A review of equid herpesvirus 1 for the veterinary practitioner. Part B: pathogenesis and epidemiology

Pharmacologic interventions for the treatment of equine herpesvirus-1 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.

REVIEW QUESTION

Does pharmacological therapy decrease either the incidence or severity of disease or infection caused by EHV-1 in domesticated horses?

METHODS

A systematic review was preformed searching AGRICOLA, CAB Abstracts, Cochrane, PubMed, Web of Science, and WHO Global Health Index Medicus Regional Databases to identify articles published before February 15, 2021. Selection criteria were original research reports published in peer reviewed journals, and studies investigating in vivo use of therapeutic agents for prevention or treatment of EHV-1 in horses. Outcomes assessed included measures related to clinical outcomes that reflect symptomatic EHV-1 infection or virus infection. We evaluated risk of bias and performed a GRADE evaluation of the quality of evidence for interventions.

RESULTS

A total of 7009 unique studies were identified, of which 9 met the inclusion criteria. Two studies evaluated valacyclovir or small interfering RNAs, and single studies evaluated the use of a Parapoxvirus ovis-based immunomodulator, human alpha interferon, an herbal supplement, a cytosine analog, and heparin. The level of evidence ranged between randomized controlled studies and observational trials. The risk of bias was moderate to high and sample sizes were small. Most studies reported either no benefit or minimal efficacy of the intervention tested.

CONCLUSIONS AND CLINICAL IMPORTANCE

Our review indicates minimal or limited benefit either as a prophylactic or post-exposure treatment for any of the studied interventions in the mitigation of EHV-1-associated disease outcome.

Prevalence of Latent Equid Herpesvirus Type 1 in Submandibular Lymph Nodes of Horses in Virginia

Equine Herpesvirus type 1 (EHV-1) typically causes mild respiratory disease, but it can also cause late-term abortion, neonatal foal death and neurologic disease. Once a horse is infected, the virus concentrates to local lymphoid tissue, where it becomes latent. The virus can be reactivated during times of stress, which can lead to the initiation of devastating outbreaks. Understanding the carriage rate of latent EHV-1 in different geographic regions is essential for managing the disease. The objective of the current study was to estimate the prevalence of latent EHV-1 and compare the frequency of each variant in the submandibular lymph nodes of horses in Virginia. Sixty-three submandibular lymph nodes were collected post-partem from horses submitted to regional labs for necropsy, and qPCR was performed. All samples were negative for the gB gene of EHV-1. The results demonstrated a low apparent prevalence of latent EHV-1 DNA in submandibular lymph nodes in this population of horses in Virginia. Despite this, the mainstay for outbreak prevention and mitigation continues to focus on minimizing risks and using appropriate and diligent biosecurity.

Characterizing the Pathogenesis and Immune Response of Equine Herpesvirus 8 Infection in Lung of Mice

Simple Summary

Equine herpesvirus 8 (EHV-8) is an important pathogen primarily affecting the horse and donkey industry, but there is little information about the pathogenicity and immune response of EHV-8 in a mouse model. We aim to investigate the pathogenicity and immune response in the lung during EHV-8 infection in BALB/c mice. The results showed that EHV-8 could effectively replicate and elicits a strong proinflammatory response in the lung tissues of a mouse model. The mouse model of viral respiratory disease proposed here will also be useful for studying the underlying mechanisms of the pathology of respiration.

Abstract

Equine herpesvirus type 8 (EHV-8), associated with abortion and severe respiratory disease in donkeys and horses, causes significant economic losses in the global equine industry. However, the pathogenicity of EHV-8 is still unknown. Mice are widely used as an animal model to evaluate virus replication and virulence. The present study aimed to evaluate the pathogenicity of the EHV-8 SDLC66 strain in BALB/c mice. Mice were used to test for infection-associated parameters (such as clinical signs, body weights, virus replication in tissues, viremia, and cytokines) and sacrificed at 0, 2, 4, and 6 days post-infection (dpi). The mice inoculated with EHV-8 exhibited lethargy, dyspnea signs, loss in body weight, and viremia. EHV-8 was detected in the liver, spleen, brain, and lung by PCR at 4 dpi and 6 dpi, effectively replicating these tissues detected by TCID₅₀ at 6 dpi. Proinflammatory cytokines, including IL-6, IL-1β, and TNF-α, were significantly increased at the 4 dpi and 6 dpi in the lung than in the control group. However, IFN-γ was only increased at 6 dpi in the EHV-8-infected group. These data showed that EHV-8 could enter the lungs of mice and cause respiratory disease in the mouse model, which helps reveal the pathogenicity of EHV-8.

