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The Virus


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Equine herpesvirus is an enveloped DNA virus. (36)  It is endemic in horse populations worldwide and causes a variety of clinical signs ranging from none to serious neurologic disease and death. (17)

Species of equids are known to be host to at least six forms of equine herpesviruses.  These can be divided into subfamilies, and each subfamily contains multiple antigenically distinct viruses:
  1. Alphaherpesvirinae, including EHV-1, -3, -4 and -9
  2. Gammaherpesvirinae, including EHV-2 and -5

These subfamilies generally have somewhat different characteristics: (25)

Alphaherpesvirinae

Alphaherpesvirinae are able to infect a wide range of hosts, replicate relatively quickly and will cause lysis of cultured cells.  They are generally neurotropic (though not all), and cause neurological disease by establishing latent infection in sensory ganglia. (25)

Gammaherpesvirinae


Gammaherpesvirinae are non-neurotropic, grow slowly in vitro, and demonstrate host tropism.  They establish latent infection in lymphatic tissue as opposed to neural tissue. (25) 

Equine herpesvirus is made up of several different antigenically distinct groups of viruses.  These include EHV-1, EHV-2, EHV-3, EHV-4, EHV-5, EHV-6, EHV-7, EHV-8 and EHV-9.


Image: Electron Micrograph on Equine herpesvirus. (3)



EHV-1
Equine herpesvirus 1 exists in horse populations worldwide and is associated with acute respiratory inflammation.  It is also responsible for abortion in mares; abortion is possible in either clinical or subclinical cases.  EHV-1 is also associated with neurological disease, though this is rare. (4)

EHV-2
Equine herpesvirus 2 (EHV-2) is a disease whose clinical significance is unclear however it is consistently found in the respiratory mucosa, conjunctiva and leukocytes of clinically normal horses. (4)  EHV-2 has been associated with keratoconjunctivitis, upper respiratory tract disease, generalized malaise and fever, pharyngitis and enlarged lymph nodes. (34)

EHV-3
Equine herpesvirus 3 (EHV-3), also called Equine coital exanthema, this disease is characterized by an acute appearance of red papules in the vaginal and vestibular mucosa as well as on the perivulvular skin of mares.  These papules quickly progress to pustules, ulcers, and ultimately result in depigmented scars after healing.  Similar lesions can be observed on the penis and prepuce of affected stallions.  EHV-3 is spread by venereal transmission from an infected stallion, and clinical signs will appear in a mare 2-10 days following infection.  There are no systemic signs associated with infection, and EHV-3 does not result in inhibited fertility, however, the disease may decrease mating incidence while clinical signs are present due to discomfort. (4)

EHV-4
Equine herpesvirus 4 (EHV-4), like EHV-1 causes acute respiratory inflammation upon infection. (4)  It also exists in populations of horses worldwide.  EHV-4 is rarely associated with neurological sequelae. (34)

EHV-5
Equine herpesvirus 5 (EHV-5) has been reported to be associated with respiratory disease in horses, but has also been isolated from clinically healthy horses. (34)  Association with disease is unclear.

EHV-6
Equine herpesvirus 6 (EHV-6) causes coital exanthema in donkeys. (8)

EHV-7
Equine herpesvirus 7 (EHV-7) is not known to be associated with disease. (8)

EHV-8
Equine herpesvirus 8 (EHV-8) is referred to in one paper, (35) but there is very little information available about this virus.

EHV-9

Equine herpesvirus 9 (EHV-9), also recognized as gazelle herpesvirus 1, was recognized as a new neurotropic herpesvirus in 2000.  It has the ability to experimentally infect both dogs and cats, resulting in clinical neurological signs in live animals as well as post mortem changes in neurological tissue both grossly and histologically (24) (28) (35).




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