Epidemiological studies of equine herpesvirus 1 (EHV-1) in Thoroughbred foals: a review of studies conducted in the Hunter Valley of New South Wales between 1995 and 1997

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.

Ocular findings in a population of geriatric equids in the United Kingdom

BACKGROUND

There are a growing number of horses, ponies, and donkeys aged 15 years or older in the United Kingdom, yet there have been no studies utilising a complete ophthalmic examination to investigate the prevalence of ophthalmic pathology within this population.

OBJECTIVES

To investigate the prevalence of ophthalmic pathology and associations with signalment, in a convenience sample of geriatric equids in the United Kingdom.

STUDY DESIGN

Cross sectional.

METHODS

Horses, ponies, and donkeys aged 15 years or older based at The Horse Trust charity underwent a full ophthalmic examination including slit lamp biomicroscopy and indirect ophthalmoscopy. Relationships between signalment and pathology were assessed using Fisher's exact and Mann-Witney U tests.

RESULTS

Fifty animals were examined ranging from 15 to 33 years (median 24, interquartile range [IQR] 21, 27). The prevalence of ocular pathology was 84.0% (95% confidence interval [CI] 73.8, 94.2%; n = 42). Four animals (8.0%) had adnexal pathology, while 37 (74.0%) and 22 (44.0%) had at least one form of anterior or posterior segment pathology, respectively. Of those with anterior segment pathology, 26 animals (52.0%) had cataract in at least one eye, with the most common location being anterior cortical (65.0% of those animals with cataract). Animals with posterior segment pathology included 21 animals (42.0%) with fundic pathology, with senile retinopathy being the most common (42.9% of all animals with fundic pathology). Despite the high prevalence of ocular pathology, all eyes examined remained visual. The most common breeds were Irish Draught (24.0%, n = 12), Shetland (18.0%, n = 9) and Thoroughbred (10%, n = 5); the majority were geldings (74.0%, n = 37). There was a statistically significant relationship between the presence of anterior segment pathology and breed (p = 0.006), with all Cobs and Shetlands examined having anterior segment pathology. The presence of posterior segment pathology and senile retinopathy were associated with older median age (posterior segment pathology: 26.0 years [interquartile range {IQR} 24.0, 30.0 years] vs. 23.5 years [IRQ 19.5, 26.5 years], p = 0.03; senile retinopathy: 27.0 years [IQR 26.0, 30 years] vs. 24.0 years [IQR 20.0, 27.0], p = 0.04). None of the pathologies investigated were more prone to affect one versus both eyes (p > 0.05; 71.4% of ocular pathologies were bilateral while 28.6% were unilateral).

MAIN LIMITATIONS

Data were obtained from a relatively small sample size of a single cohort of animals that lacked a control group.

CONCLUSIONS

There was a high prevalence and wide range of ocular lesions in this subset of geriatric equids.

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.

Seroprevalence of Equine Herpesvirus 1 (EHV-1) and Equine Herpesvirus 4 (EHV-4) in the Northern Moroccan Horse Populations

Simple Summary

This work aims to evaluate the seroprevalence of equine EHV-1/4 in horse populations in the north of Morocco and to measure the antibody titers in vaccinated horses, under field conditions, with monovalent EHV-1 vaccines. Overall, 12.8% unvaccinated, and 21.8% vaccinated horses were positive for EHV-1. All samples were positive for EHV-4 when tested with the type-specific ELISA. The virus neutralization test showed low antibody titers in samples from vaccinated horses. Our study demonstrated that EHV-1 and EHV-4 are endemic in the horse populations in the north of Morocco and highlighted the necessity of reevaluating the vaccines and the vaccination protocol used.

Abstract

This study reports the first equine herpesvirus-1 (EHV-1) and equine herpesvirus-4 (EHV-4) seroprevalence investigation in horse populations of Morocco in 24 years. It also aims to determine antibody titers in horses vaccinated under field conditions with a monovalent EHV-1 vaccine. Blood samples were collected from 405 horses, including 163 unvaccinated and 242 vaccinated animals. They were tested using a commercial type-specific enzyme-linked immunosorbent assay (ELISA) and a virus neutralization test (VNT). Overall, 12.8% unvaccinated, and 21.8% vaccinated horses were positive for EHV-1. All samples were positive for EHV-4 when tested with the type-specific ELISA. In the vaccinated group, the VNT revealed a mean antibody titer of 1:49 for EHV-1 and 1:45 for EHV-4. The present study demonstrates that EHV-1 and EHV-4 are endemic in the horse populations in the north of Morocco, with prevalence differences between regions. Furthermore, horses vaccinated with a monovalent EHV-1 vaccine had low antibodies titers. This study highlights the necessity to establish and/or support efficient biosecurity strategies based on sound management of horses and characterize further and potentially improve the efficiency of the EHV vaccines and vaccination protocol used in the field.

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.

