbit kidney cells

Herpesviruses are the main cause of abortions and respiratory or neurological disorders in horses. Various disease patterns are suspected to be associated with the A2254G point mutation in the DNA polymerase sequence (ORF30) of the herpesvirus genome, although the importance of this link is still under debate. Based on a label-free quantitative proteomic analysis, the differences in the secretion of some host proteins between rabbit kidney cells infected with A₂₂₅₄ and cells of the same line infected with G₂₂₅₄ equine herpesvirus 1 (EHV-1) strains were identified. In both groups, downregulation of proteins involved in insulin growth factor and extracellular matrix pathway regulation was observed. Among 12 proteins with increased secretion, 8 were regulated only in G₂₂₅₄ EHV-1 infection. Those were endoplasmic reticulum chaperones with calcium binding properties, related to unfolded protein response and mitochondria. It was presumed that the secretion of proteins such as calreticulin, Hspa5 or endoplasmin may contribute to the pathogenesis of EHV-1 infection.

New Insights into the Management of an EHV-1 (Equine Hospital) Outbreak

In May 2018, Wolvega Equine Hospital (WEH) experienced an EHV-1 outbreak. This outbreak caused significant economic losses and negative publicity for the hospital. How should hospitals prepare themselves for these outbreaks and prevent shedding of the virus on multiple neighboring premises? The hospital transformed most of its activities into mobile practice and the entire infected hospital population was moved to a separate remote location. The hospital was cleaned and disinfected according to the latest recommendations before reopening. Four neighboring professional equine businesses and three privately owned premises were affected by the spread of the virus from the hospital population and initiated quarantine restrictions. Equine hospitals should prepare themselves for EHV-1 outbreaks as the intake of the virus cannot be prevented. A management protocol should include public information protocols, swift client information and quarantine measures that ensure quick containment of the outbreak. Timely reopening of the hospital can be achieved by rehousing the contaminated population. It should also include good regulations with clients and a properly carried out release protocol. Equine sports organizations should establish sufficient vaccination coverage in order to decrease the frequency of EHV-1 outbreaks.

Seasonal host and ecological drivers may promote restricted water as a viral vector

In climates with seasonally limited precipitation, terrestrial animals congregate at high densities at scarce water sources. We hypothesize that viruses can exploit the recurrence of these diverse animal congregations to spread. In this study, we test the central prediction of this hypothesis - that viruses employing this transmission strategy remain stable and infectious in water. Equid herpesviruses (EHVs) were chosen as a model as they have been shown to remain stable and infectious in water for weeks under laboratory conditions. Using fecal data from wild equids from a previous study, we establish that EHVs are shed more frequently by their hosts during the dry season, increasing the probability of water source contamination with EHV. We document the presence of several strains of EHVs present in high genome copy number from the surface water and sediments of waterholes sampled across a variety of mammalian assemblages, locations, temperatures and pH. Phylogenetic analysis reveals that the different EHV strains found exhibit little divergence despite representing ancient lineages. We employed molecular approaches to show that EHVs shed remain stable in waterholes with detection decreasing with increasing temperature in sediments. Infectivity experiments using cell culture reveals that EHVs remain infectious in water derived from waterholes. The results are supportive of water as an abiotic viral vector for EHV.

Investigation of an EHV-1 Outbreak in the United States Caused by a New H752 Genotype

Here we report on an EHV-1 outbreak investigation caused by a novel genotype H752 (histidine in amino acid position 752 of the ORF 30 gene). The outbreak involved 31 performance horses. Horses were monitored over a period of 35 days for clinical signs, therapeutic outcome and qPCR results of EHV-1 in blood and nasal secretions. The morbidity of the EHV-1 outbreak was 84% with 26 clinically infected horses displaying fever and less frequently anorexia and distal limb edema. Four horses showed mild transient neurological deficits. Clinically diseased horses experienced high viral load of EHV-1 in blood and/or nasal secretions via qPCR, while subclinically infected horses had detectable EHV-1 mainly in nasal secretions. The majority of infected horses showed a rise in antibody titers to EHV-1 during the outbreak. All 31 horses were treated with valacyclovir, while clinically infected horses further received flunixin meglumine and sodium heparin. This investigation highlights various relevant aspects of an EHV-1 outbreak caused by a new H752 genotype: (i) importance of early detection of EHV-1 infection; (ii) diagnostic challenge to assess H752 genotype; (iii) apparent benefit of valacyclovir use in the early stage of the outbreak; and (iv) weekly testing of blood and nasal secretions by qPCR in order to monitor individual infection status and lift quarantine.

