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CDC Telebriefing Transcript

News Conference: Update on West Nile Virus

July 15, 2003

DR. GERBERDING: I'm the Director of the CDC, and I'm here today to give you an update on West Nile Virus activity in the United States. Joining us Dr. Jay Epstein from the Food and Drug Administration and Dr. Jim Meegan from the National Institutes of Health. Dr. Epstein will be telling us a little bit about some of the testing protocols that are now available for West Nile and Dr. Meegan will tell us about some of the exciting work going on at NIH, including the work on vaccines.

Today, we are reporting West Nile Virus in 32 states across the United States, including activity involving mosquitoes, birds and animals in these states. And in two states, Texas and South Carolina, they're reporting two human cases each, and there is an additional human case that is currently under investigation in a third state, but we are not confirming that at this point in time.

Basically, we have four human cases confirmed, and this is exactly the same number of human cases as we had a year ago, but what is a little concerning is that we have many more states that are reporting West Nile Virus this year, compared to last year. We had just over 20 states reporting at this exact same time last year. So overall the national activity level has increased.

And we also, if you remember last year, anticipate an upswing in West Nile Virus activity toward the end of July and August, so therefore we have to stay tuned to really appreciate what the full scope and magnitude of the outbreaks will be this year.

What that means to us at CDC, and I think what it means to everyone, is that we need to be prepared. It is too soon to predict the shape of the epidemic, but the signs all indicate that there is reason to anticipate a problem, and the best defense still remains the things that people can do to protect themselves, including using long-sleeve clothing and keeping your skin covered when outside, particularly in the evening and the morning, using insect repellant when appropriate and I think, very importantly, for all of us, emptying the potable water, the standing water in flower pots, and tires and other materials that collect water because that is where mosquitoes breed, and it does really make a difference to remove those mosquito breeding grounds.

Now, it's also important to appreciate what to do if you are worried about having West Nile infection. The infection typically presents with a fever, a very serious headache, often muscle aches and fatigue out of proportion to what you would see with a typical mild virus infection. So anyone who is concerned about the possibility of West Nile Virus infection should consult their clinician and seek medical treatment.

How we learn about cases of West Nile is still something that is undergoing evolution, but the bottom line is that as clinicians or laboratories make a diagnosis of West Nile Virus infection in the state, the case is reported to the state health department. The state health department may do some confirmatory testing in their laboratories because they have the special tests that help be absolutely certain that the preliminary test is accurate and also to distinguish West Nile Virus positivity from some of the other mosquito-borne infections that can present with very similar findings.

If the state identifies laboratory confirmation and agrees that the case meets the national case definition, then the case will be reported to CDC through a system called ARBONET, which is the national system for monitoring mosquito-borne infections like West Nile Virus, and then the information will be appearing on the CDC website as a confirmed case.

So the four cases that I mentioned earlier are cases that have been evaluated by the state health department, have been found to truly represent West Nile infection and have been properly reported to CDC.

We also recognize that there are other laboratory tests that have been done using commercial tests and private laboratories, and some of these tests are used to help clinicians make a diagnosis of West Nile Virus infection, but in general we are asking that those tests be confirmed by the state health department and that they be reported to the state. We will not be accepting reports that CDC have cases of West Nile unless it has been properly reported through the state health department.

So let me stop now and see if we can have a couple of minutes from Dr. Jay Epstein at the FDA. He will let us know about West Nile Virus testing and the blood supply, a very important issue. Dr. Epstein, are you available?

DR. EPSTEIN: Yes. And thank you very much, Julie. As you know, FDA has cooperated closely with CDC and NIH, as well as the blood organizations in both the laboratory and epidemiological investigations of West Nile Virus, and we continue to work cooperatively to develop appropriate responses to protect the blood supply.

Also, we've worked closely with the product manufacturers to expedite development of necessary medical products, such as screening and diagnostic tests for West Nile Virus. And I would emphasize that FDA is highly committed to working with the industry to help bring safe and effective products to market.

More specifically, since August of 2002, FDA provided alert notices, and then later in October, guidance to blood establishments on procedures to avoid the collection and use of blood that might be at risk for transmitting West Nile Virus. FDA updated its current guidance in May 2003, based on experience with the 2002 epidemic.

