RHINOPNEUMONITIS, EQUINE HERPES VIRUS 1, EQUINE ABORTION VIRUS, ALL SYNONYMS OF A SERIOUS THREAT TO THE HORSE BREEDING INDUSTRY.

The infection due to Equine Herpes Virus 1 (EHV-1) is endemic throughout the equine population worldwide and can appear in the form of respiratory disease, abortions, perinatal deaths and neurological symptoms.
This virus has the particularity of inducing the presentation of latent (silent) infectious disease in horses and so perpetuates itself. Approximately 50% of horses over 1 year of age are infected in this “silent” manner. Stress producing factors such as moves, “social” or climatic changes, overcrowding, poor quality feed, to name a few, can cause the reactivation of the latent virus and a large viral multiplication in the respiratory epithelium with a consequent dissemination of the virus to other susceptible horses. If this takes place in a group of pregnant mares in an advanced state of gestation there may be epizootic abortion and a high percentage of the mares may abort within a short period of time. This is known as an abortion storm.
A case of epizootic abortion due to EHV-1 is the most devastating manifestation of the infection caused by this virus, though both the perinatal mortality syndrome in foals and the myeloencephalopathy are serious.
The risk of dissemination of this virus in an equine population is high, an epidemic may rapidly expand and cause a great number of clinical cases.

An infection may originate:
• From an activated endogenous virus in an infected horse with the latent or “silent”form.
• By contagion, from an actively infected horse that sheds virus in its nasofaryngeal secretions.
• From the foetus, the afterbirth or a mare’s reproductive tract secretions after an abortion.
Evidence exists that the virus can travel amongst horses separated by a short distance (from one field to another, one box to another) by means of the aerosols produced by actively infected horses when they snort, etc. Virus may also be transmitted by contaminated workers hands, boots, clothes, water or feed. Iatrogenic transmission by means of inadequately disinfected diagnostic instruments (endoscopes) has also been reported.
Usually, foals undergo their first infection before they are 1 year of age, they shed large amounts of virus in their nasal secretions during the febrile period and till day 14. The respiratory symptoms in a first infection appear 3 to 6 days after natural exposure to the virus and include fever, leucopoenia, anorexia, depression, watery nasal discharge and rhinitis, faringitis, coughing and occasionally infarction of the retropharyngeal and mandibular lymph nodes. In the case of previously infected adults, the fever and the nasal discharge are minimal and short lasting, with significantly less virus shed. These cases are, in fact, generally asymptomatic.
Abortion can take place between months 7 and 11 of gestation and several months after the respiratory infection. The mare shows no premonitory signs and is left with no sequelae.
The aborted foetus may present jaundice, petechiae, subcutaneous and pulmonary oedemas, enlargement of the spleen and white necrotic focci in the liver. The etiological diagnosis is carried out by isolation of the virus from different foetal organs (thymus, lung and liver).
Neonatal death occurs when the foetus is infected near the date of birth. Foals are born alive but with severe liver, lung, lymphatic and adrenal damage. Death is caused by acute pneumonitis before 8 days of age.
The neurological presentation of the disease is infrequent, it appears about 2 weeks after natural exposure and is the consequence of the deposition of immune complexes (virus-antibodies) in the blood vessels of the central nervous system. The horses most likely to present this form of the disease are those that are immune due to previous contact with the virus.

In this article we give some practical recommendations based on current knowledge of the disease and aimed at reducing the economic impact of EHV-1 infection, the purpose of these is to:
1) Reduce the incidence and the clinical severity of the disease in horses exposed to the virus.
2) Limit the dissemination of the virus to other susceptible animals during an outbreak.

The combined implementation of a strict prophylactic vaccination program and appropriate management techniques have resulted in a significant reduction of the terrible consequences of EHV-1 infection. Vaccination is important in the prevention of epizootic EHV-1, but is not the only tool. It must be underlined that with the vaccines currently available on the market, vaccination is a complement to prophylaxis and in no case is it a substitute for “good management practices”.

Dr. George Allen, researcher at the University of Kentucky and an OIE (Office international des épizooties) expert has described prevention measures using the acronym “SISS”: segregation, isolation, subdivision and stress reduction; and control measures using the acronym “DISH”: disinfection, isolation, submission, hygiene.

Below we describe the management practices we recommend to prevent the appearance of epizootic disease caused by EHV-1 and the measures to limit the dissemination of the disease to other horses in case of an outbreak.

Prevention:
• Pregnant mares should be kept separate from other horses, specially foals and newcomers to a farm.
• Mares who are newcomers to a farm or who return after being taken to another farm to be served should be kept isolated for at least 3 weeks.
• Groups of pregnant mares should be broken down into smaller groups, according to length of gestation. Each group should be maintained separate from the others till the mares have foaled. First foaling mares should not be mixed with older mares.
• All pregnant mares should be vaccinated. The frequent practice of only vaccinating ‘valuable’ mares is a great mistake.
• Stressful situations, such as changes in the established social structure, prolonged transport, moves to other farms, shows, or inclement weather conditions and massive weanings must be avoided.
Actions recommended to halt dissemination of the infection when there is an outbreak of abortions due to EHV-1 are based on strict quarantine and hygienic measures.
• Disinfection of the areas contaminated by fluids from aborted foetuses and afterbirth. If the abortion takes place in a box, the straw bedding must be sprayed with fenolic or iodine based disinfectants and burnt. The box must be thoroughly cleaned and disinfected.
• The foetus and clinical samples must be immediately sent to a diagnostic laboratory. The etiologic diagnosis will determine the necessity of continuing or not with the containment measures.
• The mare that has aborted must be physically separated from other mares, including those in the original group. The mare that has suffered an abortion caused by EHV-1 must not come into contact with other pregnant mares for at least 2 months and must only be served when she comes into season for the second time after aborting.
• The group of mares that were in contact with the mare that aborted must, if possible, be divided into smaller groups.
• There should be no movement of horses in and out of the farm till 1 month after all the mares have foaled.
• Appropriate hygienic methods must be adopted, cleanliness, disinfection, different workers must care for different groups of horses, etc. Everything possible must be done to avoid the exposure of other pregnant mares to the aborted foetus, the afterbirth and any secretions.