Herpes Virus 4 is ubiquitous in the horse population worldwide
- Most infections with EHV-4 are sporadic, but outbreaks can
occur annually in concentrated horse populations amoung young horses,
especially under two years of age. Several
influencing factors include: age, season,
geographic distribution, compromised immune status and high
horse stocking concentration. Factors
influencing outcome also include viral strain involved and pregnancy
- Similar to other
herpes viruses, EHV-4 often causes a lifelong latent infection . These persistantly infected animals usually
serve as the source for infection of foals over two months old,
- Summary of the typical epidemiology is:
- Diffuse infection of young, susceptible horses with
morbidity generally being low
- High prevalence of latently infected carrier horses
- frequent shedding of infectious virus from carrier animals
- Infection results primarily in upper respiratory tract diseas
(rhinopharyngitis and tracheobronchitis). Thus
the disease associated with EHV-4 infection is termed
- Therefore, clinical
signs may include nasal discharge that may progress into a mucoid or
mucopurlent discharge, increased lung sounds, mild coughing, fever, and
- Entry of virus into the body and transmission through the body
initially enters the body of the horse through the upper respiratory
after it has been obtained from virus containing respiratory
fomites, or aerosols.
- Transmission of the virus occurs by direct or indirect
contact with virus-laden aborted fetuses, their placentas, and
or from infective nasal discharges from virus shedding horses. However, unlike EHV-1, EHV-4 has only been
associated with single occurrences and is not considered a risk for
- The most likely source of infection for foals prior to
weaning is latently infected mare(s), with subsequent spread to
foals. This can occur yearly allowing
the virus to transmit to each new crop of foals.
- Incubation period is from two to ten days
entering the body through the upper
respiratory tract of the horse, EHV-4 will initially infect and
mucosal epithelial cells located there.
of EHV-4 parallels that demonstrated for EHV-1, but it has not been as
extensively researched. It is known that
EHV-4 causes less severe tissue destruction than EHV-1.
Also EHV-4 rarely causes abortion when it
infects pregnant mares, unlike EHV-1. Additionally contrasting EHV-1,
detectable dissemination by EHV-4 generally does not extend beyond the
lymph nodes. As a result of this, EHV-4
results in viremia and thus as already mentioned, EHV-4 rarely results
abortion. However, EHV-4 has
occasionally been linked to leukocyte-associated viraemia, which has
- EHV-4 infections are
cleared from the respiratory tract within one to three weeks after
infection and within two to seven days of subsequent infections. Death, although rare, can be an outcome.
recovery form the upper respiratory tract disease, some are said to
â€œpoor performance syndromeâ€�. This
associated with non-specific bronchial hypersensitivity and a syndrome
resembling chronic obstructive pulmonary disease. As
a result, the major economic cost
associated with EHV-4, is not abortion like EHV-1, but rather the costs
veterinary care, training time that is lost, and any long term negative
on horse performance.
viral rhinopneumonitis, the disease caused by EHV-4 infection, cannot
clinically differentiated from equine influenza, equine viral
certain other equine respiratory infections simply by looking at
- Definitive diagnosis can be determined through virus isolation. This can be achieved through nasopharyngeal
swabs and citrated blood samples taken very early in the course of the
infection and by serologic testing of acute and convalescent serum.
- Culture identification of the
virus is time consuming, three to four weeks for serological tests, and
low sensitivity. The use of PCR for
results in higher sensitivities and specificities for identifying EHV-4
- There is no specific treatment for EHV-4.
- Isolation from the rest of the herd until
clinical signs are resolved is a good idea. For
horses with fevers over 40°C (104°F)
anti-pyretics (drugs that reduce fevers) are
recommended. Antibiotic therapy should
be initiated if a secondary bacterial infection is suspected or is
purulent nasal discharge and/or pulmonary disease.
- Immunity after natural infection with EHV-4 appears to
involve a combination of both humoral and cellular immune factors.
reinfection of the respiratory tract by EHV-4 may reach up to 3 months. However, most horses are latently infected
with EHV-4. The virus usually remains
dormant for most of the horse's life, but stress or
result in resurgence of disease and shedding of infectious virus.