Also, we've worked with device manufacturers and the blood organizations to facilitate the development and rapid availability of experimental donor screening tests for West Nile Virus. These tests were put into place in the areas of highest risk, starting in mid-June and have been available nationwide throughout the country since July 1st. Because of this screening test, as well as the recent policies on acceptance of donors, the blood supply is even safer this year than before.

Additionally, FDA recently cleared the first diagnostic test for uses and aid in the clinical laboratory diagnosis of infection with West Nile Virus, and we're also cooperating with early stage efforts to develop a vaccine for West Nile Virus and potentially to facilitate use under approved research of a possible therapy, based on use of antibodies.

Now, just to give a few specifics regarding the donor screening tests, there are two West Nile Virus test kit manufacturers, namely GemProbe, Incorporated, of San Diego, California, and Roche Molecular Systems, Incorporated, of Pleasanton, California, that are making investigational tests available to the blood collection centers under FDA-approved procedures. Although still experimental, these tests are very sensitive, and we believe that they can prevent contaminated blood from entering the nation's blood supply and therefore add a safety measure.

These tests detect nucleic acids, in particular, RNA of the West Nile Virus, and they're similar to tests already licensed to screen blood donors for evidence of infection with HIV and hepatitis C virus. In fact, these investigative procedures successfully identified the first human West Nile infection in an asymptomatic blood donor, and the products in question were removed prior to distribution, thereby, preventing exposure of blood recipients.

This was also the first case report in 2003 of a human infection by West Nile, although later reports have indicated that there were some earlier cases.

Now, in regard to the recently approved or actually cleared diagnostic test, this clearance took place on July 8th. It is a test for West Nile Virus antibodies that's manufactured by PANBIO. That's a medical diagnostic corporation located in Queensland, Australia. The test is intended to be used along with a physician's examination of the patient and other laboratory tests as part of a comprehensive evaluation of symptomatic persons to diagnose the disease.

The test measures IgM, which is a type of antibody that emerges early in the infection. The assay itself can be performed rapidly, that is to say, within a few hours. Although not a screening test, it is used if someone presents with symptoms such as fever or fatigue or a stiff neck, as Dr. Gerberding outlined. The test was evaluated in over 1,000 patients and very high sensitivity and specificity were demonstrated, in particular in patients with evidence of neurological disease, meningitis or encephalitis.

However, the test is not in itself a definitive. It's considered presumptive and must be confirmed with additional more specific testing, but its availability is thought to be a major contribution to the fight against West Nile. It also has the potential to help identify the scope and spread of the disease, to determine if there's a need for further workup in a patient, and as I said, it's rapid, and so it has reduced the turnaround time to the reporting of laboratory results.

Let me just mention very briefly a word or two about immune therapies. The National Institute of Allergy and Infectious Disease at the NIH has awarded a grant to accompany a canvass to develop a vaccine against the West Nile virus, and I think that Dr. Meegan may want to make a few remarks about that vaccine.

Also scientists at FDA are working developing a live attenuated West Nile vaccine and we have projects to try to develop improved assays to measure the neutralizing antibody that would be expected to be a protective response based on vaccination, and then we are cooperating with sponsors to permit investigational use at NIH of antibody preparations that may prove useful in the treatment of West Nile illness.

So I think I'll stop there and take questions later.

DR. GERBERDING: Thank you, Jay. Appreciate that update.

So I think what you can infer from that information is that we've made an enormous advance in terms of our ability to evaluate the safety of the blood supply, and also to make a clinical diagnosis of West Nile virus, but that the clinical test still does require confirmation to be sure that it's specific for the infection that we're talking about here today.

Before we turn to Dr. Meegan, let me just correct one piece of information that I presented earlier, because in looking at my notes I realized I misread them. There are actually three cases of West Nile virus in Texas and a single case in South Carolina, so there is still a total of four human cases, but it's three in one state and one in the other, not two and two. So please make a note of that. If you need us to get back to you one more time, it will be in the transcript corrected on our website.

Let me ask Dr. Meegan to say a couple of minutes about West Nile virus research at NIH.

DR. MEEGAN: Okay. We, as well as the other agencies have been collaborating. Our role is to support the research needed to develop the public health tools to help fight this infection, and we have laboratories here at Bethesda, but in addition we support grants and contracts to universities and companies around the U.S. In fact we're the largest supporter of infectious disease research in the U.S.

