Feature
Understanding Equine Herpes Virus (EHV) By Dr Susanna Ballinger BVSc CertAVP(EP) MRCVS
horse has been infected, as signs can be mild. Previously infected horses can become latent carriers, not exhibiting signs of clinical disease, but harbouring the virus and shedding it in their nasal secretions, particularly in times of stress. This is the main reservoir of infection and whilst shedding, carriers can appear completely normal.
About the vet:
Dr Susanna Ballinger BVSc CertAVP(EP) MRCVS is an RCVS Advanced Practitioner in Equine Practice. Veterinary Director at Ballinger Equine, the award-winning, leading first opinion ambulatory equine veterinary practice, Susanna is an FEI Permitted Treating Veterinarian and an accredited ARVS Racecourse Veterinary Surgeon. Find out more at
ballingerequine.com
The virus replicates in epithelial cells in the respiratory tract of the horse, circulates in the blood stream (viraemia) and causes inflammation of the blood vessels in the central nervous system and reproductive tract of infected animals.
EHV is a contagious viral disease, transmitted in the nasal secretions and fluids associated with abortion. Infection can occur as rapidly as 24 hours after exposure to the virus but the incubation period can be as long as 14 days, depending on the individual animal.
Clinical signs W
orldwide, there are nine identified equine herpes
viruses (EHV) with five strains causing disease in the UK horse population. The two most common types are EHV-1 which causes respiratory infection; abortion; neonatal death and neurological disease (Equine Herpes Myeloencephalopathy or EHM) and EHV-4 which usually only causes low grade respiratory infection but occasionally can cause abortion. EHV-3, known as equine coital exanthema (ECE), causes pox like lesions on the penis of stallions and vulva of mares and EHV-5 is much less common and is associated with debilitating lung scarring in adult horses.
EHV is prevalent in the UK national herd with reportedly 80-90% of horses infected with EHV before reaching two years of age. Owners may not know their
Clinical signs of EHV vary, with respiratory disease including fever, a soft cough, clear nasal discharge, conjunctivitis, enlarged lymph nodes, swollen legs and loss of appetite. A mucopurulent (green/yellow) nasal discharge can develop with a secondary bacterial infection. Infected young horses often show pronounced ‘flu-like’ symptoms and an increase in body temperature above 38.5o
C.
However, it is not uncommon for an EHV infection in an adult horse to go unnoticed. It also can cause chronic loss of performance in all competition horses and especially younger racehorses.
EHM, the neurological form of the disease caused by EHV- 1, presents initially as mild incoordination but can rapidly progress to marked ataxia (a staggering walk), paralysis, recumbency, urine dribbling and faecal incontinence. It is important to note that these cases often have no respiratory signs prior to onset of the neurological signs.
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Abortions in affected mares tend to occur in the last trimester of pregnancy, but the infection with EHV may have taken place between two weeks and several months prior to the abortion.
Definitive diagnosis of EHV infection on clinical signs alone, is problematic, as the symptoms are also typical of several other diseases. Diagnostic tests are required, including analysis of nasal swabs via qPCR to detect the DNA from the EHV organism. This highly specific test yields same-day results, facilitating rapid diagnosis which allows appropriate treatment and biosecurity measures to be employed. There is also a blood test for EHV-1 and EHV-4 which measures IgM antibodies in the blood. Follow-up sampling and testing fourteen days after the first test is required to determine whether infection and antibody production is active. The test can
be particularly useful monitoring in-contact animals during an outbreak.
Prevention and Biosecurity
EHV spreads by horse- to-horse contact and via aerosol transmission of nasal secretions up to a distance of approximately 50 metres. It is also transferred via buckets, tack, rugs, etc., and people!
Effective biosecurity measures and herd management are therefore vital in controlling spread of infection. When away from home and at competitions, keep horse to horse contact to a minimum, don’t share equipment and don’t allow grazing. Typically, the virus is persistent in the environment for no more than seven days but if conditions are perfect, the environmental
Diagnostic tests are required, including analysis of nasal swabs via qPCR to detect the DNA from the EHV organism
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