A Live-Attenuated Equine Influenza Vaccine Stimulates Innate Immunity in Equine Respiratory Epithelial Cell Cultures That Could Provide Protection From Equine Herpesvirus 1

Equine herpesvirus 1 (EHV-1) ubiquitously infects horses worldwide and causes respiratory disease, abortion, and equine herpesvirus myeloencephalopathy. Protection against EHV-1 disease is elusive due to establishment of latency and immune-modulatory features of the virus. These include the modulation of interferons, cytokines, chemokines, antigen presentation, and cellular immunity. Because the modulation of immunity likely occurs at the site of first infection—the respiratory epithelium, we hypothesized that the mucosal influenza vaccine Flu Avert^® I.N. (Flu Avert), which is known to stimulate strong antiviral responses, will enhance antiviral innate immunity, and that these responses would also provide protection from EHV-1 infection. To test our hypothesis, primary equine respiratory epithelial cells (ERECs) were treated with Flu Avert, and innate immunity was evaluated for 10 days following treatment. The timing of Flu Avert treatment was also evaluated for optimal effectiveness to reduce EHV-1 replication by modulating early immune responses to EHV-1. The induction of interferons, cytokine and chemokine mRNA expression, and protein secretion was evaluated by high-throughput qPCR and multiplex protein analysis. Intracellular and extracellular EHV-1 titers were determined by qPCR. Flu Avert treatment resulted in the modulation of IL-8, CCL2, and CXCL9 starting at days 5 and 6 post-treatment. Coinciding with the timing of optimal chemokine induction, our data also suggested the same timing for reduction of EHV-1 replication. In combination, our results suggest that Flu Avert may be effective at counteracting some of the immune-modulatory properties of EHV-1 at the airway epithelium and the peak for this response occurs 5–8 days post-Flu Avert treatment. Future in vivo studies are needed to investigate Flu Avert as a prophylactic in situations where EHV-1 exposure may occur.

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.

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.

Transcriptomic Profiling of Equine and Viral Genes in Peripheral Blood Mononuclear Cells in Horses during Equine Herpesvirus 1 Infection

Equine herpesvirus 1 (EHV-1) affects horses worldwide and causes respiratory disease, abortions, and equine herpesvirus myeloencephalopathy (EHM). Following infection, a cell-associated viremia is established in the peripheral blood mononuclear cells (PBMCs). This viremia is essential for transport of EHV-1 to secondary infection sites where subsequent immunopathology results in diseases such as abortion or EHM. Because of the central role of PBMCs in EHV-1 pathogenesis, our goal was to establish a gene expression analysis of host and equine herpesvirus genes during EHV-1 viremia using RNA sequencing. When comparing transcriptomes of PBMCs during peak viremia to those prior to EHV-1 infection, we found 51 differentially expressed equine genes (48 upregulated and 3 downregulated). After gene ontology analysis, processes such as the interferon defense response, response to chemokines, the complement protein activation cascade, cell adhesion, and coagulation were overrepresented during viremia. Additionally, transcripts for EHV-1, EHV-2, and EHV-5 were identified in pre- and post-EHV-1-infection samples. Looking at micro RNAs (miRNAs), 278 known equine miRNAs and 855 potentially novel equine miRNAs were identified in addition to 57 and 41 potentially novel miRNAs that mapped to the EHV-2 and EHV-5 genomes, respectively. Of those, 1 EHV-5 and 4 equine miRNAs were differentially expressed in PBMCs during viremia. In conclusion, this work expands our current knowledge about the role of PBMCs during EHV-1 viremia and will inform the focus on future experiments to identify host and viral factors that contribute to clinical EHM.

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.

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.

Frequency of latent equine herpesvirus type-1 infection among a sample of horses in the central North Island of New Zealand

Aim: To estimate the frequency of infection with equine herpesvirus type-1 (EHV-1) among horses from the central North Island of New Zealand, including the frequency of detection of the D₇₅₂ genotype.Methods: Samples of retropharyngeal lymph nodes (RLN) and submandibular lymph nodes (SLN) were dissected from the heads of 63 horses that were humanely killed for various unrelated reasons between March and November 2015. DNA extracted from these tissues was subjected to enrichment for EHV-1 sequences by hybridisation with biotin-labelled EHV-1 specific probe, followed by recovery of EHV-1 sequences on streptavidin-coated magnetic beads. Enriched samples were tested for the presence of EHV-1 using nested quantitative real-time PCR. The EHV-1 amplicons were sequenced to determine the genotype of the virus.Results: The median age of the horses was 6 (min 2, max 30) years, and 47/63 (75%) were Thoroughbreds. EHV-1 DNA was detected in RLN samples from 6/63 (10%) horses, and three of these horses were also positive for EHV-1 DNA in SLN. The remaining horses were negative for EHV-1 DNA in both RLN and SLN samples. The N₇₅₂ genotype was detected in all positive samples and the D₇₅₂ genotype was not detected in any of the samples.Conclusions: EHV-1 continues to circulate among horses in New Zealand. The frequency of latent EHV-1 infection among sampled horses may have been underestimated due to the sensitivity limit of the assay or because of the limited anatomical sites sampled in the study. Lack of detection of the D₇₅₂ genotype suggests that infection with this genotype is not common in horses in New Zealand.Clinical Relevance: If live animals are tested for EHV-1 using SLN biopsy it should be kept in mind that negative results do not rule out the presence of latent EHV-1 infection at other sites inaccessible for testing. The RLN appear to be the preferred sample for detection of EHV-1 DNA in horses following recent euthanasia.