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.

Isothermal Nucleic Acid Amplification Technologies for the Detection of Equine Viral Pathogens

Simple Summary

Equine viral diseases remain a prominent concern for human and equine health globally. Many of these viruses are of primary biosecurity concern to countries that import equines where these viruses are not present. In addition, several equine viruses are zoonotic, which can have a significant impact on human health. Current diagnostic techniques are both time consuming and laboratory-based. The ability to accurately detect diseases will lead to better management, treatment strategies, and health outcomes. This review outlines the current modern isothermal techniques for diagnostics, such as loop-mediated isothermal amplification and insulated isothermal polymerase chain reaction, and their application as point-of-care diagnostics for the equine industry.

Abstract

The global equine industry provides significant economic contributions worldwide, producing approximately USD $300 billion annually. However, with the continuous national and international movement and importation of horses, there is an ongoing threat of a viral outbreak causing large epidemics and subsequent significant economic losses. Additionally, horses serve as a host for several zoonotic diseases that could cause significant human health problems. The ability to rapidly diagnose equine viral diseases early could lead to better management, treatment, and biosecurity strategies. Current serological and molecular methods cannot be field-deployable and are not suitable for resource-poor laboratories due to the requirement of expensive equipment and trained personnel. Recently, isothermal nucleic acid amplification technologies, such as loop-mediated isothermal amplification (LAMP) and insulated isothermal polymerase chain reaction (iiPCR), have been developed to be utilized in-field, and provide rapid results within an hour. We will review current isothermal diagnostic techniques available to diagnose equine viruses of biosecurity and zoonotic concern and provide insight into their potential for in-field deployment.

Epidemiology of Chlamydia psittaci infections in pregnant Thoroughbred mares and foals

Late-term foal loss due to the traditional avian pathogen Chlamydia psittaci recently emerged as a threat to the Australian Thoroughbred industry. A longitudinal study of 14 stud farms was undertaken to better understand C. psittaci infection in pregnant mares and their foals by evaluating C. psittaci prevalence, equine herpesvirus-1 (EHV-1) co-infection, avian reservoirs, and potential risk factors. Mucosal swabs taken from 228 healthy pregnant mares and their foals were tested for C. psittaci and EHV-1 using species-specific qPCR assays. No foal loss was recorded due to either pathogen, and no mare tested positive to either C. psittaci or EHV-1. However, healthy newborn foals tested positive to both pathogens, at low levels, with 13.2% (n = 30/228) and 14.5% (n = 33/228) prevalence for C. psittaci and EHV-1, respectively. Co-infection occurred in 1.3% (n = 3/228) of foals. In avian environmental faecal samples collected from the same studs, C. psittaci was detected at 5.3% (n = 5/94). Multiple logistic regression modelling found that foals born in winter were more likely to be infected with C. psittaci (adjusted odds ratio = 15.83; P < 0.001; Confidence Interval 5.12-48.49). Being a maiden mare, absence of prophylactic vaginal suture, interventions in the last trimester and residing on a farm with prior history of C. psittaci abortion posed no higher risk to infection in the newborn. Analysis of all reported C. psittaci abortion cases (Hunter Valley, 2016-2019) revealed a dominant C. psittaci sequence type (denoted ST24) and a significant correlation with frost events (Spearmans' rho = 0.44; P = 0.002).

Fetal lesions of EHV-1 in equine

EHV-1 infection is responsible for huge economic losses in equines due to abortion and neonatal mortality. In this study, we describe 4 cases of abortion and neonatal deaths from pregnant mares and a she-donkey from different localities in Egypt during the period from May 2015 to October 2017. Attempts were made to isolate and identify EHV-1, in addition to compare the different pathological lesions in various tissues of the necropsied cases. EHV-1 was successfully isolated from two aborted fetuses and one dead neonatal foal from mares, beside one aborted fetus from a she-donkey. The positive cases showed cytopathic effect on embryonated chicken eggs scattered on chorioallantoic membrane. Moreover, PCR was applied for the pock lesions and revealed positive results for EHV-1. Interstitial pneumonia, bronchopneumonia and necrosis of hepatic, myocardial, microcotyledonary tissues besides disseminated thrombi were the main encountered lesions. Intranuclear inclusion bodies were demonstrated in brain, liver, placenta and pulmonary tissues. Here, we describe EHV-1 induced brain lesions represented by degenerated neurons, vascular endotheliosis with intranuclear inclusion bodies in the aborted she-donkey fetus. Lesions were more sever in the aborted fetuses from mares than the one from the she-donkey. EHV-1 antigen was detected by immunohistochemistry staining.

