COVER 1 - NMHBA SUMMER 2017 low res

y Heather Smith Thomas
Equine influenza is caused by a virus
and is one of the most common
infectious diseases of the respiratory
tract of horses. It is endemic in the
equine population of the United
States and throughout much of the world,
where young horses aged one to five are
generally the most susceptible. Older
horses may have some immunity if they
have previously encountered the virus.
Equine Influenza is one of the diseases a
horse might be exposed to when leaving
the farm and going to the racetrack.
Mark Crisman DVM, who is the Senior
Veterinarian at Equine Technical Services,
Zoetis in Blacksburg, Virginia, works for
Zoetis and also teaches at the University.
He says equine influenza has been around
for centuries—with outbreaks recorded
throughout North America and Europe.
“One incident that really got our attention
was the outbreak in Australia in 2007.
Australia had been influenza-free, so horse
owners there did not vaccinate for this
disease. It came in to that continent with
an imported horse and the native equine
population was completely susceptible,”
he says.
“More than 10,000 premises (and
all the horses on those premises) were
affected. Australia spent more than a billion
dollars getting that outbreak resolved,” says
Crisman.
“The incubation period for this
respiratory disease is very short—just a
couple of days. We’ve seen outbreaks that
literally swept through a stable or barn,
and in a teaching hospital, where within
48 hours every horse in the barn was
coughing,” he says.
The clinical signs include fever, cough
and nasal discharge. “I’ve seen fevers up
to 104 degrees and 105 and higher with
influenza. The horse also has a harsh, dry
cough. During that phase, the virus is
being aerosolized and can be spread quite
a distance just from the coughing and
sneezing.” If another horse is downwind
from the horse that is coughing, it may
inhale some of those virus-bearing droplets.
“The viral infection usually runs its
course quickly,” says Crisman. It attacks
the epithelium in the upper airways and
damages this lining (and the cilia), but this
damage will heal within one to three weeks
if there are no secondary complications.
Exercising a horse too soon after infection can
cause secondary issues. The deep breathing in of
dust and debris draws it into the airways and can
create a respiratory infection.
Ty Wyant
“The length/duration of viremia (virus
in the blood), which is the period of time
the virus is being shed and can be spread
to another horse, depends on existing
immunity of that particular horse. For
instance, in Australia, where there were
many naïve horses, the disease basically ran
amok. Here in the U.S., it’s a bit different
because many horses have had some
exposure and have some immunity because
they are generally vaccinated, which gives
some protection,” he says.
“The severity of clinical signs can vary
a great deal. Horses with some level of
immunity, such as recent vaccinations,
won’t be as sick and won’t shed the virus
as long. We tell horse owners that they
should rest the horse one week for every
day of fever.” In other words, if the horse
had a fever for only one day, a week of rest
might be adequate, whereas if the horse
had a fever for three days, you should rest
him for at least three weeks before working
him again, even if he feels good before that
time is up.
“The virus attacks the epithelium
in the upper airway. The upper airway
is designed to be a major defense
component of the horse’s immune
system,” he says. This is the first line of
defense to ensure that pathogens don’t get
down into the lungs.
Bacteria and other pathogens exist in
the upper airway, inhaled by the horse. The
tiny cilia (hair-like “fingers” that line the
epithelium) constantly move in wavelike
motion to move dust or any other inhaled
debris—including pathogens—up out of
the airway so they can be coughed out or
swallowed, keeping them out of the lungs.
30 New Mexico Horse Breeder