Equid herpesvirus-1 Distribution in Equine Lymphoid and Neural Tissues 70 Days Post Infection

Equid herpesvirus-1 (EHV-1) causes respiratory disease, abortion and myeloencephalopathy in horses worldwide. As member of the Alphaherpesvirinae, latency is key to EHV-1 epidemiology. EHV-1 latent infection has been detected in the trigeminal ganglion (TG), respiratory associated lymphoid tissue (RALT) and peripheral blood mononuclear cells (PBMC) but additional locations are likely. The aim of this study was to investigate the distribution of viral DNA throughout the equine body. Twenty-five horses divided into three groups were experimentally infected via intranasal instillation with one of three EHV-1 viruses and euthanized on Day 70, post infection. During necropsy, TG, various sympathetic/parasympathetic ganglia of head, neck, thorax and abdomen, spinal cord dorsal root ganglia, RALT, mesenteric lymph nodes, spleen and PBMC of each horse were collected. Genomic viral loads and L-(late) gene transcriptional activity in each tissue and PBMC were measured using qPCR. In addition, immunohistochemistry (IHC) was applied on neural parenchyma tissue sections. EHV-1 DNA was detected in many neural and lymphoid tissue sections, but not in PBMC. L-gene transcriptional activity was not detected in any sample, and translational activity was not apparent on IHC. Tissue tropism differed between the Ab4 wild type and the two mutant viruses.

Genome Sequences of Equine Herpesvirus 1 Strains from a European Outbreak of Neurological Disorders Linked to a Horse Gathering in Valencia, Spain, in 2021

Five equine herpesvirus 1 (EHV-1) genome sequences with links to an EHV-1 outbreak with neurological disorders after a horse gathering in Valencia, Spain, in February 2021, were determined. All strains showed the closest relationships to strains from Belgium and the United Kingdom, indicating a common source of infection.

Five equine herpesvirus 1 (EHV-1) genome sequences with links to an EHV-1 outbreak with neurological disorders after a horse gathering in Valencia, Spain, in February 2021, were determined. All strains showed the closest relationships to strains from Belgium and the United Kingdom, indicating a common source of infection.

Viral infection and allergy - What equine immune responses can tell us about disease severity and protection

Horses have many naturally occurring diseases that mimic similar conditions in humans. The ability to conduct environmentally controlled experiments and induced disease studies in a genetically diverse host makes the horse a valuable intermediate model between mouse studies and human clinical trials. This review highlights important similarities in the immune landscape between horses and humans using current research on two equine diseases as examples. First, equine herpesvirus type 1 (EHV-1) infection initiates a series of innate inflammatory signals at its mucosal entry site in the upper respiratory tract. These inflammatory markers are highly synchronized and predictable between individuals during viral respiratory infection and ultimately lead to adaptive immune induction and protection. The timing of early inflammatory signals, followed by specific adaptive immune markers correlating with immunity and protection, allow accurate outbreak tracking and also provide a foundation for understanding the importance of local mucosal immunity during other viral respiratory infections. Second, rare peripheral blood immune cells that promote allergic inflammation can be analyzed during Culicoides hypersensitivity, a naturally occurring type I IgE-mediated allergic disease of horses. Rare immune cells, such as IgE-binding monocytes or basophils, can be studied repeatedly in the horse model to unravel their larger mechanistic role in inflammation during allergic and other inflammatory diseases. We conclude with a survey of all other common equine inflammatory conditions. Together, this review serves as a reference and rationale for the horse as a non-rodent model for immunological research.

Apparent Breed Predilection for Equid Herpesvirus-1-Associated Myeloencephalopathy (EHM) in a Multiple-Breed Herd

Equid herpesvirus type 1 (EHV-1) causes several outbreaks of abortion and/or equid herpesvirus-associated myeloencephalopathy (EHM) worldwide each year. EHM is of great concern, as permanent neurological gait anomalies can leave a horse unfit for future use. The study assesses the risk factors associated with the occurrence of EHM. During an unmitigated outbreak, 141 adult horses/ponies of several distinct breeds were evaluated-using multiple Bayesian logistic regression calculating the odds ratios for breed, age, and sex. In total, 33 of the 141 horses showed signs of EHM. Fjord horses and warmblood horses were overrepresented among those developing EHM. The pony breeds, Welsh and Shetland ponies, were underrepresented. In addition, age and sex were not associated with the risk for EHM. The main limitation was that it was a retrospective analysis with some flaws of documentation. It can be concluded that breed was a significant risk factor for developing EHM during this outbreak.