Failure to detect equid herpesvirus types 1 and 4 DNA in placentae and healthy new-born Thoroughbred foals

Equid herpesvirus type 1 is primarily a respiratory tract virus associated with poor athletic performance that can also cause late gestation abortion, neonatal foal death and encephalomyelopathy. Horizontal transmission is well described, whereas evidence of vertical transmission of equid herpesvirus type 1 associated with the birth of a healthy foal has not been demonstrated. This study sampled a population of Thoroughbred mares (n = 71), and their healthy neonatal foals and foetal membranes, to test for the presence of both equid herpesvirus types 1 and 4 using a quantitative polymerase chain reaction assay. Foetal membrane swabs and tissue samples were taken immediately post-partum, and venous blood samples and nasal swabs were obtained from both mare and foal 8 h after birth. Neither equid herpesvirus type 1 nor equid herpesvirus type 4 nucleic acid was detected in any sample, and it was concluded that there was no active shedding of equid herpesvirus types 1 and 4 at the time of sampling. Consequently, no evidence of vertical transmission of these viruses could be found on this stud farm during the sampling period.

Intranasal IgG4/7 antibody responses protect horses against equid herpesvirus-1 (EHV-1) infection including nasal virus shedding and cell-associated viremia

Equid herpesvirus-1 (EHV-1) outbreaks continue despite widely used vaccination. We demonstrated previously that an ORF1/ORF71 gene deletion mutant of the EHV-1 strain Ab4 (Ab4ΔORF1/71) is less virulent than its parent Ab4 virus. Here, we describe the Ab4 challenge infection evaluating protection induced by the Ab4ΔORF1/71 vaccine candidate. Susceptible control horses developed respiratory disease, fever, nasal shedding, and viremia. Full protection after challenge infection was observed in 5/5 previously Ab4 infected horses and 3/5 Ab4ΔORF1/71 horses. Two Ab4ΔORF1/71 horses developed short-lasting viremia and/or virus shedding. Protective immunity in the respiratory tract was characterized by pre-existing EHV-1-specific IgG4/7 antibodies, the absence of IFN-α secretion and rapidly increasing IgG4/7 upon challenge infection. Pre-existing systemic EHV-1-specific IgG4/7 highly correlated with protection. T-cell immunity was overall low. In conclusion, protective immunity against EHV-1 infection including prevention of viremia was associated with robust systemic and intranasal IgG4/7 antibodies suggesting immediate virus neutralization at the local site.

Neonatal Immunization with a Single IL-4/Antigen Dose Induces Increased Antibody Responses after Challenge Infection with Equine Herpesvirus Type 1 (EHV-1) at Weanling Age

Neonatal foals respond poorly to conventional vaccines. These vaccines typically target T-helper (Th) cell dependent B-cell activation. However, Th2-cell immunity is impaired in foals during the first three months of life. In contrast, neonatal basophils are potent interleukin-4 (IL-4) producers. The purpose of this study was to develop a novel vaccine triggering the natural capacity of neonatal basophils to secrete IL-4 and to evaluate if vaccination resulted in B-cell activation and antibody production against EHV-1 glycoprotein C (gC). Neonatal vaccination was performed by oral biotinylated IgE (IgE-bio) treatment at birth followed by intramuscular injection of a single dose of streptavidin-conjugated gC/IL-4 fusion protein (Sav-gC/IL-4) for crosslinking of receptor-bound IgE-bio (group 1). Neonates in group 2 received the intramuscular Sav-gC/IL-4 vaccine only. Group 3 remained non-vaccinated at birth. After vaccination, gC antibody production was not detectable. The ability of the vaccine to induce protection was evaluated by an EHV-1 challenge infection after weaning at 7 months of age. Groups 1 and 2 responded to EHV-1 infection with an earlier onset and overall significantly increased anti-gC serum antibody responses compared to control group 3. In addition, group 1 weanlings had a decreased initial fever peak after infection indicating partial protection from EHV-1 infection. This suggested that the neonatal vaccination induced a memory B-cell response at birth that was recalled at weanling age after EHV-1 challenge. In conclusion, early stimulation of neonatal immunity via the innate arm of the immune system can induce partial protection and increased antibody responses against EHV-1.

Concurrent vaccination against equine influenza and equine herpesvirus - a practical approach

Background

There is a lack of information concerning concurrent administration of vaccines against equine influenza virus (EIV) and equine herpesvirus 1 and 4 (EHV‐1/4).

Objectives

The primary objective of this study was to determine the impact of the concurrent use of EIV and EHV‐1/4 vaccines in Thoroughbred racehorses on their humoral immune response to EIV.

Methods

This study was carried out on a population of 30 horses using an inactivated whole‐virus EIV vaccine and an inactivated EHV‐1/4 vaccine. Horses were randomly allocated to vaccination group A or B. Horses in group A were vaccinated against EIV and EHV‐1/4 2 weeks apart. Horses in group B were vaccinated against EIV and EHV‐1/4 on the same day. Whole‐blood samples were collected on the day of vaccination and 2 weeks and 6 weeks post‐vaccination. Antibody levels against EIV and EHV‐1/4 were measured using the single radial haemolysis and serum neutralisation test, respectively.