Comparative Study of the Pathogenesis of Rhinopneumonitis Induced by Intranasal Inoculation of Hamsters with Equine Herpesvirus-9, Equine Herpesvirus-1 strain Ab4p and Zebra-borne Equine Herpesvirus-1

Equine herpesvirus-9 (EHV-9), equine herpesvirus-1 (EHV-1) and zebra-borne EHV-1 are members of the family Herpesviridae and cause encephalitis and rhinopneumonitis in a range of animal species. The aim of this study was to characterize and compare the rhinopneumonitis induced by experimental intranasal inoculation of groups of hamsters with EHV-9, EHV-1 strain Ab4p or zebra-borne EHV-1 viruses. Animals inoculated with EHV-9 had earlier and more severe neurological and respiratory signs than those inoculated with EHV-1 strain Ab4p or zebra-borne EHV-1. At 4-5 days post inoculation (dpi), hamsters inoculated with EHV-9 had significantly increased expression of open reading fame (ORF) 30, the viral gene encoding the DNA polymerase, in lung tissue. ORF 30 expression at these time points was higher in the hamsters infected with EHV-9 than in those inoculated with the other two viruses. Severe, mild or very mild rhinitis was seen in animals inoculated with EHV-1 strain Ab4p, EHV-9 and zebra-borne EHV-1, respectively. Viral antigen was detected in olfactory receptor neurons, inflammatory cells and desquamated epithelial cells in animals in all groups until 5 dpi. Tracheitis was also seen in all three virus-infected groups with viral antigen detected in tracheal epithelium. Inoculated hamsters developed interstitial pneumonia of increasing severity over the course of the experiment. Bronchopneumonia and vasculitis were also seen in all three infected groups. These results confirm that, in addition to their neurotropism, EHV-9 and zebra-borne EHV-1 are pneumotropic viruses. EHV-1 strain Ab4p caused more severe upper respiratory tract disease, but no significant differences were detected in the severity of pneumonia induced by each virus.

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.

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.

Noninvasive Detection of Equid Herpesviruses in Fecal Samples

Equid herpesviruses (EHVs) are pathogens of equid and nonequid hosts that can cause disease and fatalities in captivity and in the wild. EHVs establish latent infections but can reactivate, and most EHVs are shed via the nasal passage. Therefore, nasal swabs are generally used for EHV monitoring. However, invasive sampling of wild equids is difficult. While feces is a commonly used substrate for detecting other pathogens, to our knowledge, EHVs have never been detected in feces of naturally infected equids. We systematically tested zebra feces for EHV presence by (i) establishing nested PCR conditions for fecal DNA extracts, (ii) controlling for environmental EHV contamination, and (iii) large-scale testing on a free-ranging zebra population. A dilution minimizing inhibition while maximizing viral DNA concentrations was determined in captive Grévy's zebra (Equus grevyi) fecal samples from individuals shedding EHV nasally. Sixteen of 42 fecal samples (38%) were EHV positive. To demonstrate that the EHV positivity was not a result of environmental contamination, rectal swabs of wild zebras were screened (n = 18 [Equus quagga and E. zebra]), and 50% were EHV positive, indicating that the source of EHV in feces is likely the intestinal mucosa and not postdefecation contamination. Out of 270 fecal samples of wild zebras, 26% were EHV positive. Quantitative PCRs showed that the amount of virus DNA in feces was not significantly smaller than that in other samples. In summary, fecal sampling facilitates large-scale screening and may be useful to noninvasively investigate phylogenetic EHV diversity in wild and domestic equids.IMPORTANCE Equid herpesviruses (EHVs) establish latent infections, and many EHVs are shed and transmitted via nasal discharge primarily through droplet and aerosol infection. Obtaining nasal swabs and other invasive samples from wildlife is often not possible without capture and physical restraint of individuals, which are resource intensive and a health risk for the captured animals. Fecal EHV shedding has never been demonstrated for naturally infected equids. We established the conditions for fecal EHV screening, and our results suggest that testing fecal samples is an effective noninvasive approach for monitoring acute EHV shedding in equids.