Evaluation of the Variability of the ORF34, ORF68, and MLST Genes in EHV-1 from South Korea

Equine herpesvirus-1 (EHV-1) is an important pathogen in horses. It affects horses worldwide and causes substantial economic losses. In this study, for the first time, we characterized EHV-1 isolates from South Korea at the molecular level. We then aimed to determine the genetic divergences of these isolates by comparing them to sequences in databases. In total, 338 horse samples were collected, and 12 EHV-1 were isolated. We performed ORF30, ORF33, ORF68, and ORF34 genetic analysis and carried out multi-locus sequence typing (MLST) of 12 isolated EHV-1. All isolated viruses were confirmed as non-neuropathogenic type, showing N752 of ORF30 and highly conserved ORF33 (99.7–100%). Isolates were unclassified using ORF68 analysis because of a 118 bp deletion in nucleotide sequence 701–818. Seven EHV-1 isolates (16Q4, 19R166-1, 19R166-6, 19/10/15-2, 19/10/15-4, 19/10/18-2, 19/10/22-1) belonged to group 1, clade 10, based on ORF34 and MLST analysis. The remaining 5 EHV-1 isolates (15Q25-1, 15D59, 16Q5, 16Q40, 18D99) belonged to group 7, clade 6, based on ORF34 and MLST analysis.

The Pathogenesis and Immune Evasive Mechanisms of Equine Herpesvirus Type 1

Equine herpesvirus type 1 (EHV-1) is an alphaherpesvirus related to pseudorabies virus (PRV) and varicella-zoster virus (VZV). This virus is one of the major pathogens affecting horses worldwide. EHV-1 is responsible for respiratory disorders, abortion, neonatal foal death and equine herpes myeloencephalopathy (EHM). Over the last decade, EHV-1 has received growing attention due to the frequent outbreaks of abortions and/or EHM causing serious economical losses to the horse industry worldwide. To date, there are no effective antiviral drugs and current vaccines do not provide full protection against EHV-1-associated diseases. Therefore, there is an urgent need to gain a better understanding of the pathogenesis of EHV-1 in order to develop effective therapies. The main objective of this review is to provide state-of-the-art information on the pathogenesis of EHV-1. We also highlight recent findings on EHV-1 immune evasive strategies at the level of the upper respiratory tract, blood circulation and endothelium of target organs allowing the virus to disseminate undetected in the host. Finally, we discuss novel approaches for drug development based on our current knowledge of the pathogenesis of EHV-1.

Identification of Host Factors Associated with the Development of Equine Herpesvirus Myeloencephalopathy by Transcriptomic Analysis of Peripheral Blood Mononuclear Cells from Horses

Equine herpesvirus-1 is the cause of respiratory disease, abortion, and equine herpesvirus myeloencephalopathy (EHM) in horses worldwide. EHM affects as many as 14% of infected horses and a cell-associated viremia is thought to be central for EHM pathogenesis. While EHM is infrequent in younger horses, up to 70% of aged horses develop EHM. The aging immune system likely contributes to EHM pathogenesis; however, little is known about the host factors associated with clinical EHM. Here, we used the “old mare model” to induce EHM following EHV-1 infection. Peripheral blood mononuclear cells (PBMCs) of horses prior to infection and during viremia were collected and RNA sequencing with differential gene expression was used to compare the transcriptome of horses that did (EHM group) and did not (non-EHM group) develop clinical EHM. Interestingly, horses exhibiting EHM did not show respiratory disease, while non-EHM horses showed significant respiratory disease starting on day 2 post infection. Multiple immune pathways differed in EHM horses in response to EHV-1. These included an upregulation of IL-6 gene expression, a dysregulation of T-cell activation through AP-1 and responses skewed towards a T-helper 2 phenotype. Further, a dysregulation of coagulation and an upregulation of elements in the progesterone response were observed in EHM horses.

