First Molecular Evidence of Equine Herpesvirus Type 1 (EHV-1) in Ocular Swabs of Clinically Affected Horses

Abstract

Equine Herpesvirus Type 1 (EHV-1) is a significant pathogen within the Alphaherpesvirinae subfamily, causing respiratory disease, abortions, and, in severe cases, equine herpesvirus myeloencephalopathy (EHM). While nasal swabs and blood samples are commonly used for real-time polymerase chain reaction (RT-PCR) diagnosis, variability in viral shedding necessitates exploring additional sample types. This study reports the first molecular detection of EHV-1 in ocular swabs from naturally infected horses during an outbreak in the Valencian Community in 2023. Nasal and ocular swabs were collected from ten symptomatic horses and analyzed via RT-PCR. EHV-1 was detected in all cases, with higher viral loads in nasal samples. Although nasal swabs remain the most reliable sample for EHV-1 detection, the presence of viral DNA in tear fluid suggests a previously unrecognized route of viral shedding. These findings support further investigation into the role of ocular secretions in the pathogenesis and epidemiology of EHV-1. Additional studies are needed to determine the clinical relevance and potential utility of ocular swabs in specific outbreak scenarios.

Keywords: EHM; EHV-1; Valencian Community 2023 outbreak; horses; ocular swabs.

Figure 1

Relative quantification of viral loads by measuring the Ct values of nasal and ocular swab samples obtained via RT-PCR. (A) Comparison between both swabs for all the available samples. (B) Linear correlation (defined by the Pearson correlation coefficient, referred as “r”; r = 0.414, p-value = 0.029) between both types of samples when paired data (nasal and ocular swab samples collected in the same infection day) were available. (C) Comparison of Ct values between the nasal and ocular swab samples categorized by the two infection stages: early and advanced stages of infection (1–4 and 5–8 days of the disease). In (A,C), horizontal and error lines denote mean ± standard error. A Ct value of 40 was the cutoff for positivity, as this was the maximum number of amplification cycles used in the RT-PCR protocol. ns indicates a non-significant difference, whereas the asterisk in (A,C) shows a statistically significant difference (p-value = 0.029 and 0.032, respectively, given by Student’s t-test).