Results

The pattern of EIV antibody response post‐vaccination was similar for both groups. Highest EIV antibody levels were recorded 2 weeks post‐vaccination, and a significant decrease in antibody level was observed 4 weeks later. Horses in group B demonstrated a significantly higher EIV antibody response post‐vaccination. Overall, there was no significant difference in EHV‐1/4 antibody response between the two groups post‐vaccination.

Conclusion

In this study, concurrent vaccination against EIV and EHV‐1/4 increased the response to EIV and did not compromise the humoral immune response to EHV‐1/4.

Seroprevalence and factors associated with equine herpesvirus type 1 and 4 in Spanish Purebred horses in Spain

Equine herpesvirus type 1 (EHV-1) and type 4 (EHV-4) have a worldwide distribution and cause respiratory disease, abortion, neonatal death and myeloencephalopathy in susceptible horses. Given the scarcity of serological EHV-1/EHV-4 data in Spain, the objective of this cross-sectional study was to estimate the seroprevalence of EHV-1/EHV-4 and to identify potential horse-level and stud farm-level factors associated with EHV-1/EHV-4 in the breeding Spanish Purebred (SP) horse population in central Spain. Serum samples from 334 SP unvaccinated horses, collected between September 2011 and November 2013 at 30 stud farms, were tested using a commercially available EHV-1/EHV-4 antibody ELISA and seroneutralisation as the World Organisation for Animal Health reference confirmation test. Data on factors putatively associated with seropositivity to EHV-1/EHV-4 were collected via a questionnaire and examined using logistic regression analysis. EHV-1/EHV-4 seroprevalence in the SP breeding population in central Spain, standardised for the sex distribution of the reference horse population, was 53.9 per cent (95 per cent confidence interval 44.0 per cent to 63.8 per cent). Increasing age, southern location of the stud farm, temperate climate during the summer, and a smaller surface area used for breeding activities in the farm were associated with increased odds for EHV-1/EHV-4 seropositivity, whereas EHV-1/EHV-4 vaccination of other resident horses and separation of breeding mares from youngsters were protective factors.

Evidence of widespread natural recombination among field isolates of equine herpesvirus 4 but not among field isolates of equine herpesvirus 1

Recombination in alphaherpesviruses allows evolution to occur in viruses that have an otherwise stable DNA genome with a low rate of nucleotide substitution. High-throughput sequencing of complete viral genomes has recently allowed natural (field) recombination to be studied in a number of different alphaherpesviruses, however, such studies have not been applied to equine herpesvirus 1 (EHV-1) or equine herpesvirus 4 (EHV-4). These two equine alphaherpesviruses are genetically similar, but differ in their pathogenesis and epidemiology. Both cause economically significant disease in horse populations worldwide. This study used high-throughput sequencing to determine the full genome sequences of EHV-1 and EHV-4 isolates (11 and 14 isolates, respectively) from Australian or New Zealand horses. These sequences were then analysed and examined for evidence of recombination. Evidence of widespread recombination was detected in the genomes of the EHV-4 isolates. Only one potential recombination event was detected in the genomes of the EHV-1 isolates, even when the genomes from an additional 11 international EHV-1 isolates were analysed. The results from this study reveal another fundamental difference between the biology of EHV-1 and EHV-4. The results may also be used to help inform the future safe use of attenuated equine herpesvirus vaccines.

Detection of equine herpesvirus-4 and physiological stress patterns in young Thoroughbreds consigned to a South African auction sale

Background

The prevalence of equine herpesvirus types-1 and -4 (EHV-1 and -4) in South African Thoroughbreds at auction sales is currently undefined. Commingling of young Thoroughbreds from various populations together with physiological stress related to their transport and confinement at a sales complex, may be associated with shedding and transmission of EHV-1 and -4. This prospective cohort study sampled 90 young Thoroughbreds consigned from eight farms, originating from three provinces representative of the South African Thoroughbred breeding demographic to a sales complex. Nasal swabs for quantitative real-time polymerase chain reaction (qPCR) assay to detect EHV-1 and -4 nucleic acid and blood samples for enzyme-linked immunosorbent assay for EHV-1 and -4 antibodies were collected from all horses on arrival and departure. Additional nasal swabs for qPCR were obtained serially from those displaying pyrexia and, or nasal discharge. Daily faecal samples were used for determination of faecal glucocorticoid metabolite (FGM) concentrations as a measurement of physiological stress and these values were modelled to determine the factors best explaining FGM variability.

Results

EHV-4 nucleic acid was detected in 14.4 % and EHV-1 from none of the animals in the study population. Most (93.3 %) and very few (1.1 %) of this population showed antibodies indicating prior exposure to EHV-4 and EHV-1 respectively. Pyrexia and nasal discharge were poor predictors for detecting EHV-4 nucleic acid. The horses’ FGM concentrations increased following arrival before decreasing for most of the remaining study period including the auction process. Model averaging showed that variation in FGM concentrations was best explained by days post-arrival and transport duration.

Conclusions

In this study population, sales consignment was associated with limited detection of EHV-4 nucleic acid in nasal secretions, with most showing prior exposure to EHV-4 and very few to EHV-1. The physiological stress response shown by most reflected the combination of stressors associated with transport and arrival and these are key areas for future investigation into management practices to enhance health and welfare of young Thoroughbreds during sales consignment.