Equine herpesvirus 1 elicits a strong pro-inflammatory response in the brain of mice

Equine herpesvirus type 1 (EHV-1) is an emerging pathogen that causes encephalomyelitis in horses and non-equid species. Several aspects of the immune response in the central nervous system (CNS), mainly regarding the role of inflammatory mediators during EHV-1 encephalitis, remain unknown. Moreover, understanding the mechanisms underlying extensive neuropathology induced by viruses would be helpful to establish therapeutic strategies. Therefore, we aimed to evaluate some aspects of the innate immune response during highly neurovirulent EHV-1 infection. C57BL/6 mice infected intranasally with A4/72 and A9/92 EHV-1 strains developed a fulminant neurological disease at 3 days post-inoculation with high viral titres in the brain. These mice developed severe encephalitis with infiltration of monocytes and CD8⁺ T cells to the brain. The inflammatory infiltrate followed the detection of the chemokines CCL2, CCL3, CCL4, CCL5, CXCL2, CXCL9 and CXCL-10 in the brain. Notably, the levels of CCL3, CCL4, CCL5 and CXCL9 were higher in A4/72-infected mice, which presented higher numbers of inflammatory cells within the CNS. Pro-inflammatory cytokines, such as interleukins (ILs) IL-1α, IL-1β, IL-6, IL-12β, and tumour necrosis factor (TNF), were also detected in the CNS, and Toll-like receptor (TLR) TLR2, TLR3 and TLR9 genes were also upregulated within the brain of EHV-1-infected mice. However, no expression of interferon-γ (IFN-γ) and IL-12α, which are important for controlling the replication of other herpesviruses, was detected in EHV-1-infected mice. The results show that the activated innate immune mechanisms could not prevent EHV-1 replication within the CNS, but most likely contributed to the extensive neuropathology. The mouse model of viral encephalitis proposed here will also be useful to study the mechanisms underlying extensive neuropathology.

Identification of a New Equid Herpesvirus 1 DNA Polymerase (ORF30) Genotype with the Isolation of a C2254/H752 Strain in French Horses Showing no Major Impact on the Strain Behaviour

Equid herpesvirus 1 is one of the most common viral pathogens in the horse population and is associated with respiratory disease, abortion and still-birth, neonatal death and neurological disease. A single point mutation in the DNA polymerase gene (ORF30: A2254G, N752D) has been widely associated with neuropathogenicity of strains, although this association has not been exclusive. This study describes the fortuitous isolation of a strain carrying a new genotype C₂₂₅₄ (H₇₅₂) from an outbreak in France that lasted several weeks in 2018 and involved 82 horses, two of which showed neurological signs of disease. The strain was characterised as U_L clade 10 using the equid herpesvirus 1 (EHV-1) multi-locus sequence typing (MLST) classification but has not been identified or isolated since 2018. The retrospective screening of EHV-1 strains collected between 2016 and 2018 did not reveal the presence of the C₂₂₅₄ mutation. When cultured in vitro, the C₂₂₅₄ EHV-1 strain induced a typical EHV-1 syncytium and cytopathic effect but no significant difference was observed when compared with A₂₂₅₄ and G₂₂₅₄ EHV-1 strains. An experimental infection was carried out on four Welsh mountain ponies to confirm the infectious nature of the C₂₂₅₄ strain. A rapid onset of marked respiratory disease lasting at least 2 weeks, with significant virus shedding and cell-associated viraemia, was observed. Finally, an in vitro antiviral assay using impedance measurement and viral load quantification was performed with three antiviral molecules (ganciclovir (GCV), aciclovir (ACV) and aphidicolin (APD)) on the newly isolated C₂₂₅₄ strain and two other A/G₂₂₅₄ field strains. The three strains showed similar sensitivity to ganciclovir and aphidicolin but both C₂₂₅₄ and A₂₂₅₄ strains were more sensitive to aciclovir than the G₂₂₅₄ strain, based on viral load measurement.

Outbreak of equid herpesvirus 1 abortions at the Arabian stud in Poland

Background

Equid herpesvirus 1 (EHV-1) infections are endemic worldwide, including Poland. Many are subclinical, but some are associated with respiratory disease, abortion, neonatal foal death, or neurological disease. We describe an outbreak of abortions in Arabian mares at a well-managed State stud farm in Poland.

