diagnostic insights

Equine Herpes Virus (EHV) Diagnostics

By Dr. Elizabeth Davis, Department of Clinical Sciences, KSU-CVM and Dr. Mike Moore, Kansas State Veterinary Diagnostic Laboratory

Recent news accounts across the nation have brought much attention in the equine world to the neurological form of Equine Herpes Virus (EHV). This is a brief review of laboratory methods for establishing a definitive diagnosis of EHV 1 and 4 associated disease conditions.

EHV-1 and EHV-4 are most widely recognized as causing morbidity in horses. Both EHV-1and EHV-4 can lead to the induction of upper respiratory disease in horses, while EHV-1 is also associated with abortion, neonatal disease, chorioretinopathy and severe neurologic disease, termed equine herpes myelitis (EHM). Current estimates suggest that approximately 10% of horses that develop clinical signs of disease associated with EHV-1 infection may progress to demonstrate neurologic signs. EHV-1 viral infection results in cell-associated viremia, where infected host leukocytes effectively disseminate virus throughout the infected horse. EHV-1 has been determined to exist in two distinct strains (differ in genotype) with the abortion strain referred to as the wild type strain (EHV1w) and the more pathogenic strain that has a different (mutated) genetic sequence (EHVm). In contrast, EHV-4 is predominately restricted to the upper airways and is not generally associated with abortion or neurologic disease.

Clinical signs of respiratory disease associated with EHV-1 and 4 typically include pyrexia, marked serous nasal discharge, and occasionally cough. Secondary bacterial colonization results in nasal secretions changing from mucoid to mucopurulent in nature. Polymerase chain reaction (PCR) testing has become the diagnostic test of choice due to its high analytical sensitivity and specificity. Positive PCR test results may be obtained when culture techniques are not successful due to low level viral shedding. Samples can be collected from the respiratory tract and include nasal swab sampling or nasopharyngeal lavage collection. Additionally, due to the leukocyte associated nature of the EHV-1 virus, whole blood (anticoagulated) buffy coat sampling provides an additional sample for viral PCR testing.

In EHV-1 and 4 suspect cases, nasal swabs and whole blood samples should be tested simultaneously (in parallel) to further enhance diagnostic sensitivity. Nasal swab sampling is recommended over pharyngeal lavage sampling due to greater diagnostic sensitivity3. Samples should be collected using a synthetic swab (not cotton) to maximize PCR testing accuracy. Diagnostic testing requires careful interpretation as outlined in the 2009 ACVIM Consensus Statement on this pathogen and disease process, the interested reader is referred to this particularly valuable reference for a more detailed diagnostic outline.

In the next issue of Diagnostic Insights, we will discuss reproductive and foal manifestations of equine herpes virus.


  1. Pusterla N, Mapes S, Akana N et al. (), Prevalence factors associated with equine herpesvirus type 1 infection in equids with upper respiratory tract infection and/or acute onset of neurological signs from 2008 to 2014, Vet Rec. 2016;178: 70
  2. Lunn DP, vis-Poynter N, Flaminio MJ et al. (), Equine herpesvirus-1 consensus statement, J.Vet.Intern.Med. 2009;23: 450-461
  3. Pusterla N, Mapes S, Wilson WD (), Diagnostic sensitivity of nasopharyngeal and nasal swabs for the molecular detection of EHV-1, Vet.Rec. 2008;162: 520-521

For more information about this disease or testing options, please contact KSVDL Client Care at 866-512-5650 or clientcare@vet.k-state.edu.

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