Transmission occurs
by vectors (i.e., mosquitoes), and the disease is generally
associated with areas inhabited by these vectors and where
winter temperatures are usually not severe. Vaccination
before vector season and biannually in humid areas of the
country is recommended. The vaccine for encephalomyelitis is
often polyvalent in combination with tetanus toxoid,
influenza, or other vaccines. It is important to note that
vaccination with the Venezuelan type mayinterfere with
exportation of horses to some European countries because of seroconversion.
Equine
Viral Arteritis
Equine viral arteritis is a viral disease caused by a arterivirus
that can cause respiratory disease and abortion in horses. The
vaccine is only recommended for use in stallions, open mares, and
young horses six weeks of age or older. Pregnant mares and open
mares or stallions that are within three weeks of being bred
should not be vaccinated. Vaccination may interfere with
exportation to some countries because of sero-conversion. Before
using the vaccine, veterinarians/horsemen should check with their
state veterinarian because some states regulate its use.
Influenza
Influenza is a viral disease that affects the upper respiratory
tract of horses. Vaccination with a killed virus vaccine requires
frequent administration (every two to six months) to provide
appropriate protection to susceptible horses. A modified-live
virus intranasal vaccine is also currently available.
The vaccine is available
in monovalent or polyvalent forms. A transient postvaccinal fever
occurs in some horses. Other horses also may show a loss of
appetite and depression after vaccination. Once a horse is
infected, the upper respiratory tract takes three weeks after
remission of the disease to recover fully. Thus, the significance
of immunizing the competitive and/or show horse to prevent costly
layoff is apparent. In addition, it is equally important to
vaccinate horses traveling to and from sales and breeding farms.
Current recommended
vaccination schedules are every six months for adult horses at low
to moderate risk and every one to three months for high-risk
animals. Vaccinations should not be administered two to three
weeks before a stressful event, such as a show, because of the
increased chance of the previously mentioned side effects.
Potomac
Horse Fever
Potomac horse fever (equine monocytic ehrlichiosis) is caused by
E. risticii. The disease is characterized by diarrhea, ileus, and
often laminitis. The disease has been reported mainly in the
northeastern United States and Canada but has occurred in other
countries. The disease is seasonal and usually occurs in late
spring to early fall. As soon as Potomac horse fever occurs on a
farm, there is a high risk of recurrence.
A freshwater snail is
suspected to play a role in the transmission of the disease.
Current vaccines offer short-term protection, and revaccination
every six to twelve months is recommended. If an infected horse
survives, it will maintain immunity for about two years, and
should not require immediate revaccination. Vaccine failure has
been reported in endemic areas.
Rabies
Rabies is a rhabdovirus that causes neurologic disease. The
disease can cause a large variation in clinical signs that range
from lethargy to bizarre, frightening behavior. Horses in enzootic
areas should be vaccinated, especially those exposed to wildlife.
In some horses, a local
tissue reaction occurs, and thus intramuscular vaccination is
recommended in the lower thigh, not in the neck. Some vaccines of
this type can be given subcutaneously (just under the skin).
Vaccinated horses that are exposed to a rabid animal should be
revaccinated and observed for 45 days. A licensed veterinarian
should examine the horse at the initial time of injury and perform
follow-up evaluations.
Rabies is invariably
fatal in unvaccinated horses, and it is zoonotic; therefore,
unvaccinated horses exposed to a rabid animal should be
euthanized. Vaccination of unvaccinated horses after exposure to
the disease is not recommended.
Owners unwilling to have
their horse euthanized should isolate and closely observe the
animal for six months. This isolation must occur under the
authority of the rabies control agency. Illness or behavioral
change must be reported the local rabies control agency
immediately, and the horse must be evaluated by a veterinarian. If
clinical signs are suggestive of rabies, euthanasia of the animal
is required for rabies testing.
Rabies is the one disease
that I insist that my clients vaccinate for.
