Walking On Volume 5, Issue 8, August 2018 | Page 6

For the Health of It
Importance of Protecting your Horse
against Eastern Equine Encephalomyelitis
and West Nile Encephalitis
Reprinted with permission from the July 2018 issue of Equine Disease Quarterly
The two most frequently encoun-
tered causes of equine encephalitis
or equine encepha- lomyelitis in
North America are Eastern Equine
Encephalomyelitis (EEE) and West
Nile Encephalitis (WNE) viruses.
Both are mosquito-borne and neuro-
tropic. The respective viruses are not
restricted to equids in terms of their
host range; each can be transmitted to
humans and certain other mamma-
lian and avian species. Both diseases
are a source of concern for the equine
industry not only from the poten-
tially life-threatening consequences
of either infection, but also from the
economic losses involved.
Eastern equine encephalomyelitis
poses an annual threat to equids in
the Gulf and Atlantic coastal states
and the Great Lakes region, extending
in certain years as far north as eastern
Canada. It is occasionally recorded in
some inland states such as Arkansas,
Oklahoma, Tennessee, Kentucky, and
Iowa. Evidence of EEE activity is most
often reported in Florida, in which it
has been confirmed as early as Janu-
ary, as recorded in 2018. Over the past
15 years, the yearly incidence of EEE
cases in equids has ranged from 60
(2011) to 712 (2003), with an annual
average of 206 cases. In 2017, 86 cases
of the disease were reported in 13
states.
In temperate regions, transmission
of EEE virus is seasonal, occurring in
the summer and the fall. In sub-trop-
ical regions such as Florida, there is
a year-round risk of EEE, with virus
transmission peaking in the summer
6 • Walking On
months.
Equids and humans are tangential
or dead-end hosts of EEE virus and
neither plays a role in the natural
life-cycle of the virus. Infections in
horses, mules, and donkeys are fre-
quently life-threatening; case fatality
rates can be as high as 90 percent.
West Nile encephalitis (WNE) is
also a cause of significant concern to
veterinary practitioners and members
of the equine industry. Within four
years following initial introduction
of the causal virus in New York State
in 1999, the virus had spread to 48
states and several provinces in Can-
ada. Since 1999, the yearly incidence
of WNE cases in equids has ranged
from 60 (2000) to 15,257 (2002). The
annual average number of cases over
the past 10 years was 272. In 2017,
307 equine cases were reported in 39
states.
Similar to EEE, transmission of
WNE virus is seasonal, occurring in
the summer and extending well into
the fall. Neither equids nor humans
serve as amplifying hosts for WNE
virus insofar as viremias are insufi-
cient in magnitude and duration to
infect mosquitoes. Unlike EEE, only
about 10% of WNE virus-exposed
horses will develop clinical infections.
Reported case-fatality rates in affected
horses can reach 30-40 percent, less
than half that encountered in cases of
EEE.
The American Association of
Equine Practitioners (AAEP), in ac-
cordance with criteria defined by the
American Veterinary Medical Associ-
ation with respect to “core vaccines”–
namely those that protect against
diseases that are endemic, of potential
public health significance, and repre-
sent a risk of causing severe disease—
strongly recommends that horses be
immunized against EEE and WNE.
Available inactivated whole-virus
vaccines against EEE (including West-
ern Equine Encephalomyelitis) have
been shown to be safe and effective in
protecting against this disease. Two
inactivated whole-virus vaccines, a
live canary pox vector vaccine and an
inactivated avivirus chimera vaccine
are available against WNE. All have
been confirmed safe and effective in
preventing the disease.
Despite the AAEP recommenda-
tions to horse owners to vaccinate
their horses against EEE and WNE,
regrettably many fail to do so. The
vast majority of equine cases of EEE
and WNE either have no history of
vaccination against the particular
virus or else the vaccination history
is incomplete. There is need for an
ongoing concerted effort, utilizing all
avenues of communication including
social media, to alert horse owners of
the dangers of these two vector-borne
diseases and of the importance of
vaccination as an effective means of
prevention and averting the losses
that continue to occur every year in
unprotected horses.
CONTACT:
Peter J Timoney, MVB, MS, PhD, FRCVS
ptimoney@uky edu | [phone]
Maxwell H Gluck Equine Research Center
University of Kentucky Lexington, KY