our expertise: VET COLUMN
Equine
Influenza
Tom Hutchins, DVM, DABVP
North Texas Veterinary Hospital
8283 FM 920 • Weatherford, Tx
Dr. Tom Hutchins
E
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quine influenza has a worldwide distribution
and is frequently seen in horses transported
multiple times a year. Influenza is the most
common cause of upper respiratory infections
in horses. It has a sudden onset and a short
incubation period of 1-3 days. Disease
outbreaks usually occur in horses 1-3 years
of age. This time generally corresponds with
intermingling of horses at show grounds,
racetracks and sales. Influenza affects older
horses as well but clinical signs are usually
milder and may not even be apparent. Although
infections can occur at any time of the year,
they seem to be more frequent in the winter and
spring. Lower humidity and temperatures tend
to enhance viral survival.
The initial clinical sign that is most evident
is the sudden onset of high fever and lethargy.
Rectal temperatures can climb as high as 106°F.
Cough and nasal discharge accompany the fever
but may not appear for several days following
the initial fever spike. The cough may persist
for up to three weeks. Endoscopic examination
of the upper airway reveals pharyngitis and
tracheitis. Also the development of myalgia is
diagnosed with the reluctance of many horses to
move in the course of the disease.
The influenza virus belongs to the
orthomyxovirus group. It is easily destroyed by
common disinfectants like chlorine bleaches.
Minor changes in the surface proteins of the
virus constitute a “drift” in the makeup of the
virus. Major changes in the surface proteins are
called a “shift”.
Infection in a susceptible horse begins with
inhalation of the virus. Incubation time as
mentioned previously is 1-3 days. The virus
attaches to and proliferates in the epithelial cells
of the entire equine respiratory tract. Studies
have shown that tracheal mucociliary clearance
rates are reduced in horses infected with equine
influenza for up to 32 days despite clinical
improvement. This damage apparently impairs
the transport of mucus in affected animals
from the lower airway and predisposes them to
secondary bacterial infection. A horse infected
with the virus may be unfit for competition from
50 to 100 days.
The differential diagnosis that should be
considered when a horse develops a fever
acutely and has a nasal discharge and cough
equine influenza, equine herpes virus 1 & 4,
equine viral arteritis, and Streptococcus equi
(aka Strangles) infection. A commercially
available test detects the virus and requires a
nasal swab with results available in as little as
15 minutes.
Prevention of equine influenza is vaccination.
It has been suggested that upwards of 70 percent
of the population should be vaccinated to