Virological and serological investigation of Equid herpesvirus 1 infection in New Zealand

Infection with equid herpesvirus 1 (EHV-1) may be asymptomatic, or may result in respiratory disease, abortion, neonatal death, or neurological disease. The aim of this study was to estimate the prevalence of EHV-1 infection, including differentiation between genotypes with aspartic acid (D) and asparagine (N) at position 752 of the DNA polymerase sequence, within a selected population of New Zealand horses. The second aim was to determine the predictive value of serology for detection of latently infected horses. Retropharyngeal lymph nodes (RLN) and trigeminal ganglia (TG) were dissected from 52 horses at slaughter and tested for the presence of EHV-1 DNA using magnetic bead, sequence-capture enrichment followed by nested PCR. Sera were tested for EHV-1 antibody using type-specific glycoprotein G ELISA. Overall, 17/52 horses tested positive for EHV-1 DNA. All but one positive PCR results were obtained from RLN samples. Fifteen of the EHV-1 positive horses harboured EHV-1 with N752 genotype, one of which was additionally infected with the D752 genotypes of the virus. Our data comprise the first detection of EHV-1 with D752 genotype in New Zealand and suggest that the "neurovirulent" variant of EHV-1 had been present in New Zealand for at least two years before the first reported outbreak of EHM. All sampled horses tested positive for EHV-4 antibody, and 11/52 tested positive for EHV-1 antibody. The strength of agreement between results of EHV-1 PCR and EHV-1 serology was "fair" (Kappa 0.259, 95% CI: -0.022-0.539), which was likely a reflection of low levels of both EHV-1 antibody in sera and EHV-1 DNA in tissues tested.

A review of equid herpesvirus 1 for the veterinary practitioner. Part A: clinical presentation, diagnosis and treatment

Equid herpesvirus (EHV) type 1 is a common pathogen of horses with worldwide distribution. Although severe tracheobronchitis has been described in some field outbreaks of EHV-1 respiratory disease, many EHV-1 infections occur asymptomatically or are accompanied only by signs of mild respiratory disease. However, EHV-1 infection can also result in outcomes other than respiratory disease such as abortion, neonatal death or neurological disease. This review provides an overview of the diagnosis, treatment and prognosis for EHV-1-associated diseases, with an emphasis on neurological presentations of EHV-1 infection.

Development of a peptide ELISA for discrimination between serological responses to equine herpesvirus type 1 and 4

A peptide-based enzyme-linked immunosorbent assay (ELISA) for discrimination between serological responses to equine herpesvirus type 1 (EHV-1) and 4 (EHV-4) was developed. Three and four peptides for EHV-1 and EHV-4, respectively, were designed and studied initially in the ELISA using sera from foals infected experimentally. The most promising peptide pair, derived from EHV-1 glycoprotein E and EHV-4 glycoprotein G, was evaluated further using acute and convalescent sera from horses infected experimentally and naturally as well as a panel of horse field sera. Ten pre- and post-vaccination serum pairs were similarly tested in the type-specific ELISA. The peptide ELISA was able to identify horses which had been infected with EHV-1 or EHV-4 as derived from the results using acute and convalescent sera collected from natural outbreaks. When applied to a set of field samples, the assay proved robust with respect to determining the EHV-1 and EHV-4 antibody status. Also, the peptide ELISA was able to detect type-specific seroconversion for EHV-1 in vaccinated animals. With further validation, the EHV-1/EHV-4 peptide ELISA described in this study could serve as a reliable and cost-effective alternative to current methods for serological EHV-1 and EHV-4 diagnosis.

Molecular data of UL24 homolog gene (ORF37) from Brazilian isolates of equine herpesvirus type 1

Equine herpesvirus type 1 (EHV-1) is associated with abortions, respiratory distress, and neurological disturbances in horses. The ORF37 of EHV-1 encodes a protein homolog to UL24 gene product of human herpesvirus that has been associated with neurovirulence. In the present work, ORF37 PCR fragments derived from two Brazilian EHV-1 isolates, a German isolate and an American reference strain were sequenced and characterized by molecular phylogenetic analysis. This genomic region is highly conserved an allowed to infer genetic distances between EHV-1 strains and other animal herpesvirus.

Infection of peripheral blood mononuclear cells with neuropathogenic equine herpesvirus type-1 strain Ab4 reveals intact interferon-α induction and induces suppression of anti-inflammatory interleukin-10 responses in comparison to other viral strains

