Equine Herpes Virus (EHV) is a widespread virus found in most horse population throughout the world. The virus is commonly found with 2 strains known as EHV-1 and EHV-4. The EHV-1 typically causes respiratory symptoms, abortions and neurological signs and the EHV-4 is usually of the respiratory form but can cause abortions.
EHV respiratory form is common in weanlings and yearlings, whereas older horses tend to have a more insidious disease causing loss of athletic performance and can be asymptomatic carriers.
The neurological disease (equine herpes myeloencephalopathy) is caused by the virus damaging the blood vessels especially at the blood brain barrier. This is in most outbreaks isolated cases, but has been known to cause multiple occurrences.
The disease causing abortion is when the virus spreads to the small blood vessels within the endometrial lining of the uterus. Subsequent thrombosis /vasculitis of the placenta can then affect its circulation which if extreme causes abortion. If abortion does not occur, the placenta being compromised will have an adverse effect on the foal resulting in a weak and sickly neonate.
The usual clinical sign of the disease is an elevated temperature which in most cases precedes the other clinical forms as previously mentioned. The elevated temperature may be the only sign.
The respiratory form commonly shows a s fever, nasal discharge and coughing, which is common with many respiratory diseases both of viral and bacterial in origin.
Abortions occur without any warning and are usually later than 8 months of gestation. If the foals are carried to term they are often very weak, jaundiced and show neurological and or respiratory signs. These foals often die within several days.
The herpes myeloencephalopathy may start as a respiratory disease with elevated temperature, but then show incoordination of the hindquarters sometimes with incontinence. When recumbent they have difficulties in rising and often show the typical dog sitting position.
The viral incubation period is often 24 hours and lasts several days. Abortions can occur 1 - 2 weeks, or several months after infection. The transmission which lasts 10 - 21 days is by a erosol i.e. coughing. Abortions will also distribute the virus via the foetus and foetal membranes.
Poor hygiene, sharing of equipment, infected clothing, infected stables and improper disposal of aborted material all are responsible for spread of the disease.
The diagnosis of EHV is based on the clinical findings but has to be confirmed by virus isolation, serology and molecular testing. This would be done at a laboratory such as the Animal Health Trust.
Treatment is in most cases supportive, possibly antibiotics for secondary bacterial infections, anti inflammatory medication and fluids if required. Ataxic horses may require catheterisation of the bladder and slings if having trouble getting up.
Vaccination programmes are recommended. We use Duvaxyn EHV 1 - 4 (protects against strains 1&4) at 5, 7 and 9 months of pregnancy. It is also recommended that all animals on the premises are vaccinated. Horses in training are given 2 vaccinations 4 - 6 weeks apart and if possible every 6 months (or 3 months if training can be interrupted). In the event of an outbreak foals can be vaccinated as early as 3 months of age.
Equine influenza is a highly contagious viral disease that affects horses, donkeys and mules throughout the world. The virus has an incubation period of 2 - 4 days. There is a sudden onset with pyrexia up to 107oF or 420C which lasts several days, or longer if a secondary bacterial infection sets in. Coughing occurs concurrently and may last several weeks. A slight nasal discharge is present, enlarged glands under the throat and runny eyes are also commonly observed. There is also weakness, anorexia, elevated respiratory rate and general muscle stiffness. Secondary bacterial infections can exacerbate these symptoms.
Slight infections can result in recovery after 2 - 3 weeks, however severely infected animals can take up to 6 months to fully recover.
Diagnosis is by observing the clinical signs followed by laboratory analysis to isolate the virus or specific antibody titre determination.
Prevention of the disease is by vaccination.
There still seems to be much confusion over the rules and regulations with regard to vaccinations against equine influenza. In 1980 the rules of racing, initiated a compulsory vaccination program which has been copied by most competition and show horses . The rules stipulate that two vaccinations are given 21 - 92 days apart followed by a third vaccination 150 - 210 days later. Annual boosters are given thereafter. They are allowed to compete after the initial two vaccinations providing the time for the third has not expired. Horses are not allowed to compete within 7 days of receiving the vaccine. Most governing bodies that control the various competitions adhere to these rules, but from time to time these can alter so it is also recommended that checks are made to ensure that the vaccination policy complies with the individual ruling.