Environmental stressors may cause equine herpesvirus reactivation in captive Grévy's zebras (Equus grevyi)

Equine Herpesviruses (EHV) are common and often latent pathogens of equids which can cause fatalities when transmitted to non-equids. Stress and elevated glucocorticoids have been associated with EHV reactivation in domestic horses, but little is known about the correlation between stress and viral reactivation in wild equids. We investigated the effect of an environmental stressor (social group restructuring following a translocation event) on EHV reactivation in captive Grévy's zebras (Equus grevyi). A mare was translocated by road transport from Zoo Mulhouse, France, to join a resident group of three mares in Tierpark Berlin, Germany. We used an indirect sampling method to assess the frequency of EHV shedding for 14 days immediately after the translocation event (termed the 'experimental period'). The results were compared with those from two control periods, one preceding and one subsequent to the experimental period. In addition, we measured fecal glucocorticoid metabolite (fGCM) concentrations daily in all individuals from 6 days before, to 14 days after translocation. We found significantly higher EHV shedding frequencies during the experimental period, compared to each of the two control periods. All animals showed significantly elevated fGCM concentrations, compared to fGCM levels before translocation. Finally, we found that an increase in fGCM concentration was significantly associated with an increased likelihood of EHV shedding. Although the small number of animals in the study limits the conclusions that can be drawn from the study, taken together, our results support the hypothesis that environmental stressors induce viral reactivation in wild equids. Our results suggest that potentials stressors such as group restructuring and translocation should be considered in the management of zoological collections to reduce the risk of fatal EHV infections in novel hosts. Moreover, environmental stressors may play an important role in EHV reactivation and spread in wild equid populations.

Equine behavioral enrichment toys as tools for non-invasive recovery of viral and host DNA

Direct collection of samples from wildlife can be difficult and sometimes impossible. Non-invasive remote sampling for the purpose of DNA extraction is a potential tool for monitoring the presence of wildlife at the individual level, and for identifying the pathogens shed by wildlife. Equine herpesviruses (EHV) are common pathogens of equids that can be fatal if transmitted to other mammals. Transmission usually occurs by nasal aerosol discharge from virus-shedding individuals. The aim of this study was to validate a simple, non-invasive method to track EHV shedding in zebras and to establish an efficient protocol for genotyping individual zebras from environmental DNA (eDNA). A commercially available horse enrichment toy was deployed in captive Grévy's, mountain, and plains zebra enclosures and swabbed after 4-24 hr. Using eDNA extracted from these swabs four EHV strains (EHV-1, EHV-7, wild ass herpesvirus and zebra herpesvirus) were detected by PCR and confirmed by sequencing, and 12 of 16 zebras present in the enclosures were identified as having interacted with the enrichment toy by mitochondrial DNA amplification and sequencing. We conclude that, when direct sampling is difficult or prohibited, non-invasive sampling of eDNA can be a useful tool to determine the genetics of individuals or populations and for detecting pathogen shedding in captive wildlife.

Antiherpetic Drugs in Equine Medicine

Since vaccination may not prevent disease, antiherpetic drugs have been investigated for the therapy of several equine herpesviruses. Drug efficacy has been assessed in horses with disease, but most evidence is in vitro, in other species, or empirical. Oral valacyclovir is most often administered in the therapy of equine herpesvirus type-1 (EHV-1) to protect adult horses from equine herpesvirus myeloencephalopathy, while oral acyclovir is frequently administered for EHV-5 infection in the therapy of equine multinodular pulmonary fibrosis. Other antiherpetic drugs are promising but require further investigation. Several topical drugs are also empirically used in the therapy of equine viral keratitis.

Zebra Alphaherpesviruses (EHV-1 and EHV-9): Genetic Diversity, Latency and Co-Infections

Alphaherpesviruses are highly prevalent in equine populations and co-infections with more than one of these viruses’ strains frequently diagnosed. Lytic replication and latency with subsequent reactivation, along with new episodes of disease, can be influenced by genetic diversity generated by spontaneous mutation and recombination. Latency enhances virus survival by providing an epidemiological strategy for long-term maintenance of divergent strains in animal populations. The alphaherpesviruses equine herpesvirus 1 (EHV-1) and 9 (EHV-9) have recently been shown to cross species barriers, including a recombinant EHV-1 observed in fatal infections of a polar bear and Asian rhinoceros. Little is known about the latency and genetic diversity of EHV-1 and EHV-9, especially among zoo and wild equids. Here, we report evidence of limited genetic diversity in EHV-9 in zebras, whereas there is substantial genetic variability in EHV-1. We demonstrate that zebras can be lytically and latently infected with both viruses concurrently. Such a co-occurrence of infection in zebras suggests that even relatively slow-evolving viruses such as equine herpesviruses have the potential to diversify rapidly by recombination. This has potential consequences for the diagnosis of these viruses and their management in wild and captive equid populations.