The recent increase in incidence, morbidity, and mortality of neurological disease induced by equine herpesvirus type 1 (EHV-1) has suggested a change of virulence of the virus. The exact mechanisms by which EHV-1 induces neurologic disease are not known. Environmental, viral, and host risk factors might contribute to neurological manifestation. Here, we investigated innate interferon-α (IFN-α), interleukin-10 (IL-10) and IL-4 responses after infection of equine peripheral blood mononuclear cells (PBMC) with EHV-1 using an available cytokine multiplex assay. Three viral strains representing an older isolate (RacL11), a recent abortigenic (NY03) and a neuropathogenic isolate (Ab4) were compared to identify differences in cytokine induction that might explain the increased pathogenicity of Ab4. Cytokine concentrations were also compared between foals, mares after birth, pregnant and non-pregnant mares to investigate whether immune responses to EHV-1 infection are influenced by age or pregnancy status. PBMC from all groups secreted high concentrations of anti-viral IFN-α in response to EHV-1. A reduced response was observed in foals compared to non-pregnant mares. EHV-1 infection induced moderate IL-10 and overall low IL-4 secretion. Ab4 infection resulted in a significant reduction of IL-10 responses in adult horses. IL-10 and IL-4 responses were lower in foals than in most mare groups. These data suggested that EHV-1 induces robust IFN-α secretion without major differences between viral strains. However, anti-inflammatory IL-10 production was significantly reduced after infection with neuropathogenic Ab4. The ability of this EHV-1 isolate to down-regulate IL-10 production might contribute to increased local inflammation and a higher risk for neurological manifestation of the disease after infection with Ab4.

Evaluation of the usefulness of a PCR assay performed at a clinical laboratory for the diagnosis of respiratory disease induced by equine herpesvirus type 1 in the field

A PCR assay for the diagnosis of respiratory disease induced by equine herpesvirus type 1 (EHV-1) was performed at the clinical laboratory in the Racehorse Clinic of the Ritto Training Center of the Japan Racing Association from December 2007 to March 2008. The assay was performed without the trouble of contamination throughout the study and its turnaround time was approximately 6 hr. The PCR detection rates of EHV-1 among seroconverted horses were 22.2% for nasal swabs and 33.3% for blood samples. However, EHV-1 DNA was also detected in horses without seroconversion at a low rate. These results indicated that the PCR assay should be used as an adjunct method, but would help to make an early diagnosis of EHV-1 infection.

Control of EHV-1 viremia and nasal shedding by commercial vaccines

Equine herpesvirus-1 is a cause of outbreaks of abortion and neurological disease. The pathogenesis of both these diseases depends on establishment of viremia. An experiment was performed to determine the protective efficacy of two commercially available vaccines used with an optimized 3-dose vaccination regime: a modified-live viral (MLV) and a high antigen load killed vaccine licensed for abortion control. The study design was a blinded, randomized challenge trial. Three groups of 8 yearling ponies received one of three treatments: MLV vaccine (Rhinomune, Boehringer Ingelheim Vetmedica, Inc.); killed vaccine (Pneumabort-K, Pfizer Animal Health); or a placebo (control group). Three vaccinations were administered at intervals of 27 and 70 days followed by challenge infection 24 days later. Clinical disease after challenge was significantly reduced in both vaccine groups; the reduction was greater in the MLV vaccine group. Nasal shedding was reduced by at least 1-2 logs in both vaccine groups. The number of days of viremia was significantly reduced in the killed vaccine group only. This study demonstrated that both commercial vaccines significantly suppressed EHV-1 disease and nasal viral shedding, and one vaccine suppressed days of viremia.

Equine herpesvirus-1 consensus statement

Equine herpesvirus-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). In recent years, there has been an apparent increase in the incidence of EHM in North America, with serious consequences for horses and the horse industry. This consensus statement draws together current knowledge in the areas of pathogenesis, strain variation, epidemiology, diagnostic testing, vaccination, outbreak prevention and control, and treatment.

Clinical, serological and molecular investigations of EHV-1 and EHV-4 in 15 unweaned thoroughbred foals

Fifteen unweaned thoroughbred foals, born on a stud farm to vaccinated mares, were clinically monitored during their first six months of life and repeatedly tested for equine herpesvirus type 1 (EHV-1) and equine herpesvirus type 4 (EHV-4). Nasopharyngeal swabs and blood samples were collected and screened respectively by PCR and seroneutralisation to detect the presence of the virus, explore its role as a possible cause of respiratory disease, and to assess the efficiency of the pcr for the diagnosis of this disease. The foals were divided into three groups on the basis of their clinical signs and whether they had seroconverted to EHV-1 and/or EHV-4: first, foals with no clinical signs of disease that had not seroconverted; secondly, foals with clinical signs that had seroconverted, and thirdly, foals with clinical signs that had not seroconverted. The results indicated that the viruses circulated on the stud farm despite stringent vaccination regimens against them, and confirmed their association with respiratory disease. The absence of significantly different pcr results among the three groups of foals showed that the pcr was effective in confirming the circulation of the viruses on the premises without being particularly helpful as a diagnostic tool.

Prevalence of equine herpesvirus-1 infection among Thoroughbreds residing on a farm on which the virus was endemic

OBJECTIVE

To determine the incidence of equine herpesvirus-1 (EHV-1) infection among Thoroughbreds residing on a farm on which the virus was known to be endemic.

DESIGN

Prospective cohort study.

ANIMALS

10 nonpregnant mares, 8 stallions, 16 weanlings, 11 racehorses, and 30 pregnant mares and their foals born during the 2006 foaling season.

PROCEDURES

Blood and nasopharygeal swab samples were collected every 3 to 5 weeks for 9 months, and placenta and colostrum samples were collected at foaling. All samples were submitted for testing for EHV-1 DNA with a PCR assay. A type-specific EHV-1 ELISA was used to determine antibody titers in mares and foals at birth, 12 to 24 hours after birth, and every 3 to 5 weeks thereafter.

