Walking On Volume 6, Issue 4, April 2019 | Page 14
For the Health of It
Equine Influenza
Reprinted with permission from the April 2019 issue of Equine Disease Quarterly
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 evi-
dence 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 recommenda-
tion.
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 ac-
tivity surged in the last three months of 2018 with out-
breaks 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 Califor-
nia, Arizona, Ohio, Indiana, and Washington State.
Some of the horses in these outbreaks, in both the
14 • Walking On
USA and Europe, had been vaccinated for EI, raising
the question: Is this a new strain that is not in the vac-
cines? The answer appears to be no. While the virus
causing the Nigerian EI event is still uncharacterized,
genetic 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 Eu-
ropean circulation means their horses have no natu-
ral 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 unvaccinat-
ed 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 usu-
ally watery (serous) at first and then turns thick and
yellow (mucopurulent). The horse may show unusu-
ally rapid breathing (tachypnea) or lose its appetite
(anorexia). Sometimes there may be enlarged subman-
dibular lymph nodes or dependent limb edema. Keep