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

The Effect of Vaccination Status on Total Lymphocyte Count in Horses Affected by Equine Herpes Virus-1 Myeloencephalopathy

Equine herpesvirus 1-induced myeloencephalopathy has a significant impact on the equine industry. Nevertheless, the clinical variables that may affect the severity of the disease are still under investigation. The objective of this research is studying the relationship between the level of lymphopenia and vaccination status with the severity of the disease in horses at an event, considering whether they had been correctly vaccinated or not prior to exposure to EHV-1. Ten horses were admitted to a veterinary teaching hospital following an equine herpesvirus myeloencephalopathy outbreak during an international show jumping competition in Spain. Data were collected from passport vaccination records, daily analyses, and the clinical histories of the affected horses. Correctly vaccinated horses had a significantly longer hospitalization duration (6/10, 15.5 ± 1.2 days) compared to incorrectly vaccinated horses (4/10, 12.5 ± 1.2 days; p = 0.01). Lymphopenia (<1.6 × 103 lymphocytes/µL) was the most common leukogram abnormality. Correctly vaccinated horses demonstrated a higher lymphocyte count compared to incorrectly vaccinated horses within 24 h of admission (p < 0.01). This difference remained significant from days 1 to 4 and on day 6 post-admission (p =0.03). This study found that lymphopenia is a common leukogram alteration in equine herpesvirus 1-infected horses, and horses correctly vaccinated prior to an equine herpesvirus myeloencephalopathy outbreak tend to have a longer hospitalization time. Correctly vaccinated horses exhibited higher lymphocyte counts during the first 24 h and throughout hospitalization compared to incorrectly vaccinated horses. The immune system could play a relevant role in influencing the severity of equine herpesvirus myeloencephalopathy outbreaks, highlighting the need for further studies in this area.

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.

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.