RESULTS

Results of the PCR assay were positive for only 4 of the 1,330 samples collected (590 blood samples, 590 nasopharyngeal swab samples, 30 placentas, and 30 colostrum samples), with EHV-1 DNA detected in nasal secretions from 3 horses (pregnant mare, stallion, and racehorse) and in the placenta from 1 mare. Seroconversion was detected in 3 of 27 foals during the first month of life.

CONCLUSIONS AND CLINICAL RELEVANCE

Results suggested that there was a low prevalence of EHV-1 infection among this population of Thoroughbreds even though the virus was known to be endemic on the farm and that pregnant mares could become infected without aborting. Analysis of nasopharyngeal swab samples appeared to be more sensitive than analysis of blood samples for detection of EHV-1 DNA.

Evidence-Based Immunization in Horses

Evidence of vaccine efficacy is essential for practitioners when giving advice to clients about the relative merits of different vaccines or when trying to evaluate the economic benefits of instituting a vaccine program. In equine veterinary medicine, this sort of data, which are necessary to make informed decisions about vaccine use and effectiveness, are often not available. Veterinarians need to consider the epidemiology of the disease in question, the type of vaccine that they are administering to the animal, the immunologic constraints of the vaccine technology, and the available evidence of efficacy when they are evaluating which vaccine to use or whether to vaccinate at all.

Equine herpesvirus-1-specific interferon gamma (IFNgamma) synthesis by peripheral blood mononuclear cells in thoroughbred horses

REASONS FOR PERFORMING STUDY

An assay has been developed that measures EHV-1 specific interferon gamma synthesis (IFNgamma), a cytokine produced following the activation of memory T lymphocytes and therefore a measure of cell mediated immunity. The method requires validation in the field.

OBJECTIVES

To measure the frequency of EHV-1 specific, IFNgamma synthesising peripheral blood mononuclear cells (PBMC) in a population of Thoroughbred horses, and examine its relationship with age, gender, premises and history of vaccination or field infection with EHV-1.

METHODS

Lymphocytes from 200 Thoroughbred horses were stimulated with EHV-1 in vitro, and IFNgamma detected using a monoclonal antibody and indirect immunofluorescence. Percent positive cells were enumerated by flow cytometric analysis and the results described and compared statistically between groups.

RESULTS

The frequency of IFNgamma+ PBMC was significantly higher in animals age >5 years compared with 2-4 years, in females vs. males, on stud farms vs. training yards and following vaccination of 2-year-olds with inactivated virus compared with nonvaccinates. Age strongly confounded all these associations and care must therefore be taken interpreting these results. Mares exposed to a field infection with EHV-1 also had higher frequencies of IFNgamma+ PBMC than other vaccinated horses.

CONCLUSIONS

The frequency of EHV-1 specific, IFNgama+ PBMC among the sample Thoroughbred population was diverse but lowest in young, unvaccinated horses-in-training.

POTENTIAL RELEVANCE

The frequency of EHV-1 specific lymphocytes synthesising IFNgamma in this population may be associated with its susceptibility to infection with this virus. This easy technique may be applied to monitor the antigenicity of vaccines and their effectiveness at stimulating cellular immunity.

Frequency and phenotype of EHV-1 specific, IFN-gamma synthesising lymphocytes in ponies: the effects of age, pregnancy and infection

Equine herpesvirus-1 (EHV-1) infects horses, causing acute respiratory disease, neurological signs, and is also a leading cause of abortion. Protection from EHV-1 infection and disease depends on both humoral (virus neutralising antibody) and cellular (mainly cytotoxic T lymphocytes, CTL) immune responses. CTL activity after EHV-1 infection has been extensively investigated and is closely associated with an alternative measure of cell mediated immunity (CMI), interferon-gamma (IFN-gamma) synthesis. This study investigates EHV-1-specific IFN-gamma synthesising cells in potentially immunocompromised horses; foals, pregnant mares and aged animals, after field or experimental infection with EHV-1. In foals and pregnant mares, the kinetics after experimental infection were similar and the phenotype of IFN-gamma+ synthesising cells after EHV-1 stimulation was mainly CD8alpha+. In contrast, in samples collected from primed healthy ponies exposed to EHV-1 several months previously or in old ponies (28 years old), the majority of EHV-1-specific IFN-gamma+ lymphocytes expressed a CD5+, CD8alpha- phenotype. This study highlights the complexity of the relationship between EHV-1, a common pathogen in horses, and the virus-specific cellular immune response as measured using IFN-gamma synthesis.

Meningitis and Encephalomyelitis in Horses

This article provides an overview of meningitis and encephalomyelitis in horses, including diagnostic tests, treatment developments, and preventative measures reported in the equine and human medical literature of the past few years.

Equine herpesvirus type 1 modified live virus vaccines: quo vaditis?

