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These guidelines are intended to be a reference used by veterinarians utilizing vaccines in their respective practices. They are neither regulations nor directives and should not be interpreted as such. It is the responsibility of attending veterinarians, through an appropriate veterinarian-client-patient relationship, to utilize relevant information coupled with product availability to determine optimal health care programs for their patients. Based on the professional judgment of those involved with the development of these guidelines, the recommendations for vaccine administration in this document may differ from the manufacturer’s recommendation. However, it is incumbent on each individual practitioner to reach a decision on vaccine usage based on the circumstances of each unique situation and his or her professional experience.
Information provided in these guidelines addresses only those products licensed by the United States Department of Agriculture (USDA) for use in horses (including draft and pony breeds). There are limited data regarding the use of vaccines in other equidae (i.e. asses, donkeys, mules, miniature horses, and zebra); vaccination of these animals is at the discretion of the attending veterinarian.
The AAEP thanks the members of the Infectious Disease Committee for their work in preparing these guidelines: Mary C. Scollay, DVM, Chair; D. Craig Barnett, DVM; William Bernard, DVM, Dip. ACVIM; Gordon W. Brumbaugh, DVM, Ph.D.; Brian S. Carroll, DVM; Noah Cohen, VMD, Ph.D.; Tim Cordes, DVM; Roberta M. Dwyer, DVM, MS, Dip. ACVPM; Robert E. Holland Jr., DVM, Ph.D.; Daniel G. Kenney, VMD, Dip. ACVIM; Maureen T. Long, DVM, Ph.D.; Paul Lunn, BVSc, MS, Ph.D., MRCVS, Dip. ACVIM; Peter Timoney, MVB, Ph.D., FRCVS; Josie L. Traub-Dargatz, DVM, MS, Dip. ACVIM; Julie Wilson, DVM, DACVIM; and W. David Wilson, MRCVS.
Copyright AAEP 2008
About Equine Herpesvirus - From AAEP (American Association of Equine Practitioners
Equine herpesvirus is a common DNA virus that occurs in horse populations worldwide. The two most common strains are EHV-1, which causes abortion, respiratory disease and neurologic disease; and EHV-4, which usually causes respiratory disease only but can occasionally cause abortion.
Respiratory disease caused by EHV is most common in weaned foals and yearlings, often in autumn and winter. Older horses are more likely than younger ones to transmit the virus without showing signs of infection. Although EHV-1 causes outbreaks of abortion, EHV-4 has only been associated with single occurrences and is not considered a risk for contagious abortions
EHV-1 myeloencephalitis results from widespread vascular injury after damage to the vascular endothelium of the blood-brain barrier. Neurologic signs result from vasculitis, thrombosis, and necrosis of neurologic tissue. Herpesvirus myeloencephalitis cases occur singly or as outbreaks affecting 20-50% of the population. They may or may not be associated with a previous or ongoing EHV-1 respiratory disease outbreak.
Fever—commonly precedes other clinical signs, but may be the only clinical sign and goes undetected.
Neonatal foals infected in utero are usually abnormal from birth:
CNS signs (occasionally)
Death commonly occurs within 3 days.
Older foals: nasal discharge is most common sign of illness.
Usually no warning signs of impending abortion
Typically occurs late pregnancy (8+ months); occasionally as early as 4 months.
Incoordination of the hind (and occasionally fore) limbs
Neurologic signs may be preceded by fever and respiratory signs.
After infection by any route, incubation period may be as short as 24 hours, is typically 4-6 days, but can be longer.
EHV abortion can occur from two weeks to several months following infection with the virus.
Aerosol transmission (most common route)
Inhalation of droplets from coughing and snorting.
Note: EHV is not as easily spread by this route as is Equine Influenza virus.
Mares who have aborted, or whose foals have died, transmit infection via the respiratory route.
Shedding by the respiratory route typically lasts for 7-10 days, but can persist much longer.
Therefore a 28-day isolation period is recommended after diagnosis.
Virus can be viable for several weeks in the environment once it has been shed by the horse.
Fomites are a significant factor in EHV contagion as compared to influenza virus.
Aborted fetuses, fetal membranes and/or fluids are significant sources of infection.
Infected foals are highly contagious and can transmit infection to other horses via the respiratory route and by shedding virus into the environment.
Serological diagnosis using Viral Neutralization titers (synonym: Serum Neutralization titers) cannot distinguish between EHV-1 and EHV-4. Nevertheless, in combination with specific clinical signs, a four-fold rise in titer can assist in diagnosis. When a single virus neutralization
titer is very high (typically 1:1024 to 1:2048 or greater) this is likely to be the result of recentinfection rather than vaccination.
Complement fixation titers have been reported to be useful in diagnosis of recent infection; however, commercial testing is currently unavailable in N. America.
A commercial test kit, suitable for use in practice, is available for detection and differentiation of
EHV-1 and EHV-4 specific antibodies directed against a viral glycoprotein called gG (Svanovir™: www.svanova.comhttp://www.svanova.com/).
Shedding Time of Organism Past Resolution of Clinical Signs
Likely for up to a week, possible for 21 days or more. Recovered horses typically develop latent infections and are capable of shedding virus (with or without
clinical disease) particularly at times of stress for the remainder of their lives.
Environmental transmission plays a minor role in the maintenance of virus in the horse population since environmental persistence of EHV-1 is short, estimated to be no more than 35 days under ideal conditions and probably less than 7 days in most practical field situations
Specific Control Measures
Booster vaccination of healthy animals in primary and secondary contagion control perimeter may have some value, and is not known to lead to complications. If animals are unvaccinated prior to series during the risk period.
clinically normal horses housed within the primary perimeter may be permitted segregated exercise periods outside the perimeter. Precautions should be taken, and may include:
Exercise scheduled after general population’s exercise period to avoid potential virus transfer to unaffected horses/barns by exercise riders
Access to starting gate or similar equipment denied
Restricted use of ponies/outriders’ horses—horses housed within the primary perimeter may only be escorted by horses housed within the same facility.
Direct horse-to-horse contact is to be avoided.
Prompt post-contact use of hand sanitizer by individuals having contact with horses during exercise
Release of animals from isolation
Maintain isolation procedures (primary perimeter) for 28 days after last suspected new infection.
In the absence of clinical disease, the risk of exposure decreases with time.
Biosecurity Issues for Receiving Animals
Horses having been housed within primary perimeter:
Isolate from general population for 28 days
Horses having been housed within secondary biosecurity perimeter:
After having determined its level of risk-aversion, the recipient facility may consider the following:
Vaccination requirements for entrance into facility
Health certificate specifications
Testing (negative PCR from nasal swab)
Update vaccination for animals at recipient facility before arrival of potentiallyinfected/exposed animal.