Immunization with Attenuated Equine Herpesvirus 1 Strain KyA Induces Innate Immune Responses That Protect Mice from Lethal Challenge

Equine herpesvirus 1 (EHV-1) is a major pathogen affecting equines worldwide. The virus causes respiratory disease, abortion, and, in some cases, neurological disease. EHV-1 strain KyA is attenuated in the mouse and equine, whereas wild-type strain RacL11 induces severe inflammation of the lung, causing infected mice to succumb at 4 to 6 days postinfection. Our previous results showed that KyA immunization protected CBA mice from pathogenic RacL11 challenge at 2 and 4 weeks postimmunization and that KyA infection elicited protective humoral and cell-mediated immune responses. To investigate the protective mechanisms of innate immune responses to KyA, KyA-immunized mice were challenged with RacL11 at various times postvaccination. KyA immunization protected mice from RacL11 challenge at 1 to 7 days postimmunization. Immunized mice lost less than 10% of their body weight and rapidly regained weight. Virus titers in the lungs of KyA-immunized mice were 1,000-fold lower at 2 days post-RacL11 challenge than virus titers in the lungs of nonimmunized mice, indicating accelerated virus clearance. Affymetrix microarray analysis revealed that gamma interferon (IFN-γ) and 16 antiviral interferon-stimulated genes (ISGs) were upregulated 3.1- to 48.2-fold at 8 h postchallenge in the lungs of RacL11-challenged mice that had been immunized with KyA. Murine IFN-γ inhibited EHV-1 infection of murine alveolar macrophages and protected mice against lethal EHV-1 challenge, suggesting that IFN-γ expression is important in mediating the protection elicited by KyA immunization. These results suggest that EHV-1 KyA may be used as a live attenuated EHV-1 vaccine as well as a prophylactic agent in horses.

IMPORTANCE

Viral infection of cells initiates a signal cascade of events that ultimately attempts to limit viral replication and prevent infection through the expression of host antiviral proteins. In this study, we show that EHV-1 KyA immunization effectively protected CBA mice from pathogenic RacL11 challenge at 1 to 7 days postvaccination and increased the expression of IFN-γ and 16 antiviral interferon-stimulated genes (ISGs). The administration of IFN-γ blocked EHV-1 replication in murine alveolar macrophages and mouse lungs and protected mice from lethal challenge. To our knowledge, this is the first report of an attenuated EHV-1 vaccine that protects the animal at 1 to 7 days postimmunization by innate immune responses. Our findings suggested that IFN-γ serves as a novel prophylactic agent and may offer new strategies for the development of anti-EHV-1 agents in the equine.

The effect of gold nanoparticles on the diagnostic polymerase chain reaction technique for equine herpes virus 1 (EHV-1)

We experimented the effect of 15 nm unmodified citrate coated GNPs on the key PCR reactants to see if these would enhance the overall outcomes of the reaction. Thus, the optimized GNPs-assisted PCR could be used for more efficient diagnosis of EHV-1.

Effect of equine herpesvirus type 1 (EHV-1) infection of nasal mucosa epithelial cells on integrin alpha 6 and on different components of the basement membrane

The respiratory mucosa is the common port of entry of equine herpesvirus type 1 (EHV-1) and several other alphaherpesviruses. An important prerequisite for successful host invasion of the virus is to cross the epithelial cell layer and the underlying basement membrane barrier. In the present study, an analysis was performed to see if an EHV-1 infection of nasal mucosa epithelial cells leads to damage of the underlying extracellular matrix proteins. Nasal mucosa explants were inoculated with EHV-1 and collected at 0, 24 and 48 hours post-inoculation (hpi). Then, double immunofluorescence staining was performed to detect viral-antigen-positive cells on the one hand and integrin alpha 6, laminin, collagen IV and collagen VII on the other hand. The area of these extracellular matrix proteins was measured in regions of interest (ROIs) at a magnification of 200X by means of the software imaging system ImageJ. ROIs were defined beneath uninfected and infected regions. In uninfected regions, 22-28 % of the ROI was stained for integrin alpha 6, 18-37 % for laminin, 14-38 % for collagen IV and 18-26 % for collagen VII. In infected regions, the percentage positive for integrin alpha 6 was significantly decreased to 0.1-9 % and 0.1-6 % after 24 and 48 hours of inoculation, respectively. Infection did not alter the percentages for laminin and collagen IV. For collagen VII, an increase in the percentage (from 18-26 % to 28-39 %) could be observed underneath EHV-1-infected plaques at 48 hours of inoculation. In conclusion, the results revealed a substantial impact of EHV-1 infection on integrin alpha 6 and collagen VII, two important components of the extracellular matrix, which are associated with the basement membrane and may facilitate virus penetration via hijacked leukocytes to underlying tissues.

