EQUINE | Equine Disease Quarterly
Anna Hollis, BVetMed, DACVIM, DipECEIM, MRCVS
[email protected]
Animal Health Trust
Newmarket, UK
Equine Influenza
Equine influenza (EI) is considered endemic in both
the USA and Europe, but the viruses causing EI differ
slightly. For many years, the viruses in circulation in
the USA have been ‘Florida clade 1’ (FC1) whereas in
Europe they have been ‘Florida clade 2’ (FC2). These
clades split apart in 2003; circulation of FC2 ceased
in the USA around 2005, and by 2010 there was no
evidence of FC1 in circulation in Europe. Because of
the extensive movement of horses between North
America and Europe, the international panel of EI
experts has recommended for the last ten years that
EI vaccines contain representatives of both FC1 and
FC2. Some (not all) available EI vaccines meet this
recommendation.
EI virus activity has recently increased in the USA,
Europe, and Nigeria. Normally, the virus circulates at
a variable, but fairly low, level in the USA, but virus
activity surged in the last three months of 2018 with
outbreaks in 12 states. An extensive EI event occurred
in a donkey sanctuary in Nigeria. For the first time since
2015, multiple outbreaks of EI were reported during
January and February of 2019 in France, Belgium, the
Netherlands, Germany, Ireland, England, and Scotland.
In England, it resulted in a temporary lockdown of at
least 174 premises and cancellation of racing for 6 days
in February. Outbreaks were also reported in California,
Arizona, Ohio, Indiana, and Washington State.
Some of the horses in these outbreaks, in both the USA
and Europe, had been vaccinated for EI, raising the
question: Is this a new strain that is not in the vaccines?
The answer appears to be no. While the virus causing
the Nigerian EI event is still uncharacterized, genetic
28
analysis of isolates from both England and the USA
confirms these as FC1. There are two mutations that
make them different from the recommended vaccines
strains, but are these important? That is still under
investigation. The absence of FC1 from European
circulation means their horses have no natural immunity
and are dependent on vaccination for protection. The
reports from England indicate that the clinical disease
is of shorter duration and less severe in vaccinated
horses; this would indicate the vaccines are working,
at least partially. Milder disease in vaccinated horses
may reflect an inadequate level of protective immunity
following exposure to unvaccinated horses shedding
large quantities of virus.
What should owners and veterinarians do to protect
their horses from EI?
1. Familiarize yourself with the clinical signs of EI.
Often the very first sign is a harsh cough. Other
signs include fever and nasal discharge, which
is usually watery (serous) at first and then turns
thick and yellow (mucopurulent). The horse may
show unusually rapid breathing (tachypnea) or
lose its appetite (anorexia). Sometimes there
may be enlarged submandibular lymph nodes or
dependent limb edema. Keep in mind that not all
these signs may be present. Horses can be infected
and still appear normal (subclinical infection),
especially if they have been previously vaccinated.
Other infectious agents can produce clinical signs
that look like EI, but aren’t. EHV-1/4 or strangles
(Streptococcus equi) are examples. Have your
veterinarian collect a nasal swab, or ideally a
nasopharyngeal swab (which goes beyond the
nostrils into the back of the throat), and send it
to a veterinary diagnostic laboratory to confirm
a diagnosis. Information on nasal-swabbing can
be found at http://vetsci.ca.uky.edu/services. If
your horse does develop clinical signs of EI, then
• Equine Health Update •