And in regard to West Nile, we're funding three--four basic areas. One is to improve diagnosis, one is in the area of prevention, using or developing vaccines to prevent the disease. Another is in therapy, developing antiviral medicines to help treat the disease, and another is in basic research to learn about the virus itself.

Basic research also looks at the virus as it replicates in animals and man and mosquitoes, and one interesting discovery from that basic research portfolio of grants has been the development of a hamster model. In addition to mice, we now have a good hamster model. If you were to give West Nile to 100 hamsters, about 50 would die with a disease that looks just like the encephalitis seen in humans. So that model will end up helping us test potential vaccines and potential antiviral drugs later on.

In the area of prevention, we're supporting three different approaches to vaccines. After the initial outbreak in '99 we weren't sure what the picture would look like with West Nile, whether it would continue to spread or whether it would disappear. We therefore funded a number of grants looking at modern methods to develop vaccines. The three approaches are a live vaccine made by mixing West Nile virus with the already-established yellow fever vaccine. This is called a chimeric virus, so we take the advantages of the well-established yellow fever vaccine and put on the outside of that vaccine West Nile code proteins. So by substituting West Nile we can take advantage of this vaccine that's been around for 70 years or so and has an excellent safety record.

That approach, which has been most successfully applied by A. Cambis Company in Boston, Massachusetts, has been taken all the way through testing in primates, so the vaccine, the chimeric vaccine has been made, tested for safety in small animals, and then larger animals, and then all the way up through monkeys, and has also been tested for efficacy in again, small animals, the hamster model, and also in horses and in monkeys. And as shown so far in those tests, those preclinical tests, to be safe and efficacious. And the company is now applying to test that vaccine in humans probably by the end of the summer if everything goes all right.

As you know, there's a series of tests for a new vaccine a Phase I trial in a small number of people, and then if everything goes right, a Phase II trial in larger numbers, and a Phase III in even larger numbers. So it will be quite a while until that's developed, but right now, that particular vaccine looks like our best bet.

In addition, there are two other approaches to vaccine. One is an approach where we just make inactive protein. This isn't a live virus that replicates in you; it's a system that produces live West Nile protein that's been inoculated into you. That hasn't been tested beyond animals yet. It's somewhat behind the chimeric virus approach, but it would have advantages in that it could be given to anyone, even those that you normally wouldn't want to give a live vaccine to, people who are pregnant or immune suppressed. So that approach has looked promising but is still a ways away from being tested in humans.

And the final approach has been looking at a very modern method called nucleic acid of DNA vaccines, and I think it's fair to say that that his even further back in the development chain, back at the laboratory bench level, but that shows, does show promise as well.

In the area of therapy, we've been testing many compounds in cell cultures--this is in Petri dishes--and looking for those that inhibit West Nile replication in those cells, and those that have shown positive activity are being tested now in mice and hamsters. We've gone through about 600 different compounds and about between 20 and 30 show activity, and like I said, those are being transitioned into animal testing.

The other area that we've been working on in therapy has been using immune globulin. This is--people who have recovered from West Nile donate blood. We extract the antibodies from that blood and use that as a preparation to treat people. We have submitted an IND to FDA, as Jay mentioned earlier, and they're hoping this year to do a study where we treat patients with the encephalitis, with the encephalitic complication with this immune globulin. So that study is ongoing as a therapeutic study.

DR. GERBERDING: Thank you very much for that perspective. I think there's an obvious enormous amount of work that's going on in this area, and vaccine work is something that we're looking forward to, but as I said at the beginning of this update, we're right on track with the epidemic last year, and in fact, we have more states showing evidence of West Nile virus activity. So for right now the most important messages is that people need to be prepared and take the steps necessary to prevent mosquito bites so that we can avoid exposure until such time as we have treatment or vaccines to add additional layers of protection.

There are a couple of brief points to add about the issue of the testing that you heard about, and that is because these tests are available this year, more people are likely to be tested, including people with very mild cases of West Nile virus. And so that means that our case count may be higher than last year because we're including people with milder stages of infection in the illness. The commercial test that is available now is one that does require a confirmation test, and so the state laboratories will be doing that.

The reason this is particularly important is that we know that there are other mosquito-borne viruses circulating this year that also cause encephalitis. They can create an illness that looks very much like West Nile virus. So we've had reports of St. Louis virus encephalitis, as well as Eastern Equine encephalitis, and those viruses, particularly the St. Louis encephalitis virus can be confused on some of the screening tests with West Nile. So this is a year for mosquitoes, and mosquito-borne illnesses, and people need to take the steps to fight the bight.

