Page 703 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

678 CHAPTER 3

VetBooks.ir Immunostimulants (e.g. inactivated Propionibacterium to susceptible horses. The disease is OIE listed and is
notifiable in many countries. EVA can be controlled
acnes, mycobacterial cell wall abstracts or interferon
alpha) are used by some practitioners in horses with
suspected chronic post-viral disease, but such prod- by vaccination.
ucts are not licensed in all countries and are not Aetiology/pathophysiology
indicated for acute disease. In cases that develop EVA is caused by a togavirus and has worldwide dis-
neurological disease supportive care should be pro- tribution. The virus is mainly transmitted directly
vided. Nucleoside analogues antiviral drugs can be from horse to horse by aerosolised respiratory secre-
used in those cases, such as p/o or i/v acyclovir and tions and venereally from carrier stallions to mares.
p/o valacyclovir, but these compounds have poor Indirect transmission via fomites is also possible.
bioavailability in horses and reports with convincing There is initial virus replication in the respiratory
clinical benefits in horses are currently lacking. tract followed by cell-associated viraemia, which dis-
Vaccination against EHV-1 and EHV-4 assists in seminates virus to the reproductive tract. Viraemia
disease control by decreasing nasal virus shedding, is associated with a leucopenia, principally a lym-
thus reducing the spread of the virus, and reduc- phopenia, and a leucocytosis, mainly a lymphocy-
ing the frequency of abortion. Current vaccines are tosis, develops on recovery. Virus replication in the
licensed for control of respiratory disease and abor- walls of small arteries and veins causes vasculitis.
tion. No vaccines possess a marketing authorisation In stallions, persistent infection of the accessory
for control of neurological disease. However, the sex glands, mainly the ampulla, is established and
licensed vaccine in Europe and the licensed vaccines infectious virus is shed in semen for several years,
in the rest of the world have all demonstrated the although semen quality and fertility are not affected.
ability to reduce virus replication in vaccinated horses Infection of mares via venereal or respiratory routes
and so may be helpful in preventing neurological dis- at the time of covering causes early embryonic
ease. Practitioners should be aware that the use of death. Infection of pregnant mares at any time of
vaccines to aid control of neurological disease consti- gestation via the respiratory route causes fetal death.
tutes off-label use, which in Europe requires informed Serological surveys in the UK have revealed marked
consent from the owner or keeper of the animal. breed differences in the incidence of seropositive
horses and carrier stallions, presumably reflecting
Prognosis differences in stud management and practice, with
The prognosis is good for respiratory disease, Standardbreds having a significantly higher inci-
although some horses may develop chronic fatigue/ dence than other breeds.
poor performance syndromes. Mares that have
aborted usually successfully breed in the next season. Clinical presentation
The prognosis for neurological cases varies signifi- The incubation period has an average of 7 days but
cantly, with more severely affected horses (recum- seems to be highly variable in the field, possibly up to
bent) sometimes requiring euthanasia on welfare 21 days. Initial clinical signs are pyrexia, depression
grounds and less severely affected horses making a and anorexia. Clinically obvious respiratory disease
full recovery. may develop, but in many mares and stallions disease
is mild or subclinical. Respiratory signs, if present,
EQUINE VIRAL ARTERITIS are typical of infectious URT disease and include
nasal discharge, lymphadenopathy and coughing.
Definition/overview There may be marked conjunctivitis and lacrima-
EVA causes respiratory disease and abortion and tion. In some horses there are clinical signs relating
thus shares many clinical features in common with to vasculitis including conjunctival and periorbital
EHV-1 infection. Stallions become carriers, shed- oedema, scrotal and preputial oedema and oedema
ding infectious virus in semen, and they are the of the limbs. There are usually no clinical signs
main reservoir of infection and transmission of virus relating to early embryonic death. Where abortion