Commentary on Equine Herpesvirus

The recent outbreaks of paralytic equine herpesvirus type-1 (EHV-1) infection at racetracks and training centers in Kentucky, Maryland, and Pennsylvania have illustrated the benefits of combining the old with the new. The old is the regular, twice-daily recording of a horse's temperature to determine if it is febrile and incubating an infection. The new is the PCR (polymerase chain reaction) test to confirm diagnosis of EHV-1 infection.

Taking a horse's temperature is a simple means of assessing an animal's state of health. Each animal has a normal temperature range which, when elevated (usually above 101.5°F), requires investigation. It is not foolproof, as older horses may not exhibit a temperature rise following infection or may do so for a short period (several hours). In the past, temperatures were invariably taken once a disease outbreak had occurred to monitor the spread of an infection. Increasingly, it has become part of the daily management routine to identify the initial case(s) of disease, enabling prompt diagnosis and prevention.

The horse that has spiked a fever within the last 24 hours is the ideal candidate from which to obtain a nasal swab and blood sample for a laboratory diagnosis. Frequently, when an outbreak of an infectious disease occurs, the laboratory is overwhelmed with samples. By identifying animals that are most likely to provide a positive result, the resources of the laboratory are put to more effective use.

The PCR test was developed several decades ago, but its application as a routine diagnostic procedure in the field for EHV-1 is recent compared to its use on post mortem material and as a research tool. PCR does not measure infectious virus, but the presence of viral particles. It requires considerable technical skill on behalf of the operator, and the test system must be validated and controlled to avoid false positive and negative results. Automation can facilitate the rapid throughput of large numbers of samples, but such equipment is expensive and requires a trained, skilled operator.

If these hurdles can be overcome, the PCR provides an accurate and rapid result within 48 to 72 hours of the laboratory receiving the sample. Utilizing overnight express mail, material packaged in insulated containers reaches the laboratory within 24 hours from widely dispersed geographical areas. This express delivery to equipped laboratories is important, as not all laboratories are geared up to undertake PCR testing for equine pathogens.

By providing an early, reliable diagnosis, the ability to reduce the spread of a specific infection is considerably hastened. Isolation of the sick horse(s) and restriction of movement minimizes the number of in-contact animals that subsequently develop illness. As observed during recent outbreaks, racing continued at affected racetracks, albeit under strict conditions of biosecurity resulting in a slight reduction in the number of runners and the cancellation of several race days and prestigious handicap races. These limitations were primarily a consequence of restrictions of the movement of out-of-state "ship in" horses.

With experience, application of the old and the new will be refined, allowing the attending veterinarian to provide an accurate, prompt diagnosis, thus reducing the spread of equine infectious disease and enabling the diagnostic laboratory to provide a rapid and cost-effective service.

Reprinted from the April 2006 Equine Disease Quarterly. Author: Dr. David G. Powell, 859/257-4757,, Maxwell H. Gluck Equine Research Center, University of Kentucky, Lexington, Ky.

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Equine Disease Quarterly

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