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CDC Telebriefing Transcript
This Week's MMWR Looks at West Nile Virus Activity
June 13, 2002
CDC MODERATOR: Thank you, Paul. Good afternoon. Today, we will have a
discussion with two CDC experts in reference to the West Nile virus. You
will get this afternoon a copy of the MMWR which will feature the lead
article on West Nile virus, which will essentially dictate our discussion
We have joining us Drs. Daniel O'Leary, a West Nile expert here at CDC, and
Dr. Stephen Ostroff, the acting deputy director for the National Center for
Infectious Disease at CDC. I will spell both their names, and then we will
proceed with Dr. O'Leary providing an overview of today's article, followed
by comments from Dr. Ostroff about West Nile virus.
Dr. O'Leary is Daniel, D-a-n-i-e-l O, apostrophe, L-e-a-r-y. Steven Ostroff.
S-t-e-p-h-e-n O-s-t-r-o-f-f. And now I'd like to turn it over to Dr.
DR. O'LEARY: Good morning and thank you for the opportunity to talk about
this new report.
I would like to summarize by saying that in 2001, West Nile virus activity
was reported to the Centers For Disease Control from 359 counties in 27
states and the District of Columbia.
This was an increase from 2000, when activity was reported from 138 counties
in 12 states and the District of Columbia.
There was new West Nile virus activity noted in 16 states. Human cases of
disease were reported from ten states in 2001, and seven of these ten states
had human activity, or human West Nile illness for the first time.
In 2001, horses were the only nonhuman mammals reported with West Nile
infection. There were 733 reports of horse illnesses.
The apparent geographic limit of West Nile virus activity in the United
States in 2001 stretched as far north as the southern part of Maine, as far
west as western Arkansas, and as far south as Florida, and this was an
observation that demonstrated that West Nile virus has moved westward and
southward since 1999, when it was detected only in New York, and in 2000,
when it was only detected in New York, New Jersey and Connecticut.
I want to talk a little bit about West Nile surveillance. The report
stresses that dead birds remain an important indicator of West Nile virus
activity and that the collection and testing of dead birds for West Nile
virus by state and local health departments is an essential component of
West Nile surveillance.
Also an important component is the capture and the testing of mosquitoes for
West Nile infection. Both of these components can provide somewhat of an
early detection system that will give warning that human risk is increasing.
Additionally, surveillance for West Nile illness in horses has proven
somewhat valuable in select areas as providing early warning of West Nile
Moving on a little bit--oh, excuse me. Backing up one moment to talk about
human surveillance activities, the article stresses that at a minimum, human
surveillance should include the West Nile testing of patients with
encephalitis where no other cause can be found, and that this surveillance
should take place well into the fall months, since West Nile virus activity
in humans was noted later in the season in 2001 than in previous years.
Moving on to prevention, prevention of West Nile virus depends on detecting
the activity of West Nile virus in an area as early as possible, then
educating the public on the ways to avoid mosquito bites such as wearing of
protective clothing and the use of insect repellant when outside and when
mosquitoes are biting.
And also what should be stressed is that there need to be strong local
mosquito control activities that monitor levels of mosquito populations and
can guide the communities in decreasing these populations when risk is
The bottom line message of the article is that West Nile activity has been
detected through the eastern half of the United States now. The article will
show you a map that shows the 27 states that are affected, or where West
Nile virus activity's been found. But it is basically, we believe it has now
established itself in the Eastern United States, and so this is the bottom,
this is what we feel is our main point, and also that prevention of human
and animal illness will depend on early detection of the virus through
surveillance efforts at the state and local level, and also that personal
protection against mosquitoes and effective local level community mosquito
control programs will help to prevent human illness.
And now, Bernadette, I'll turn it over to you and Dr. Ostroff.
CDC MODERATOR: Okay. Thank you, Dan. We're going to have a few comments from
Dr. Ostroff in reference to the movement of the virus this year and after
that, we will open things up for questions and turn it back over to you,
DR. OSTROFF: Yeah. Let me just make a couple of very brief comments. One of
them is that what's happening and what we're seeing with West Nile is a
prime example of an emerging infectious disease, this one in the United
States. One thing that's important to note is that in any area that, in a
given year, has reported West Nile activity either in horses, birds, or in
humans, in essentially all of those areas, the following year, the virus has
come back. I think we only have one state that reported a single dead bird
in 2000, that being the State of Vermont, which last year did not report any
findings of West Nile.