Case presentation

Eight of 30 pregnant mares aborted and one gave birth to a weak foal that died within 72 h after birth. EHV-1 was isolated from all fetuses as well as from the diseased foal. All viruses belonged to the N₇₅₂ variant based on the predicted open reading frame (ORF) 30 amino acid sequence. All were identical to each other and to previous EHV-1 viruses from the same stud based on the ORF68 sequence analysis. The outbreak coincided with the lapse in the routine yearly EHV-1/4 vaccinations of the mares.

Conclusions

Multiple abortion due to EHV-1 infection can occur in well-managed groups of horses. Reactivation of latent EHV-1 in one of the resident mares followed by a horizontal spread was considered the most likely explanation for the outbreak. Routine vaccination is an important part of a herd-heath program.

Identification of antiviral compounds against equid herpesvirus-1 using real-time cell assay screening: Efficacy of decitabine and valganciclovir alone or in combination

Equid herpesvirus-1 infections cause respiratory, neurological and reproductive syndromes. Despite preventive treatments with vaccines, resurgence of EHV-1 infection still constitutes a major threat to equine industry. However, no antiviral compound is available to treat infected horses. In this study, 2891 compounds were screened against EHV-1 using impedance measurement. 22 compounds have been found to be effective in vitro against EHV-1. Valganciclovir, ganciclovir, decitabine, aphidicolin, idoxuridine and pritelivir (BAY 57-1293) are the most effective compounds identified, and their antiviral potency was further assessed on E. Derm, RK13 and EEK cells and against 3 different field strains of EHV-1 (ORF30 2254 A/G/C). We also provide evidences of synergistic interactions between valganciclovir and decitabine in our in vitro antiviral assay as determined by MacSynergy II, isobologramm and Chou-Talalay methods. Finally, we showed that deoxycytidine reverts the antiviral effect of decitabine, thus supporting some competition at the level of nucleoside phosphorylation by deoxycytidine kinase and/or DNA synthesis. Deoxycitidine analogues, like decitabine, is a family of compounds identified for the first time with promising antiviral efficacy against herpesviruses.

Equine herpesvirus 1 infection orchestrates the expression of chemokines in equine respiratory epithelial cells

The ancestral equine herpesvirus 1 (EHV1), closely related to human herpes viruses, exploits leukocytes to reach its target organs, accordingly evading the immune surveillance system. Circulating EHV1 strains can be divided into abortigenic/neurovirulent, causing reproductive/neurological disorders. Neurovirulent EHV1 more efficiently recruits monocytic CD172a⁺ cells to the upper respiratory tract (URT), while abortigenic EHV1 tempers monocyte migration. Whether similar results could be expected for T lymphocytes is not known. Therefore, we questioned whether differences in T cell recruitment could be associated with variations in cell tropism between both EHV1 phenotypes, and which viral proteins might be involved. The expression of CXCL9 and CXCL10 was evaluated in abortigenic/neurovirulent EHV1-inoculated primary respiratory epithelial cells (ERECs). The bioactivity of chemokines was tested with a functional migration assay. Replication of neurovirulent EHV1 in the URT resulted in an enhanced expression/bioactivity of CXCL9 and CXCL10, compared to abortigenic EHV1. Interestingly, deletion of glycoprotein 2 resulted in an increased recruitment of both monocytic CD172a⁺ cells and T lymphocytes to the corresponding EREC supernatants. Our data reveal a novel function of EHV1-gp2, tempering leukocyte migration to the URT, further indicating a sophisticated virus-mediated orchestration of leukocyte recruitment to the URT.

Detection of Coxiella burnetii and equine herpesvirus 1, but not Leptospira spp. or Toxoplasma gondii, in cases of equine abortion in Australia - a 25 year retrospective study

Equine abortion is a cause of severe economic loss to the equine industry. Equine herpesvirus 1 is considered a primary cause of infectious abortion in horses, however other infectious agents can also cause abortion. Abortions due to zoonotic pathogens have implications for both human and animal health. We determined the prevalence of Coxiella burnetii, Leptospira spp. and Toxoplasma gondii in 600 aborted equine foetal tissues that were submitted to our diagnostic laboratories at the University of Melbourne from 1994 to 2019. Using qPCR we found that the prevalence of C. burnetii was 4%. The highest annual incidence of C. burnetii was observed between 1997-2003 and 2016-2018. The prevalence of C. burnetii in Victoria and New South Wales was 3% and 6% respectively. All the samples tested negative for Leptospira spp. and Toxoplasma gondii DNA. Equine herpesvirus 1 DNA was detected at a prevalence of 3%. This study has provided evidence for the presence of C. burnetii in equine aborted foetal tissues in Australia, but the role of C. burnetii as potential cause of abortion in Australia requires further investigation. C. burnetii is a zoonotic disease agent that causes the disease 'Q fever' in humans. We recommend that appropriate protective measures should be considered when handling material associated with equine abortions to reduce the risk of becoming infected with C. burnetii.

