Equine Disease Quarterly | EQUINE
seasonal, occurring in the summer and the fall. In sub-
tropical regions such as Florida, there is a year-round
risk of EEE, with virus transmission peaking in the
summer 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 frequently 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 Canada. 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 insufficient
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 accordance with criteria defined by the
American Veterinary Medical Association with
respect to “core vaccines”–namely those that protect
against diseases that are endemic, of potential
public health significance, and represent a risk of
causing severe disease—strongly recommends
that horses be immunized against EEE and WNE.
Available inactivated whole-virus vaccines against EEE
(including Western 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 flavivirus chimera vaccine are available
against WNE. All have been confirmed safe and
effective in preventing the disease.
Despite the AAEP recommendations 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, (859) 218-1094, Maxwell H. Gluck Equine Research Center,
University of Kentucky, Lexington, KY
Rabies in the Horse and Beyond in Kentucky:
A Look at the Last 29 Years
Rabies virus exposure typically occurs following
the bite of an infected animal. Depending on the
anatomic site of exposure, an incubation period of
variable duration follows as the virus evades the
immune response by hiding in the central nervous
system. Amplification of the virus occurs in the
dorsal root ganglion after which it travels towards the
brain via the spinal cord. At this point, clinical signs
• Volume 20 Issue 3 | October 2018 •
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