Unmasking the ongoing challenge of equid herpesvirus- 1 (EHV-1): A comprehensive review

Management of an Equine Herpesvirus-1 Outbreak During a Multi-Week Equestrian Event

The present study reports on the management of an EHV-1 outbreak at a large, multi-week equestrian event with ongoing showing. Within a 48 h period, 8 horses out of a cohort of 38 horses from the same trainer displayed elevated rectal temperatures ranging from 38.4 to 39.0 °C. Initial testing using a point-of-care PCR assay detected EHV-1 in 2/8 horses, with the results being confirmed at a later time by qPCR. As a precautionary measure and because of the inability to isolate the entire at-risk population, the 38 horses were relocated to an equine facility outside the equestrian event for daily monitoring and weekly EHV-1 qPCR testing of nasal secretions. Overall, 22/38 (58%) horses tested EHV-1 qPCR-positive in nasal secretions over the monitoring period of 28 days, with only one additional horse developing fever. Once all 38 horses tested EHV-1 qPCR-negative twice, 7 days apart, 17 horses returned to the equestrian event to compete for the remaining 2 weeks of the circuit. The present study highlights the importance of isolating and testing horses with fever but also subfebrile horses, as EHV-1 can cause silent infection. The relocation of the exposed horses to an outside facility allowed close monitoring of these horses while reducing the risk of direct and indirect exposure to other show horses. The regular testing for EHV-1 through nasal secretions during the outbreak, coupled with proper biosecurity protocols, allowed the safe return of the show horses to the event. The key elements in reducing the spread of EHV-1 were the routine assessment of rectal temperature, early isolation of horses with elevated rectal temperature and on-site EHV-1 PCR testing.

Increase in Virus-Specific Mucosal Antibodies in the Upper Respiratory Tract Following Intramuscular Vaccination of Previously Exposed Horses Against Equine Herpesvirus Type-1/4

BACKGROUND/OBJECTIVES

Equine herpesvirus type-1 (EHV-1) enters through the upper respiratory tract (URT) and causes respiratory disease, abortions, and myeloencephalopathy in equids. Pre-existing immunity at the viral entry site, especially mucosal IgG4/7 antibodies, has recently been shown to correlate with protection from disease and incomplete viral replication at the URT. Here, we tested whether intramuscular (i.m.) vaccination with a commercial inactivated EHV-1/4 vaccine can induce mucosal antibodies (mucAbs) at the URT.

METHODS

Adult horses with complete EHV-1 vaccination and/or exposure histories were vaccinated i.m. six times within eight months. Before and after each vaccination, blood and nasal swab samples were obtained. Serum and mucAbs were measured in fluorescent bead-based EHV-1 assays.

RESULTS

All horses still had existing EHV-1 specific serum and mucAbs prior to vaccination, which were mainly composed of IgG4/7 antibody isotypes. Serum IgG4/7 significantly increased after the first vaccination and stayed high until the end of the study. An additional short-lasting serum IgG1 response was only induced by the first vaccine application. At the URT, mucAbs increased after five out of six i.m. vaccine injections. Like the systemic antibody response, mucAbs were dominated by IgG4/7 and a small IgG1 increase after the first vaccination.

CONCLUSIONS

Our data emphasize that robust EHV-1 specific mucAb levels are obtained after i.m. vaccination with the inactivated EHV-1/4 vaccine used here. The findings have important implications for evaluating EHV-1/4 vaccines for their ability to induce and maintain protective mucosal IgG4/7 antibodies.

The Genomic Characterization of Equid Alphaherpesviruses: Structure, Function, and Genetic Similarity

Equine herpesvirus 1 (EHV-1), EHV-4, EHV-8, and EHV-9, are classified within the subfamily Alphaherpesvirinae and are recognized as causative agents of respiratory, urogenital, and neurological disorders in horses. These viruses, collectively referred to as αEHVs, exhibits both unique and shared characteristics in terms of host interaction, pathogenesis, epidemiology, and immune evasion, which arise from both the identities and discrepancies among respective genomic homologs. The genomic architecture of αEHVs is similar to other members of the same subfamily, such as well-known HSV-1, VZV, and PRV. However, research on the molecular mechanisms underlying αEHV infection and immune response remains significantly less advanced compared to studies on human, porcine, and bovine herpesviruses. This paper systematically describes the genomic structure, function, and genetic similarities of αEHVs and conducts a comparative analysis of selected αEHVs through pairwise sequence alignments of nucleotides and amino acids. This review offers an extensive synthesis of the current understanding related to the study of αEHVs, highlighting the challenges and potential solutions for future research endeavors.