But that's the only example we have, where we had a state report West Nile
activity, and then not report it the following year. And so we would fully
anticipate that all of the states that you [audio drop] in the map that's in
this week's article, would, over the course of the 2002 season, once again
report activity related to the West Nile virus, and so we would anticipate
seeing activity, once again, throughout the eastern half of the United
States. It's just a matter of when and where for any individual community or
any particular state.
As far as where the virus will potentially move this season, as Dr. O'Leary
mentioned, it expanded its geographic range considerably over calendar year
2001, and we were somewhat surprised by how far it went to the West and how
far it went in the southern part of the United States, and we have to keep
in the back of our mind that we can't always predict when and where this
virus may appear.
We would anticipate fully, over the course of the 2002 season, that there
will be new locations that will identify West Nile. The tendency for this
virus has been for it to spread to contiguous locations, and if we were to
predict, at all, where we may see activity in 2002, where it wasn't
previously seen, I would most predict those states that are adjacent to the
areas that have previously seen West Nile activity.
However, it remains somewhat of an unpredictable virus and there really,
based on what we've seen the last couple of years, is no reason that the
virus wouldn't continue to expand its geographic range within the United
States. Exactly how far and over what time sequence is what we really cannot
For those individuals that live in the zone where the virus has previously
been, we would certainly make the same recommendations that have been made
by Dr. O'Leary in his comments, and, in particular, for those precautionary
activities, in terms of personal protection are most appropriate for those
individuals that fall into the high-risk group for human illness, and that
is our senior citizens, persons with a compromised immune system, for one
reason or another, and these individuals should particularly heed the
precaution measures which are prudent precautionary measures concerning use
of insect repellents when engaged in outdoor activities, when feasible, to
use long sleeves and long shirts, and for those individuals who may be at
particularly high risk, they may want to alter their activities so that
they're not outdoors during peak times of the day when the mosquitoes that
are most likely to transmit the disease are active, which is early morning
and in the evening.
And those messages were messages that have previously been distributed, and
they're still absolutely relevant for those individuals who wish to reduce
their risk of being exposed to this virus, and, with that, we'll open it up
AT&T MODERATOR: Ladies and gentlemen, if you wish to ask a question, press
the one on your touchtone phone. You'll hear a tone indicating that you've
been placed in queue and you may remove yourself from queue at any time by
pressing the pound key.
If you're using a speaker-phone, please pick up your handset before pressing
the numbers. Once again, if you wish to ask a question press one at this
time, and one moment, please, for the first question.
Once again, ladies and gentlemen, if you have a question please press one at
We have a question from the line of Erin McLinn [ph], Associated Press.
MR. : Yes. Hi. You talked about how this virus is expanding geographically.
Could you talk about how it's expanding sort of across the calendar. We've
seen cases later and later. Do you expect it to go later into the fall this
DR. OSTROFF: Well, I think there are a couple of comments that I would make
about last season. I think it's a testament to the public health system,
that given the events of last fall, particularly in those locations where
most of the human cases of West Nile were identified, which is up in the
urban Northeast, and in Florida, that our surveillance systems functioned as
well as they did, given the anthrax events, and given the 9-11 events.
But we do feel that despite those other events, that the surveillance system
gave us a relatively accurate depiction of the time sequence, and certainly
where there was evidence that the virus was present.
We have always recognized that as the virus moves into different geographic
areas ,that there are likely to be differences in terms of when human
illness may occur and where it may occur, because the types of mosquitoes
that are present, the climatic conditions, the ecology in different places
is just different, and the mosquito transmission season, as one example,
down in the southern part of the United States are considerable longer than
they are in northern climates.
In addition, last fall was a bit unusual in that it remained quite warm,
even in the northern part of the country, well into the fall, and there was
human illness detected in places like Massachusetts and Connecticut, well
into the autumn, during periods when I think if you were to talk to most
experts on mosquitoes, would tell you are sort of beyond the usual period
where one would have to concern themselves with mosquito-transmitted
diseases in a place like Massachusetts in the month of November.
