Equine Herpesvirus Type I (EHV-1)

Test codes:
S0071 - Qualitative detection of Equine Herpesvirus Type I by polymerase chain reaction
P0013 - Equine respiratory panel (includes EHV-1 and other pathogens)
P0014 - Equine neurological panel (includes EHV-1 and other pathogens)
P0024 - Equine breeding panel (includes EHV-1 and other pathogens)

Equine herpesvirus 1 (EHV-1) is a major cause worldwide of epidemic abortion, perinatal mortality, respiratory disease and neurological disorders in horses. EHV-1 is a member of the Alphaherpesvirinae subfamily that also includes herpes simplex virus (HSV) types 1 and 2 and varicella zoster virus (VZV). As with other herpesvirus infections, lifelong latency of the virus in the nervous system of the host occurs, and periodical reactivation of the virus can cause new outbreaks (Slater et al., 1994).

Although primary infection generates a humoral response and the production of neutralizing antibodies, infected animals do not develop long lasting protection against EHV-1 and remain susceptible to re-infection throughout life, although the severity of secondary infection is reduced.

Upper respiratory infection is the most common manifestation of EHV-1 infection. Commonly, young horses (weanlings, yearlings, and 2 year olds) have depression, poor appetite, nasal discharge and cough. If young horses are housed or pastured together, many horses in the herd may experience disease from which they recover uneventfully. Disease may be mild or unapparent in older horses. Neurological signs occur infrequently as a result of EHV-1 infection; however, outbreaks of neurologic disease associated with fever, nasal discharge and cough have been reported in the USA and elsewhere. Neurologic signs may include incoordination that can progress to an inability to stand. Horses may be unable to urinate or may dribble small volumes of urine. Horses may have difficulty producing manure. Sometimes the neurologic signs are accompanied by cellulitis (inflammation or swelling of the limbs) and petechiae (small hemorrhages on the gums).

The transmission of EHV-1 from the mare to the foal is one of the major causes of spreading infection. Lactating mares having the potential to infect their suckling foals within 30 days of age. These foals can then spread EHV-1 to other foals within their group prior to and throughout weaning. The weaning process involves the separation of the foal from its dam and intensive human contact, placing foals under stressful conditions, a contributing factor in the reactivation of EHV. The virus can then spread through contact with other foals. Farm management practices such as routine handling of mares and foals and the mixing of paddock groups can also encourage the introduction and spread of EHV.

Vaccinated mares are still able to transmit EHV-1 to their unweaned unvaccinated foals, despite having received three vaccinations during the previous gestation. The vaccine may result in a reduction in the period of excretion of the virus, however it is not sufficient to prevent EVH-1 infecting new foals.

A diagnosis of EHV-1 abortion can only be made by postmortem examination of the fetus. EHV-1 is readily isolated from a wide range of tissues in aborted fetuses. Infection of endothelial cells in the uterus and placenta plays a significant role in the pathogenesis of abortion and in dissemination of the virus. Testing the mare's serum for antibodies is of little value, because virus-infected foals can be born from mares that show no evidence of recent antibody activity, and noninfected foals can be born from dams that do show recent antibody rises.

The virus can be isolated from pharyngeal or nasal secretions by culture. Isolation of the virus from respiratory secretions strongly suggests that the virus is the cause of the disease. However, this method is slow and has low sensitivity. Molecular detection by PCR can identify the virus in pharyngeal or nasal secretions. Blood cells can also be tested by PCR, with detection of the virus indicating that the horse is currently or has recently been viremic. However, it is important to note that a negative PCR test does not rule out EHV-1 as the cause of the disease, as viremia is intermittent.

Utilities:

• Confirm the disease causing agent
• Shorten the time required to confirm a clinical diagnosis of EHV-1 infection
• Ensure that horse populations are free of EHV-1
• Early prevention of spread of this virus
• Minimize personnel exposure to this virus
• Safety monitoring of biological products that derive from horses

References:
Slater, J. D., Borchers, K., Thackray, A. M. and Field, H. J. (1994) The trigeminal ganglion is a location for equine herpesvirus 1 latency and reactivation in the horse. J. Gen. Virol. 75: 2007–2016.

Specimen requirements: Nasopharyngeal swab, or 1 ml whole blood in EDTA (purple top) or ACD (yellow top) tube, or tissue, shipped overnight at room temperature; or tissue shipped frozen.

For specimen types other than those listed here, please call to confirm specimen acceptability and shipping instructions.

For all specimen types, if there will be a delay in shipping, or during very warm weather, refrigerate specimens until shipped and ship with a cold pack unless more stringent shipping requirements are specified. Frozen specimens should be shipped so as to remain frozen in transit. See shipping instructions for more information.

Turnaround time: 2 business days

Methodology:
S0071 - Qualitative PCR
P0013 - Qualitative PCR, qualitative reverse transcription PCR and qualitative real time PCR

Normal range: Nondetected