|West Nile VirusHow Real is the Danger?|
by Beth A. Valentine, DVM, PhD
West Nile virus (WNV) has been making headlines and striking fear
into the hearts of horse owners. Hardly a day goes by when someone doesn't
mention WNV. First identified years ago in Africa, WNV was foreign to the United
States until the fall of 1999, when birds, horses, and people in New York State
developed signs of encephalitis, an infection of the brain (enceph = brain, itis
= inflammation). It probably came here in a mosquito, perhaps having hitched a
ride on a cargo ship. At first health officials thought the outbreak was one of
the encephalitis viruses already known to occur in this country, the most common
of which are Eastern Equine Encephalitis (EEE) and Western Equine Encephalitis
(WEE), but this virus soon proved to be different and before long was identified
Initial hopes were that this virus would not continue to
spread, but would die out during the winter of 1999 to 2000. That expectation
did not materialize. WNV is able to overwinter in mosquitoes and birds,
re-emerging when mosquito season starts again. Since WNV infects migratory
birds, it is readily spread from state to state. WNV is therefore not likely to
remain geographically contained, as has EEE, WEE, and VEE.
By the fall
of 2000 WNV had been confirmed in 12 states: Connecticut, Delaware, Maryland,
Massachusetts, New Hampshire, New Jersey, New York, North Carolina,
Pennsylvania, Rhode Island, Vermont, Virginia, and Washington, DC.
of mid August of 2002, it has been confirmed in 37 states and the District of
Columbia. Confirmed human cases are those documented to be WNV by the Centers
for Disease Control and Prevention (CDC) and confirmed equine cases have been
tested at the USDA National Veterinary Services (NVSL) at Ames, IA. Since its
first appearance the virus has spread to include more than half the United
States, extending as far west as Texas, Colorado, New Mexico, and Wyoming.
Some folks on the West Coast are convinced WNV will never be a problem in their
area, but they may well be proven wrong. Surveillance for WNV has been
established in the western states, and dead birds and animals with suspicious
neurologic diseases are already being tested for WNV.
Beware of Numbers
An awful lot of inaccurate information is
being spread about WNV. Some comes from inaccurate interpretation of media
reports, but some just seems to crop up like blackberries in Oregonout of
nowhere. The internet has allowed the rapid dissemination of WNV information,
some good and some not so good. I have seen "confirmed" reports of
entire barns of horses dying or being put down because of WNV. Well, the facts
are that not all exposed horses (or people, for that matter) develop the
disease. Only between 3% and 14% of exposed horses actually develop disease, and
of those only 30% to 40% will be fatal cases. For WNV to infect an entire barn
of horses is highly unlikely.
Sometimes the person posting an erroneous message later recognizes and
corrects the error, as in the case in which one horse was infected and 12 others
on the farm were first said to have died from WNV, later corrected to say 12
others had been vaccinated for WNV. Another recent post, in which a group of
horses on a Pennsylvania farm were supposedly buried due to WNV has never been
corrected, even though the USDA reports only 2 confirmed equine cases of WNV as
of September 2002 in Pennsylvania.
The large numbers published regarding WNV might refer to confirmed WNV
in dead birds (3,128 as of August, 2002), to infected horses or people who
subsequently recovered, to animals that are being tested for WNV (many of which
end up being negative), or to the number of horses that were vaccinated
following detection of WNV in the area. Other diseases cause neurologic signs in
horses, so a horse with these signs is not necessarily a case of WNV, and every
dead bird does not have WNV.
The most accurate and up-to-date WNV information and case numbers are
published by the
Although the CDC reports some equine cases, the USDA is specifically tracking
animal cases of WNV and has more accurate case numbers. But, even these numbers
will be less than actual cases, because the USDA will wait for confirmatory
tests before listing a case, and also because veterinarians in areas of WNV
outbreaks don't always test every suspicious case. So, it is true that the
numbers listed on state and government websites will be less than the actual
number of equine cases.
West Nile Virus and Its Signs
West Nile Virus Prevention
and Answers about West Nile Virus
Nile Virus in Canada
Valentine, DVM, PhD, is an equine pathologist at Oregon State University,
Corvallis, and co-author of
Horses, an Owner's Manual.