Genome-wide association study for host genetic factors associated with equine herpesvirus type-1 induced myeloencephalopathy

Background

Equid herpesvirus (EHV‐1) infections in horses can lead to equine herpesvirus myeloencephalopathy (EHM), characterised by neurological clinical signs. The sporadic occurrence of the disease in horse herds suggests a host genetic component. A recent study reported an association between the occurrence of EHM and genetic markers on horse chromosome 6 (ECA6).

Objectives

To investigate the association of EHM with genetic host factors, especially with reference to the association reported for ECA6.

Study design

Genome‐wide association study (GWAS) was conducted based on 94 horses that had EHV‐1 infections and comparing the 27 developing clinical EHM to the 67 which did not.

Methods

DNA samples were tested from 94 horses for 382,529 single nucleotide polymorphisms (SNPs) with the Affymetrix Axiom 670K SNP array to identify possible associations with EHM. The data analysis included tests for basic, additive, dominant and recessive modes of inheritance, haplotype associations and runs of homozygosity (ROH).

Results

Results from this study did not identify significant SNPs, haplotypes or ROH associations with the development of EHM following EHV‐1 infections and excluded the involvement of a recessive genetic factor in the susceptibility to develop EHM.

Main limitations

Sample size and complex phenotype.

Conclusions

The results exclude the involvement of a recessive genetic factor in the susceptibility to develop clinically apparent EHM but do not have the power to exclude the involvement of other, complex host genetic factors. Furthermore, there was no association between development of EHM and genes on equine chromosome 6, as previously reported.

Susceptibility of rat immortalized neuronal cell line Rn33B expressing equine major histocompatibility class 1 to equine herpesvirus-1 infection is differentiation dependent

Equine herpesvirus-1 (EHV-1), which causes encephalomyelitis in horses, shows endotheliotropism in the central nervous system of horses, and generally does not infect neurons. However, little is known about the mechanism underlying the resistance of neuron to EHV-1, due to the lack of convenient cell culture systems. In this study, we examined EHV-1 infection in immortalized Rn33B rat neuronal cells, which differentiate into neurons when cultured under nonpermissive conditions. Because murine cell lines are resistant to EHV-1 infections due to the lack of functional entry receptors for EHV-1, we used an Rn33B-derived cell line that stably expresses the equine MHC class 1 molecule, which acts as EHV-1 entry receptor (Rn33B-A68B2M cells). EHV-1 infected undifferentiated Rn33B-A68B2M cells more efficiently than differentiated cells, resulting in the production of progeny virus in the former but not in the latter. By contrast, both differentiated and undifferentiated cells infected with herpes simplex virus-1 produced infectious viral progeny. While EHV-1 infection induced stronger expression of IFN alpha gene in differentiated cells than in undifferentiated cells, downstream IFN responses, including phosphorylation of STAT1 (signal transducer and activator of transcription 1) and expression of IFN-stimulated genes, were not activated regardless of whether cells were differentiated or not. These results suggest that neuronal differentiation of RN33B-A68B2M cells reduced their susceptibility to EHV-1, which is not due to different IFN responses. This culture system may be useful as an in vitro model for studying neuron-specific resistance to EHV-1, by investigating viral and host factors responsible for the difference in susceptibility between differentiated and undifferentiated cells.