Having said that, I wouldn't jump to the conclusion that this represents a
climate change phenomenon or global warming or anything like that.
We do know that there are anomalies from year to year to year in terms of
when disease may occur and what the climatic conditions may be, and it may
not follow the same pattern this year.
We did notice, and it has been reported, that there were West Nile findings
down in the southern part of the United States, in the State of Florida at
very low levels throughout the winter season, basically indicating that in
some of these locations the virus will set up year-round transmission. And
so for those individuals who live in some of these areas that might
experience much longer transmission seasons, the prevention messages are
very relevant for them, not only in the summer months but also during times
of year where mosquitoes may be active.
MR. : Thank you.
AT&T MODERATOR: We have a question from Adam Merkis [ph] with Health Scope
MR. : Hi. I have a few questions. I don't know how, if you'd like to take
them all at once, or I can ask them in series.
But the first is do you have any new information on the total number of
human cases, including the number of deaths? Have there been any signs this
year, so far, of the virus, and any indication of how it spreads?
DR. OSTROFF: Dan, you want to take those?
DR. O'LEARY: Okay. As far as the number of human cases in 2001, there were
66 human, cases of human illness. We have not had any cases of human illness
reported yet this year.
In terms of the number of deaths, in 2001 there were nine deaths among those
66 cases. What we have seen over the past three years, 1999, 2000, and 2001,
is that about 14 percent of humans who develop encephalitis actually died
But we don't have any new data in 2001. There are no human cases reported at
DR. OSTROFF: The other thing that I'll mention is that that 14 percent
figure compares to a figure of 11 percent in each of the previous two
seasons. That difference is really not significant.
The mortalities for West Nile, that have been stated in the medical
literature are--have usually been between 10 and 20 percent, individuals
that develop clinical illness, and so far the experience in the United
States has been exactly in that particular range.
As far as your third question about how is the virus spreading from place to
place, the experts pretty universally agree that the spread of the virus is
occurring through migratory birds and this is one of the reasons why there
isn't a good way to prevent the virus from moving into additional geographic
locations, and this is why we would continue to expect to see the geographic
range of the virus expand.
MR. : Can you still hear me?
DR. OSTROFF: Yeah.
MR. : If it is indeed spreading through bird migration, why would you see an
increasing pattern of spreading, unless there's some sort of threshold
effect where you need to have a certain level of the virus, you know, in the
background, because the birds migrate, presumably the same patterns every
DR. OSTROFF: Well, yes and no. As they migrate, for instance, north-south,
in the spring and in the fall, they also tend to mix with birds from other
locations, and then what happens is those birds, as they migrate north back
to their home areas, may move the virus with them. We think that's how a lot
of the spread could well occur.
MR. : Thank you.
DR. O'LEARY: I might add just a comment to that, and when infected--the
thought is that when infected birds do travel, carrying the virus in their
blood to a new area, that bird is fed upon my mosquitoes who establish the
virus in a new area, and then can infect additional birds in that area.
AT&T MODERATOR: We have a question from the line of Paul Neal [ph] of
MR. : Yes. You mentioned infections in Canada and the Cayman Islands in the
report. Do you expect the virus to spread to other parts of the Americas,
the way it is in this country?
DR. OSTROFF: Again, this is information from most of the experts that have
tried to examine what the potential, eventual range of the virus may be, and
all we can say at this point is that it has been detected in the Caribbean,
certainly it has been detected in Canada, and most of us would feel that
there's no reason that the virus wouldn't
continue to move southward. Certainly, the ecologic conditions that are
present in the Caribbean and Central America and in South America are
conducive to--I mean, they have the right environment conditions, they have
the right mosquitoes for the virus to continue to be, to continue to move
The question for us is whether or not the systems will be in place in some
of these areas to be able to identify its presence, because some of these
locations, particularly if you get down into areas of South America, are not
heavily populated, and we note from a lot of experience that the best
indicator of the presence of the virus seems to be detection of dead birds,
and it's one of the same issues that we have with further westward expansion
of the virus into some parts of the Great Plains where the population
densities of humans are not particularly large, and if a bird dies and drops
to the ground in the middle of an area where there aren't any people,
nobody's going to notice that it's there and the less populated an area is,
the less likely it is that we're going to detect evidence of the presence of
But, ultimately, we think that there's really nothing that would limit the
ability of the virus to continue to move southward.