Infections of horses with equine herpesvirus type 1 (EHV-1) have garnered new attention over the last few years. Devastating outbreaks occurring worldwide, primarily of the neurologic form of the disease, have resulted in a reassessment of the control strategies, and particularly the prophylactic measures, that are necessary to keep the infection and spread of disease in check. Most of the available EHV-1 vaccines are based on preparations of inactivated virus, which are applied monovalently for prevention of EHV-1-caused abortion in pregnant mares or as part of multivalent vaccines to prevent respiratory disease. Despite the importance of an induction of cytotoxic immune responses for protection against EHV-1-induced disease, only two modified live virus vaccine preparations, which are both based on the avirulent EHV-1 strain RacH and were developed more than 40 years ago, are commercially available. Current efforts focus on exploiting the available infectious bacterial artificial chromosome clones of various EHV-1 strains to engineer a new generation of modified live virus vaccines. Both more efficient and long-lasting anti-EHV-1 immunity and delivery of immunogens of other pathogens are attempted and within immediate reach. The improvement of modified live virus vaccines will likely be a major focus of research in the future, and will hopefully help to more completely protect horses against one of the most important and devastating viral diseases.

The equine immune response to equine herpesvirus-1: The virus and its vaccines

Equine herpesvirus-1 (EHV-1) is an alphaherpesvirus which infects horses, causing respiratory and neurological disease and abortion in pregnant mares. Latency is established in trigeminal ganglia and lymphocytes. Immunity to EHV-1 lasts between 3 and 6 months. Current vaccines, many of which contain inactivated virus, have reduced the incidence of abortion storms in pregnant mares but individual animals, which may be of high commercial value, remain susceptible to infection. The development of effective vaccines which stimulate both humoral and cellular immune responses remains a priority. Utilising data generated following experimental and field infections of the target species, this review describes the immunopathogenesis of EHV-1 and the interaction between the horse's immune system and this virus, both in vivo and in vitro, and identifies immune responses, highlighting those which have been associated with protective immunity. It then goes on to recount a brief history of vaccination, outlines factors likely to influence the outcome of vaccine administration and describes the immune response stimulated by a selection of commercial and experimental vaccines. Finally, based on the available data, a rational strategy designed to stimulate protective immune responses by vaccination is outlined.

Antibody and cellular immune responses following DNA vaccination and EHV-1 infection of ponies

Equine herpesvirus-1 (EHV-1) is the cause of serious disease with high economic impact on the horse industry, as outbreaks of EHV-1 disease occur every year despite the frequent use of vaccines. Cytotoxic T-lymphocytes (CTLs) are important for protection from primary and reactivating latent EHV-1 infection. DNA vaccination is a powerful technique for stimulating CTLs, and the aim of this study was to assess antibody and cellular immune responses and protection resulting from DNA vaccination of ponies with combinations of EHV-1 genes. Fifteen ponies were divided into three groups of five ponies each. Two vaccination groups were DNA vaccinated on four different occasions with combinations of plasmids encoding the gB, gC, and gD glycoproteins or plasmids encoding the immediate early (IE) and early proteins (UL5) of EHV-1, using the PowderJect XR research device. Total dose of DNA/plasmid/vaccination were 25 microg. A third group comprised unvaccinated control ponies. All ponies were challenge infected with EHV-1 6 weeks after the last vaccination, and protection from clinical disease, viral shedding, and viremia was determined. Virus neutralizing antibodies and isotype specific antibody responses against whole EHV-1 did not increase in either vaccination group in response to vaccination. However, glycoprotein gene vaccinated ponies showed gD and gC specific antibody responses. Vaccination did not affect EHV-1 specific lymphoproliferative or CTL responses. Following challenge infection with EHV-1, ponies in all three groups showed clinical signs of disease. EHV-1 specific CTLs, proliferative responses, and antibody responses increased significantly in all three groups following challenge infection. In summary, particle-mediated EHV-1 DNA vaccination induced limited immune responses and protection. Future vaccination strategies must focus on generating stronger CTL responses.

Immune responses to commercial equine vaccines against equine herpesvirus-1, equine influenza virus, eastern equine encephalomyelitis, and tetanus

Horses are commonly vaccinated to protect against pathogens which are responsible for diseases which are endemic within the general horse population, such as equine influenza virus (EIV) and equine herpesvirus-1 (EHV-1), and against a variety of diseases which are less common but which lead to greater morbidity and mortality, such as eastern equine encephalomyelitis virus (EEE) and tetanus. This study consisted of two trials which investigated the antigenicity of commercially available vaccines licensed in the USA to protect against EIV, EHV-1 respiratory disease, EHV-1 abortion, EEE and tetanus in horses. Trial I was conducted to compare serological responses to vaccines produced by three manufacturers against EIV, EHV-1 (respiratory disease), EEE, and tetanus given as multivalent preparations or as multiple vaccine courses. Trial II compared vaccines from two manufacturers licensed to protect against EHV-1 abortion, and measured EHV-1-specific interferon-gamma (IFN-gamma) mRNA production in addition to serological evidence of antigenicity. In Trial I significant differences were found between the antigenicity of different commercial vaccines that should be considered in product selection. It was difficult to identify vaccines that generate significant immune responses to respiratory viruses. The most dramatic differences in vaccine performance occurred in the case of the tetanus antigen. In Trial II both vaccines generated significant antibody responses and showed evidence of EHV-1-specific IFN-gamma mRNA responses. Overall there were wide variations in vaccine response, and the vaccines with the best responses were not produced by a single manufacturer. Differences in vaccine performance may have resulted from differences in antigen load and adjuvant formulation.