The
2002-2003 Region 11 officers are Jeff
Gilberston (President), Jerry
Erickson (Vice President), Ron
Pray (Secretary/Treasurer) and Torben
Hansen (Trustee).
West
Nile Virus
by
Dr.
Charlie Hjerpe
During
the past couple of days, I have received 4 phone calls and/or
E-mail messages asking me questions about West Nile Virus (WNV)
and what to do about it. The current situation has implications
for our sport (see section C., at the end of this encyclical).
Consequently, I decided that I needed to become better informed,
and I am passing on to you some of what I have learned.
A. FACTS
ABOUT WNV:
1. WNV is known to cause clinical disease and death only in
birds (many different species), horses and man.
2. WNV was first discovered in Uganda (Africa) in 1937.
It was not known to be present in the United States, prior to
1999. During the initial (1999) outbreak of "West Nile fever"
in the western hemisphere (in southern New York State), 7 people
and 9 horses died.
3. Currently, WNV is known to exist in nearly all states
east of the Mississippi River (except Minnesota, Wisconsin and
Vermont). To the west of the Mississippi, it has been detected
only in Iowa, Missouri, Arkansas, and Texas.
4. The reservoir for WNV in nature is in wild birds. Most
infected birds do not show evidence of clinical disease, but often
develop high and persistent concentrations of WNV in their blood.
Mosquitoes become infected by feeding on infected birds, and then
pass the virus on to other (non-infected) birds, horses and people.
At least some infected birds (that do not die of the
infection) probably become permanent carriers of WNV.
5. Horses and people are so-called "dead-end hosts",
because WNV can only be detected in their blood for a short period
of time following infection, and then only in relatively low concentrations.
Consequently, equine and human infections are unlikely to contribute
significantly to (new) mosquito infections, which could initiate
an outbreak or contribute to the severity of an existing outbreak.
Parenthetically, the disease cannot ordinarily be directly or
indirectly transmitted between horses, between people, or between
horses and people (even by mosquitoes).
6. The discovery of WNV in Texas on June 19th of this year
has touched off quite a bit of concern in California public health
circles. This is because certain east coast bird species, that
travel through Texas during the spring and fall migrations, are
known to share "migratory pathways" with a number of
California's migratory bird species. This suggests that there
is considerable risk that WNV may soon appear in California.
7. The "Corvidae", a family of birds that includes
jays, magpies and crows/ravens, appear to be more likely to sicken
and die as a result of WNV infections than most other bird species.
Consequently, they can be used as "sentinel species"
for early detection of new WNV disease outbreaks. The phone calls
I have been receiving were prompted by public service announcements
on local radio and television stations, asking listeners who might
observe unusual numbers of dead or dying birds to contact local
public health authorities, especially if affected birds are Corvids.
8. Human beings appear to be quite a bit less susceptible
to severe disease caused by WNV than are horses. Only about 1
out of every 100 infected people will develop clinical signs of
disease, and only about 1 out of every 1000 infected people will
die from their infection. There is no specific treatment for WNV
infection, either in people or horses.
9. In contrast to the situation in man, about 1 out of every
10 infected horses will develop clinical signs of disease. (Parenthetically,
9 out of 10 horses infected by mosquitoes will develop subclinical
infections, and will remain perfectly healthy). About 2 to 4 out
of every 100 infected horses will die from their infection.
10. Clinical signs associated with severe WNV infections in
horses result from a severe viral meningoencephalomyelitis, and
include fever, depression, stumbling, staggering, wobbly gait,
incoordination, circling, head pressing, trembling, posterior
weakness, paralysis, prostration and death. These clinical manifestations
are not diagnostic, and can be caused by a fairly long list of
other equine diseases in which the central nervous system becomes
involved.
11. There is no particular breed, sex or age of horse that
is any more susceptible to WNV than any other.
B. PREVENTION OF WNV INFECTIONS IN HORSES:
Prevention of WNV infections is based on mosquito control
and vaccination of horses.
1. Mosquito Control: To be honest, mosquito control is
not likely to be 100% effective in preventing WNV infections in
situations where large proportions of the local bird and mosquito
populations are infected with WNV. The elements of mosquito control
are as follows:
a. Eliminate standing water in which mosquitoes can breed,
in such places as bird baths, old discarded car and truck tires,
empty garbage cans, etc.
b. Where stagnant water cannot be eliminated, such as in
ponds, stock watering troughs, etc., stock these locations with
Gambusia sp. (mosquito fish).
c. Utilize insect repellant sprays and wipes directly on
your horses.
d. Stable horses at night, from well before dusk until
well after dawn. For this strategy to be effective, the barns
must be screened and fogged with an insecticide, before the horses
are placed in them.
2. Vaccination:
a. Fort Dodge Laboratories markets a vaccine which has
received "conditional" approval for use in California.
This means that the vaccine (in preliminary studies) has been
shown to be safe and to produce an antibody response in vaccinated
horses, but has not actually been tested and shown to be effective
in preventing the disease under actual field conditions.
b. Fort Dodge recommends that two doses of vaccine be administered
at a 3 to 6-week interval between doses, and that a booster dose
be administered annually.
c. Detectable WNV antibodies do not appear in the blood
of vaccinated horses until several weeks following administration
of the second dose of the vaccine. Therefore, it is likely that
initiation of vaccination following initial discovery of the presence
of WNV in California will be less that optimally effective in
preventing clinical disease. Consequently, if you plan to vaccinate,
it is probably advisable to begin immediately, and to booster
again in early spring, prior to the first arrival of migratory
bird species in March of 2003. I plan to go in to the Veterinary
Medical Teaching Hospital tomorrow morning, pick up 8 doses of
vaccine, and begin to vaccinate my horses immediately, if not
sooner.
C. IMPLICATIONS OF WNV FOR THE SPORT OF FIELD TRIALING:1.
I can almost guarantee that, if WNV were to be found in California
during the next few weeks, we would not be able to take our California
horses with us into Canada this summer. The European Union now
prohibits the entry of U.S. horses that originate from any state
in which WNV is known to exist. It is also possible that non-infected
states, such as Montana and North and South Dakota, might decide
to start doing the same thing.
2. It is possible that, at some time in the future, various
states and/or countries could begin requiring that a horse must
have a negative blood test for WNV antibodies in order to enter
or pass through their particular real estate. This would be a
problem for previously vaccinated horses, because it is NOT now
possible to determine whether a positive WNV reaction is a result
of vaccination or from actual infection.
3. It is likely that the agriculture departments of most states
will now begin to enforce "the letter of the law" with
respect to interstate health charts and Coggins tests for horses.
If you plan to travel across state lines with your horses this
summer, you should plan to have all of your "papers' current
and in order.
4. WNV infection can cause a very bad disease, and if it becomes
established in California, and if we don't vaccinate against it
(or if it turns out to be the case that the vaccine is not very
effective), some of our field trial horses are going to be dying
from "West Nile fever".
For those of you who want to know more about this, please check
out the following web sites: www.vetmed.ucdavis.edu/ceh
and www.cdfa.ca.gov/ahfss/ah/wnv_info.htm.
Sincerely,
Dr. Charlie Hjerpe
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