EHV-1 Diagnostic Testing

Your veterinarian can perform a nasal swab and blood collection and send it to a laboratory where an assay (real-time TaqMan® polymerase chain reaction, PCR) for neurological EHV-1 virus can be performed. Use of other tests may not be as reliable, based on the most recent scientific publications.

UC Davis has two laboratories that are capable of performing diagnostic tests:

California Animal Health and Food Safety Laboratory
Submission forms and shipping requirements
www.cahfs.ucdavis.edu
(click on "Submission Form" on top tool bar, then click on "Standard Submission Form")

Real-Time PCR Research and Diagnostics Core Facility
Submission forms and information
http://www.vetmed.ucdavis.edu/vme/taqmanservice/diagnosics.html
(click on "Diagnostic Tests and Panels, Submission Form" at bottom of the page)

Samples for EHV-1 testing should consist of nasal swabs and whole blood samples drawn in EDTA tubes.

The Gluck Equine Research Center in Lexington, Kentucky, is the OIE reference laboratory for EHV-1. Further information regarding EHV-1 can be obtained at www.ca.uky.edu/gluck/BiblioEHV1.asp.

Cautionary Note Regarding Diagnostic Testing

The California Animal Health and Food Safety Laboratory (CAHFS) offers two real-time PCR assays for the detection and differentiation of neuropatholgenic Equine Herpesvirus-1 from non-neuropathogenic EHV-1. The assays developed by Dr. George Allen at the Gluck Equine Research Center, University of Kentucky, detect viral DNA and distinguish between the two strains by identifying the genetic difference located in the polymerase gene. Dr. Allen's work is described in Equine Veterinary Journal 3:252-257 (2006).

While these real-time PCR results indicate the presence or absence of viral DNA in the specimen tested, they do not predict clinical outcome. Dr. Allen's work with experimentally infected foals suggests a five-fold higher risk of a horse developing neurological disease when infected with the form of EHV-1 containing the neuropathogenic marker. The real-time PCR testing can be performed on nasal-pharyngeal swabs, EDTA blood, or postmortem tissues. CAHFS will routinely perform both assays for all samples submitted for EHV-1 detection.

The Real-Time PCR Research and Diagnostics Core Facility also offers routine diagnostic testing for equine viral pathogens using real-time PCR technology.

For EHV-1 detection, the Core Facility offers:

Quantitation of genomic DNA level to allow veterinarians to indirectly assess the infectious nature of an animal shedding EHV-1. The results in blood would also help to determine the level of viremia as well as the risk for a horse to eventually develop neurologic signs or abortion.

Detection of the different genotypes to differentiate between the neurotropic and non-neurotropic strains. The results will be reported as presence of neurotropic, non-neutrotropic or dual EHV-1 strains. Please keep in mind that up to 24% of horses with neurologic disease can be infected with "non-neurotropic" form, suggesting that other factors may also contribute to the onset of equine herpesvirus myeloencephalopathy.

Detection of EHV-1 transcripts at the cDNA level. This approach will help characterize the virral state and determine if the virus is lytic or nonreplicating. The results will be qualitatively reported as positive (evidence of active replication = lytic virus) or negative (absence of active replication). This assay is available only when a sample is positive for EHV-1 at the genomic DNA level.

PCR results for EHV-1 are expressed qualitatively (positive or negative) and quantitatively in the case of a positive result. The viral load of a sample is calculated by the absolute number of EHV-1 (glycoprotein B gene) per million cells (either blood cells or nasopharyngeal cells). Diseased horses (neurological or febrile) commonly have high viral loads in blood and nasal secretions. Subclinical horses commonly have low to moderate viral loads only in nasal secretions. The absolute number should be used judiciously and allow comparisons between samples taken at different time points.

Appropriate use of PCR Testing for EHV-1

Since EHV-1 is considered to be endemic within the horse population, random testing of normal horses for EHV-1 by PCR diagnostics can and likely will detect horses with nonreplicating (dead) viral DNA; latent, low-level, transient carriage of virus; or viral levels that are not sufficient to pose a significant risk for disease transmission. Until more research data regarding the pathogenesis and epidemiology of equine herpesvirus-1 myeloencephalopathy (EHM) is acquired, random testing of horses for the virus may very well result in interference with the free movement of horses and staging of their athletic competitions, which later may be demonstrated to have been unnecessary.

It must be understood that the positive predictive value of any medical diagnostic test is the relationship between those individuals who test positive to the number of those testing positive that actually develop clinically significant disease. Only if that relationship is very close does the diagnostic test have a high positive predictive value.

For example, with equine infectious anemia (EIA), the number of horses testing positive (Coggins test) matches very closely with those that are actively infected and so have clinical significance for transmission of disease. Thus, the Coggins test has a high positive predictive value. By contrast, with equine protozoal myeloencephalitis (EPM), there is a marked difference between the numbers of horses testing positive in the Western Blot test and those who actually exhibit symptoms of the disease. Consequently, it is well accepted that the Western Blot test has a low positive predictive value for EPM.

With our current outbreaks of EHV-1, the interpretation of the positive predictive value of the diagnostic technology employed and the "test result" obtained are problematic at this time. Even given the sophistication of our current molecular testing capabilities, the interpretation of PCR viral detection for EHV-1 should be done only in the context of the presenting clinical signs for disease in the horse being tested.

At this time the significance of a positive PCR in an asymptomatic horse is unknown, regardless of the test being employed or the laboratory performing the test. There is simply not sufficient information yet acquired to justify or recommend control measures or quarantine procedures for horses testing positive for EHV-1 in the absence of clinical signs of disease.

Horses with high fevers and/or clinical signs of coughing or mild nasal discharge, with or without neurologic symptoms, should be tested for EHV-1 by PCR diagnostics if other explanations for these signs of disease are not apparent. Detecting a positive PCR for EHV-1 in these circumstances may warrant some degree of patient isolation and limited movement of exposed horses.

 
 
Center for Equine Health, School of Veterinary Medicine, UC Davis, Tel (530) 752-6433