MR. : Thanks.
AT&T MODERATOR: We have a question from the line of Eri Peck [ph] with WHYY.
MS. : Hi. Dr. O'Leary, could you elaborate on the virus establishing itself,
how that happens, and what it means to sort of be established, and is it
established in New Jersey and in the Philadelphia area?
DR. O'LEARY: By "established" we mean that it has turned, it has--activity
has been detected by surveillance efforts at the state level, season after
season, since 1999, and New Jersey is one area. Well, actually, New Jersey
first detected--I'd have to look to see what years New Jersey detected the
DR. OSTROFF: It was detected--
DR. O'LEARY: Back in '99, was it, Dr. Ostroff?
DR. OSTROFF: Yeah; it was in '99.
DR. O'LEARY: Okay. So the virus has turned up, has gone through a winter,
between 1999 and 2000, turned up again in 2000, overwintered in 2000-2001,
and then was detected again in 2001.
So in this way we are--from this, we are concluding that West Nile is
established, or that it will--and we have no reason to believe that it will
not continue to reemerge in future seasons.
Dr. Ostroff, would you like to add anything?
DR. OSTROFF: Yeah. I think that's correct. I mean, when we use the term that
it's become established in a particular location, it means from one year to
the next it's been identified, indicating that it is present in that area
and we would fully anticipate that it will be part of the range and gamut of
what we call vector-transmitted diseases, that individuals in places like
New Jersey and Philadelphia will have to worry about, not only this season,
but probably in the years to come.
And it's worth pointing out that there are other vector-borne agents that
are also capable of causing human disease in the Northeast United States.
That doesn't mean that they always do. They tend oscillatory patterns in
terms of when and where disease occurs, and generally it's at pretty low
levels, and so as I think was reported last year in some of the newspapers
in New Jersey, this is yet another disease present within the greater
environment in New Jersey, that individuals in that state will just have to
factor in to their activities, and we've always said, from the very
beginning, that very prudent, precautionary measures are all that are
necessary to reduce your risk of West Nile, and, in general, for a large
proportion of the population, particularly in the younger age groups, the
risk of developing West Nile is really quite low.
It's certainly not zero, and there are other types of vector-borne diseases
which are also preventable with the same type of precautionary measures,
that occur in younger age groups.
And so we would urge that all people take reasonable precautions to reduce
their risk of being bitten by mosquitoes. It's not only good public health
advice but it also makes things a lot more pleasant for you when you're out
doing outdoor activities, not to be constantly be bitten by mosquitoes.
MS. : Great. Just one clarification. When you said it overwinters, does that
mean it stays in the native bird population, or it comes back every year
with migratory birds, or do we not know?
DR. OSTROFF: Dan, you want to take that one?
DR. O'LEARY: Well, we have, our research scientists here have detected West
Nile in mosquitoes that were found in a dormant stage during the winter. So
we don't know at what levels the virus is overwintering in mosquitoes but we
do know that it does occur at at least a low level and we believe that this
may well be one contributing mechanism for the persistence of West Nile from
season to season. It would be speculation, but we think that migrating birds
can also bring, move the virus, and if that is true, they could certainly
move the virus back into an area that had activity the previous season.
MS. : Thank you.
DR. OSTROFF: But the one thing that I would add to that is that at this
point it's pretty unlikely that in a place like New Jersey, that there needs
to be reintroduction of the virus in migratory birds to have the virus
appear again during the season in the State of New Jersey, because of this
phenomenon that appears to occur with West Nile, which is that there are
dormant overwintering mosquitoes that probably do carry the virus and then
bring it back out in the springtime.
MS. : Great; thank you.
AT&T MODERATOR: At this time there are no further questions in queue.
CDC MODERATOR: Do you all have any comments you'd like to add?
All right. Thank you very much for your participation today. If you have any
follow-up questions, you can contact the CDC Press Office at area 
639-3286. Thank you and have a good afternoon.
AT&T MODERATOR: That does conclude our conference for today. Thank you for
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