Diagnosis

Respiratory disease

Equine herpesvirus causes respiratory symptoms that are often indistinguishable from other respiratory infections of horses, such as influenza, equine viral arteritis, and strangles. Common symptoms include fever, coughing, nasal discharge, and discharge from the eyes. In order to definitively diagnose rhinopneumonitis, the veterinarian will have to take samples and submit them to a laboratory for testing. As of quite recently, labs are able to tell the difference between EHV 1 and 4 due to the development of an ELISA test that differentiates between the two
(1).

Looking for equine herpesvirus (2) Looking for antibodies to equine herpesvirus (2)
- virus isolation (from blood samples and samples obtained from nasopharyngeal swab)
- polymerase chain reaction (PCR) from a blood sample
- antibody counts (using ELISA tests) from consecutive blood samples (if the amount of antibody increases, it indicates the horse is responding to this infection)


Abortion

Other causes of abortion include leptospirosis, Salmonella abortusequi, placentitis from Streptococcus zooepidemicus or Escherichia coli, or abnormalities in the fetus (1). However, abortions in late pregnancy should be treated as if they were from equine herpesvirus until proven otherwise, as there are likely to be other mares at risk.

Testing the aborted fetus (1, 2) Testing the mare after abortion (2)
- characteristic lesions (both visible to the naked eye and microscopic) on the fetus (include hemorrhage on the respiratory mucosa, clear yellow fluid in the chest and abdominal cavities)
- virus isolation
- immunohistochemistry (showing the presence of virus in the tissues of the fetus)
- blood testing the mare after abortion has very little value


Neurological disease

Since respiratory symptoms might not be present beforehand, the neurologic symptoms of equine herpesvirus can look like equine protozoal myeloencephalitis (EPM), spinal cord trauma, or polyneuritis equi (3). Common symptoms include incoordination, stumbling, toe dragging, and laxity of the tail. Diagnosis in the live horse is difficult, and definitive diagnosis in an outbreak often relies on post-mortem analysis. However, if a highly contagious neurological disease is occurring on a property, it may be advised to presume it is equine herpesvirus until proven otherwise.

Diagnosis in live horses  (1,3) Diagnosis after death (1)
- presumptive, based on symptoms
- analysis of the cerebrospinal fluid
- polymerase chain reaction (PCR) from a blood sample (may be negative even if the horse is infected)
- antibody counts from consecutive blood samples (if they increase, it indicates the horse is responding to this infection)
- characteristic lesions in sections of nervous tissue


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