Equid herpesvirus 1 (EHV1) infection of equine mesenchymal stem cells induces a pUL56-dependent downregulation of select cell surface markers

Equid herpesvirus 1 (EHV1) is an ubiquitous alphaherpesvirus that can cause respiratory disease, abortion and central nervous disorders. EHV1 is known to infect a variety of different cell types in vitro, but its tropism for cultured primary equine mesenchymal stem cells (MSC) has never been explored. We report that equine MSC were highly permissive for EHV1 and supported lytic replication of the virus in vitro. Interestingly, we observed that an infection of MSC with EHV1 resulted in a consistent downregulation of cell surface molecules CD29 (β1-integrin), CD105 (endoglin), major histocompatibility complex type I (MHCI) and a variable downregulation of CD172a. In contrast, expression of CD44 and CD90 remained unchanged upon wild type infection. In addition, we found that this selective EHV1-mediated downregulation of cell surface proteins was dependent on the viral protein UL56 (pUL56). So far, pUL56-dependent downregulation during EHV1 infection of equine cells has only been described for MHCI, but our present data indicate that pUL56 may have a broader function in downregulating cell surface proteins. Taken together, our results are the first to show that equine MSC are susceptible for EHV1 and that pUL56 induces downregulation of several cell surface molecules on infected cells. These findings provide a basis for future studies to evaluate the mechanisms underlying for this selective pUL56-induced downregulation and to evaluate the potential role of MSC during EHV1 pathogenesis.

The equine immune responses to infectious and allergic disease: a model for humans?

The modern horse, Equus caballus has historically made important contributions to the field of immunology, dating back to Emil von Behring's description of curative antibodies in equine serum over a century ago. While the horse continues to play an important role in human serotherapy, the mouse has replaced the horse as the predominant experimental animal in immunology research. Nevertheless, continuing efforts have led to an improved understanding of the equine immune response in a variety of infectious and non-infectious diseases. Based on this information, we can begin to identify specific situations where the horse may provide a unique immunological model for certain human diseases.

Equine herpesvirus type 1 replication is delayed in CD172a+ monocytic cells and controlled by histone deacetylases

Equine herpesvirus type 1 (EHV-1) replicates in the epithelial cells of the upper respiratory tract and disseminates through the body via a cell-associated viraemia in monocytic cells, despite the presence of neutralizing antibodies. However, the mechanism by which EHV-1 hijacks immune cells and uses them as 'Trojan horses' in order to disseminate inside its host is still unclear. Here, we hypothesize that EHV-1 delays its replication in monocytic cells in order to avoid recognition by the immune system. We compared replication kinetics in vitro of EHV-1 in RK-13, a cell line fully susceptible to EHV-1 infection, and primary horse cells from the myeloid lineage (CD172a(+)). We found that EHV-1 replication was restricted to 4 % of CD172a(+) cells compared with 100 % in RK-13 cells. In susceptible CD172a(+) cells, the expression of immediate-early (IEP) and early (EICP22) proteins was delayed in the cell nuclei by 2-3 h post-infection (p.i.) compared with RK-13, and the formation of replicative compartments by 15 h p.i. Virus production in CD172a(+) cells was significantly lower (from 10(1.7) to 10(3.1) TCID50 per 10(5) inoculated cells) than in RK-13 (from 10(5) to 10(5.7) TCID50 per 10(5) inoculated cells). Less than 0.02 % of inoculated CD172a(+) cells produced and transmitted infectious virus to neighbouring cells. Pre-treatment of CD172a(+) cells with inhibitors of histone deacetylase activity increased and accelerated viral protein expression at very early times of infection and induced productive infection in CD172a(+) cells. Our results demonstrated that the restriction and delay of EHV-1 replication in CD172a(+) cells are part of an immune evasive strategy and involve silencing of EHV-1 gene expression associated with histone deacetylases.