Let me now take some questions, and we'll start with a question from someone here in the room.

Do you have a question?

QUESTION: Hi. What kind of a vaccine program do you foresee? It's interesting to hear the development that's been going on, but some may say there's been a very large vaccination program that went on last year and that all of those people who were bitten and didn't even know they had, may have had West Nile Virus, they may have inadvertently been vaccinated just naturally just by getting a bite.

So basically, if you develop a vaccine, how do you foresee it would be used? Would everyone just go to the drugstore and get themselves vaccinated or only people who were very sick? Who are you making the vaccine for?

DR. GERBERDING: I think the first step is to get a vaccine that protects against disease, and while we're doing that, try to evaluate what would be the most likely population in which it would be useful and effective. I mean, this is a challenge because the population of people at risk for mosquito bites is very large, but we also know that the population of people at risk for the severe forms of West Nile Virus infection is slightly skewed to include those people who are in older age groups or who have other medical conditions.

So the older you are, the more likely you are to develop the brain inflammation from the infection. Whereas, most healthy young people who are bitten have less severe disease, but not always.

So everyone needs to be protected against mosquitoes. The vaccine development program will have to take into consideration exposure risk, as well as disease severity. And until we have a product, we won't really know what the duration of protection is or how often we would need to deliver it. Those are really good questions, and I'm sure all of HHS will be working together to try to solve that dilemma as we go forward.

There's a question here.

QUESTION: Looking at the map there, we see that it's concentrated in the Midwest, and the West Nile Virus isn't out West or in the New England States primarily. Do you see a migration to either of those areas or is it being isolated in the Midwest?

DR. GERBERDING: Well, if you remember at the very beginning of West Nile, it started right there in Manhattan the first year, and then it moved down the Eastern seaboard the next year. Last year it moved across the United States, and it's very early to predict what we're going to see on the West Coast. So, obviously, the surveillance system to look for mosquitoes and infected birds and animals is very active in the Western states right now because we won't be at all surprised to see that it does emerge there, and it may emerge very quickly.

Could I have a telephone question, please.

OPERATOR: Ladies and gentlemen, on the phone, if you would like to ask a question, please press the star, then one, on your touch-tone phone. Our first call is from Diedre Henderson of the Denver Post. Please go ahead.

QUESTION: Hi. My question has to do with some of the tests that were done before the experimental test was rolled out to be used on the blood supply. From what I understand, the prevalence testing found 5 positives per 10,000 Red Cross banked blood samples made available to GenProbe; 5 per 10,000 were positive for West Nile. What's the status of a follow-up test on those archived blood samples? And if those were confirmed as true positives, what happened with the blood and the patients who received it.

DR. GERBERDING: Dr. Epstein, did you hear the question, and can you answer it?

DR. EPSTEIN: Yes. The studies that were done on banked samples that were collected in 2002 have confirmed an overall frequency of about 7 per 10,000. Some of that repository testing is still ongoing, and that result conformed, very nicely with an estimated that CDC had generated, that the expected rate was about 8 per 10,000 for samples that were collected in Cleveland and Detroit.

In terms of the overall risk, the estimate, based on incidence West Nile illness, meningoencephalitis, was that the average U.S. rate was in the neighborhood of .4 per 10,000 or 4 per 100,000 during the nationwide peak epidemic, but that in certain states, of course, it was much higher, and the state that had the highest risk estimate was Michigan, with a rate of about 10.5 per 10,000 or, that is to say, about 105 per 100,000.

So I think the general conclusion, although these studies are ongoing, is that the risk estimates that were generated before repository testing have proved accurate, based on the repository testing that's gone on since 2002.

DR. GERBERDING: I'll have another question from the phone, please.

OPERATOR: And that's from the line of Rob Stein of the Washington Post. Please go ahead.

QUESTION: Hi, Dr. Gerberding. Thanks for doing this.

What I was wondering was the increased activity in more states this time, this summer, do you think that that's a reporting phenomenon or that it's really off to a quicker start this year.

DR. GERBERDING: The information I presented to you is corrected for the reporting time. There is usually a lag in the time that we identify a new state and the time that it appears up on the map because of the steps that I outlined earlier, where the sample has to be confirmed and so forth. But we are looking at same time last year, in terms of reported cases of human disease and states known to have West Nile Virus infection in it.