Equine Herpesvirus-1 Induced Respiratory Disease in Dezhou Donkey Foals: Case Study from China, 2024

Equine herpesvirus-1 (EHV-1) is a significant pathogen that causes substantial economic losses in the equine industry worldwide, which leads to severe respiratory diseases and abortions in horses. However, reports of EHV-1 infection in donkeys are limited, particularly in China. This case study reported an EHV-1-induced respiratory disease in Dezhou donkey foals in Shandong Province, China, in July 2024. Three one-month-old foals exhibited high fever, nasal discharge, and respiratory distress, with a 100% mortality rate. The causative agent, strain LC126, was isolated from a one-month-old donkey foal exhibiting severe respiratory disease. Phylogenetic analysis of the EHV-1 isolate LC126 showed close similarity to EHV-1. Overall, our study revealed that EHV-1 can cause respiratory distress as well as death in donkeys. The study underscores the emerging threat of EHV-1 in donkeys and highlights the need for veterinarians and breeders to give proper attention to the potential threat of EHV-1 outbreaks.

Alphaherpesvirus in Pets and Livestock

Herpesviruses are a group of DNA viruses capable of infecting multiple mammalian species, including humans. This review primarily summarizes four common alphaherpesviruses found in pets and livestock (feline, swine, canine, and bovine) in aspects such as epidemiology, immune evasion, and latency and reactivation. Despite the fact that they primarily infect specific hosts, these viruses have the potential for cross-species transmission due to genetic mutations and/or recombination events. During infection, herpesviruses not only stimulate innate immune responses in host cells but also interfere with signaling pathways through specific proteins to achieve immune evasion. These viruses can remain latent within the host for extended periods and reactivate under certain conditions to trigger disease recurrence. They not only affect the health of animals and cause economic losses but may also pose a potential threat to humans under certain circumstances. This review deepens our understanding of the biological characteristics of these animal alphaherpesviruses and provides an important scientific basis for the prevention and control of related diseases.

Breaking Latent Infection: How ORF37/38-Deletion Mutants Offer New Hope against EHV-1 Neuropathogenicity

Equid alphaherpesvirus 1 (EHV-1) has been linked to the emergence of neurological disorders, with the horse racing industry experiencing significant impacts from outbreaks of equine herpesvirus myeloencephalopathy (EHM). Building robust immune memory before pathogen exposure enables rapid recognition and elimination, preventing infection. This is crucial for effectively managing EHV-1. Removing neuropathogenic factors and immune evasion genes to develop live attenuated vaccines appears to be a successful strategy for EHV-1 vaccines. We created mutant viruses without ORF38 and ORF37/38 and validated their neuropathogenicity and immunogenicity in hamsters. The ∆ORF38 strain caused brain tissue damage at high doses, whereas the ∆ORF37/38 strain did not. Dexamethasone was used to confirm latent herpesvirus infection and reactivation. Dexamethasone injection increased viral DNA load in the brains of hamsters infected with the parental and ∆ORF38 strains, but not in those infected with the ∆ORF37/38 strain. Immunizing hamsters intranasally with the ∆ORF37/38 strain as a live vaccine produced a stronger immune response compared to the ∆ORF38 strain at the same dose. The hamsters demonstrated effective protection against a lethal challenge with the parental strain. This suggests that the deletion of ORF37/38 may effectively inhibit latent viral infection, reduce the neuropathogenicity of EHV-1, and induce a protective immune response.

Novel Gammaherpesvirus Infections in Narrow-Ridged Finless Porpoise (Neophocaena asiaeorientalis) and False Killer Whales (Pseudorca crassidens) in the Republic of Korea

A female narrow-ridged finless porpoise (Neophocaena asiaeorientalis) stranded on a beach on Jeju Island showed epithelial proliferative skin lesions on its body. Two false killer whales (Pseudorca crassidens), caught using nets near Gangneung and Samcheok, respectively, had multiple plaques on their penile epidermis. Histological examination of the epidermis revealed that all the lesions had common features, including accentuated rete pegs, ballooning changes, and eosinophilic intranuclear inclusion (INI) bodies. Based on the histopathological results, herpesvirus infection was suspected, and thus further analysis was conducted using herpesvirus-specific primers. Based on nested polymerase chain reaction (PCR) tests using the herpesvirus-detectable primers, the PCR products demonstrated two fragments: a 222-base-pair (bp) sequence of the DNA polymerase gene, SNUABM_CeHV01, showing 96.4% identity with a bottlenose dolphin herpesvirus from the Jeju narrow-ridged finless porpoise; and a 222 bp sequence of the DNA polymerase gene, SNUABM_CeHV02, showing 95.95% identity with the same bottlenose dolphin herpesvirus from the Gangneung and Samcheok false killer whales. The significance of this study lies in its ability to demonstrate the existence of novel cetacean herpesviruses in South Korean seawater, representing an important step forward in studying potentially harmful pathogens that affect endangered whale and dolphin populations.