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 "Standard Submission Form")
Lucy Whittier Molecular and Diagnostic Core Laboratory
Submission forms and information
www.vetmed.ucdavis.edu/vme/taqmanservice/diag_home.html
(click on "Submission Forms" in left column)
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 (buffy coat), or postmortem tissues. CAHFS will routinely perform both assays for all samples submitted for EHV-1 detection.
The Lucy Whittier Molecular and Diagnostic Core Laboratory also offers routine diagnostic testing for equine viral pathogens using real-time PCR technology.
The Whittier Laboratory offers a well-validated assay, which detects all strains of EHV-1 by targeting the glycoprotein β gene. This assay detects the EHV-1 virus but does not differentiate between the various neurogenic strains or the EHV-1 abortion strain, which is considered to be less risk for neurological disease. The sub-differentiation of various strains will be possible in the near future, but until then the molecular detection of EHV-1 by real time PCR should be correlated with clinical presentation (abortion, pneumonia, myeloencephalopathy). For example, a neurological horse with fever and positive PCR has a strain of EHV-1 and should be treated as contagious with all handling guidelines followed.
The PCR assay is performed on either whole blood or nasal secretions in order to document viremia and nasal shedding, respectively.
Research groups have recently identified a region of variation in the genome of different EHV-1 strains that correlates directly with their ability to cause a more virulent form of neurologic disease. Current estimates for the neurogenic strains isolated from exposed horses that have this mutation range from 70 to 94%. In addition, the nonmutant strains of EHV-1 also have historically been implicated as a causative agent for neurological disease.
Currently, molecular assays to differentiate between neurogenic and non-neurogenic strains have been developed and used for research purposes only. While these assays are informative, we believe it is too early to use these types of assays in the diagnostic setting without additional validation. The assay we offer for all strains of EHV-1 will be our standard at this time.
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.