Whole genome sequencing of Herpes Simplex Virus 1 directly from human cerebrospinal fluid reveals selective constraints in neurotropic viruses

Herpes Simplex Virus type 1 (HSV-1) chronically infects over 70 per cent of the global population. Clinical manifestations are largely restricted to recurrent epidermal vesicles. However, HSV-1 also leads to encephalitis, the infection of the brain parenchyma, with high associated rates of mortality and morbidity. In this study, we performed target enrichment followed by direct sequencing of HSV-1 genomes, using target enrichment methods on the cerebrospinal fluid (CSF) of clinical encephalitis patients and from skin swabs of epidermal vesicles on non-encephalopathic patients. Phylogenetic analysis revealed high inter-host diversity and little population structure. In contrast, samples from different lesions in the same patient clustered with similar patterns of allelic variants. Comparison of consensus genome sequences shows HSV-1 has been freely recombining, except for distinct islands of linkage disequilibrium (LD). This suggests functional constraints prevent recombination between certain genes, notably those encoding pairs of interacting proteins. Distinct LD patterns characterised subsets of viruses recovered from CSF and skin lesions, which may reflect different evolutionary constraints in different body compartments. Functions of genes under differential constraint related to immunity or tropism and provide new hypotheses on tissue-specific mechanisms of viral infection and latency.

Viral Equine Encephalitis, a Growing Threat to the Horse Population in Europe?

Neurological disorders represent an important sanitary and economic threat for the equine industry worldwide. Among nervous diseases, viral encephalitis is of growing concern, due to the emergence of arboviruses and to the high contagiosity of herpesvirus-infected horses. The nature, severity and duration of the clinical signs could be different depending on the etiological agent and its virulence. However, definite diagnosis generally requires the implementation of combinations of direct and/or indirect screening assays in specialized laboratories. The equine practitioner, involved in a mission of prevention and surveillance, plays an important role in the clinical diagnosis of viral encephalitis. The general management of the horse is essentially supportive, focused on controlling pain and inflammation within the central nervous system, preventing injuries and providing supportive care. Despite its high medical relevance and economic impact in the equine industry, vaccines are not always available and there is no specific antiviral therapy. In this review, the major virological, clinical and epidemiological features of the main neuropathogenic viruses inducing encephalitis in equids in Europe, including rabies virus (Rhabdoviridae), Equid herpesviruses (Herpesviridae), Borna disease virus (Bornaviridae) and West Nile virus (Flaviviridae), as well as exotic viruses, will be presented.

A novel PCR protocol for detection and differentiation of neuropathogenic and non-neuropathogenic equid alphaherpesvirus 1

Equid alphaherpesvirus 1 (EHV-1) infections can have a major impact on the horse industry and equine welfare by causing abortion or respiratory or neurologic disease. A single nucleotide polymorphism (A₂₂₅₄→G₂₂₅₄) in open reading frame (ORF) 30, encoding the catalytic subunit of the DNA polymerase, has been shown to be a strong predictive marker for neuropathogenicity. Given that a previously established real-time PCR (rtPCR) protocol yielded unsatisfactory results concerning determination of the EHV-1 genotype, we developed and evaluated a new conventional PCR protocol enabling identification of the genotype by sequencing and restriction enzyme analysis (REA). Thirty samples from horses with signs typical for EHV-1 infection were tested by rtPCR and our new conventional PCR. The results showed that compared to rtPCR, the conventional PCR protocol combined with sequencing and REA was more reliable concerning unambiguous determination of the EHV-1 genotype. Results of our new assay confirmed previous findings, according to which the non-neuropathogenic genotype A₂₂₅₄ is predominantly found in animals with fever, respiratory signs, and abortions or perinatal mortality, whereas the neuropathogenic genotype G₂₂₅₄ is primarily detected in animals suffering from neurologic disease. In some samples, results pointed towards coinfection with both genotypes. Further studies are required in order to elucidate the significance of infections with genotype A₂₂₅₄ and G₂₂₅₄ in neurologic and non-neurologic cases, respectively.

EHV-1: A Constant Threat to the Horse Industry

Equine herpesvirus-1 (EHV-1) is one of the most important and prevalent viral pathogens of horses and a major threat to the equine industry throughout most of the world. EHV-1 primarily causes respiratory disease but viral spread to distant organs enables the development of more severe sequelae; abortion and neurologic disease. The virus can also undergo latency during which viral genes are minimally expressed, and reactivate to produce lytic infection at any time. Recently, there has been a trend of increasing numbers of outbreaks of a devastating form of EHV-1, equine herpesviral myeloencephalopathy. This review presents detailed information on EHV-1, from the discovery of the virus to latest developments on treatment and control of the diseases it causes. We also provide updates on recent EHV-1 research with particular emphasis on viral biology which enables pathogenesis in the natural host. The information presented herein will be useful in understanding EHV-1 and formulating policies that would help limit the spread of EHV-1 within horse populations.