Worldwide, 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 2 years of age. Owners may not know their 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.
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 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 38.5oC. 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.
Abortions in affected mares tend to occur in the last trimester of pregnancy, but the infection with EHV may have taken place between 2 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 7 days but if conditions are perfect, the environmental survivability of the virus can be up to one month. It is however easily inactivated by disinfectants, so be sure to scrub tack and transport, wheelbarrows and wellingtons with an appropriate product.
Infected premises must be quarantined immediately, with horses confined to the premises in accordance with the attending veterinary surgeon’s instructions. There may be quarantining of different groups with the farm to prevent spread amongst stock at one site. Quarantine means staying on the property with no trips out to competitions, vet clinics or hacking off site until any outbreak is contained.
Vaccination for EHV-1 and 4 assists in controlling infection by decreasing the amount of virus shed, the duration of shedding in nasal secretions and limiting the clinical signs of infection and abortion. This in turn decreases environmental contamination with virus. An inactivated vaccine is available for EHV-1 and 4 but should not be administered to in-contact animals during a disease outbreak as there are possible contraindications if they are already infected, and it will interfere with disease monitoring on blood samples. The vaccine gives some protection against the respiratory disease and reduces virus shedding. Importantly though, no EHV vaccine to date has proved effective against EHM, although by decreasing virus shedding and viraemia, it is thought to offer some protection.
The protocol for vaccination for EHV-1 and EHV-4 is a primary course with a vaccine given on day one and then a second given four to six weeks later. Non-breeding horses should receive booster vaccines every six months. Following a primary course, pregnant mares should be vaccinated at five, seven and nine months of pregnancy to reduce incidence of abortion. Good biosecurity is vital in prevention of EHV and particularly so for pregnant mares. New mares should be quarantined for 3 weeks before introduction to the herd. Herd groups should be kept the same where possible with stress and movement minimised in the final trimester of the pregnancy.
Treating EHV infections, respiratory and neurological, means providing supportive care to the infected animal as there is currently no drug to cure the viral infection. Such veterinary care includes drugs to bring down the fever, symptomatic treatment for the cough, and eye drops for the conjunctivitis. Sometimes in cases where there is a secondary bacterial infection, usually of the respiratory tract, antibiotics may be appropriate. Drugs to control the circulating viraemia include oral anti-viral medication, but unfortunately are of only limited effect.
If EHM develops, it is important to provide nursing care, often with referral to an isolation unit at an equine hospital. This includes a deep bedded stable, catheterisation of the bladder, hydration via intravenous fluids and if the horse has difficulty standing or is recumbent, then a full body sling is necessary to provide support.
The respiratory form of EHV tends to self-limit and the horses may recover in a few days with supportive treatment. Sadly, even with prompt action and supportive care, the neurological form is often fatal.