This is not surprising. As we said, this is moving West, and so the fact that if it was there last year, it's going to show up earlier there this year does not surprise anyone. I think what remains to be seen is to what extent the population of mosquitoes, and birds, and animals were interact to be an effective means of transmitting to large numbers of people. And there are so many variables that affect that that it's just too soon to tell.

Just because a state lights up on the map doesn't necessarily mean that every jurisdiction in the state has a high density of infected mosquitoes. So our surveillance systems are working on developing more precision, but fundamentally the message that I just keep coming back to is be prepared and take the steps necessary to protect yourselves against mosquito bites.

Let me take a question from here in the room.

QUESTION: Thanks. I had a question about can you give us an update on the Eastern Equine encephalitis activity? How concerned are you all on this? How many human cases? And is there any like vaccine or screening test under development for that?

DR. GERBERDING: For those who are not familiar with Eastern Equine encephalitis, it's an arbovirus that's in the same family of all the viruses we've been talking about today. And we have had two cases of Eastern Equine encephalitis virus, one in Illinois and one in Georgia. That is not something that is surprising. Again, we see sporadic cases of this illness and sometimes there are years where there's more of an outbreak picture, but it's under surveillance and we are certainly on the lookout for all of these encephalitises this year. So one of the advantages of having improved surveillance for one thing is that you pick up a lot of other infections and illnesses as you go forward, and as our testing improves, out ability to make these diagnoses is also improved. It's all part of the new normal of emerging infectious disease. So we're on the lookout and we will be alert to anything that looks like it could be an arbovirus infection.

Is there another telephone call, please?

OPERATOR: And that's from the line of Ray Weiss, Daytona Beach News Journal. Please go ahead.

QUESTION: Yes. I was wondering if there's been any research done in terms of immunity. Once a person has it are they immune to it or can they acquire a different variety or offshoot of it?

DR. GERBERDING: Dr. Meegan, I'll let you take that question about the duration of immunity once infected with West Nile virus.

DR. MEEGAN: All right. West Nile is in a family of viruses that includes yellow fever. And if you look at the yellow fever, both the vaccine and natural infection from yellow fever, immunity is very, very long, measured at over 4 years. There were studies done on people who were immunized during World War II and they still show protective antibodies. So although we don't have complete data for West Nile we would guess that it would be long-term immunity from both a live vaccine and from those recovering from the disease.

DR. GERBERDING: Thank you.

Another telephone question?

OPERATOR: And that's from Daniel Daning [ph] with Web MD. Please go ahead.

QUESTION: Dr. Meegan, you mentioned that one of the cases was related to identification of--or your testing of the blood supply. Could you elaborate on that please?

DR. GERBERDING: I'm sorry. I had a very difficult time hearing your question. If I understand it, you asked if one of the cases was related to testing of the blood supply?

QUESTION: That's correct.

DR. GERBERDING: Okay. Dr. Epstein, can you take that question?

DR. EPSTEIN: Yes. With the implementation of the nucleic acid test for West Nile virus under these investigational protocols, a donor, an asymptomatic donor was identified in the Gulf Coast Regional Blood Center. And this was then announced publicly in a news release on July 3rd.

Basically this demonstrated that the test worked. The screening test result was of course confirmed, and follow up on the donor has confirmed antibody seroconversion, so we know it's a real case. The donor has remained asymptomatic. And it's not a complete surprise where this detection occurred. The donor lives in Harris County. It was known that west Nile activity had been present in that area.

And I should mention that the FDA worked closely with the CDC to identify the areas at highest risk, so that when these investigational tests first became available, they were of course in limited supply and we were able to target them to the areas of highest risk. So this was a successful detection which led to preventing a risk to the blood supply in that area.

DR. GERBERDING: Thank you.

A question here in the room?

QUESTION: Just wanted to clarify with the question. But I believe that that asymptomatic blood donor case in Texas wasn't one of the two or three cases you've confirmed of West Nile. Was that correct?

DR. GERBERDING: Jim, can you clarify that?

Yes, that is correct. A case has to have clinical symptoms in order to be considered a case of West Nile virus disease, and to nationally recognized case definition includes the symptoms or signs that would be ill, and so in this particular example the patient was found to have the virus without illness, so it is not considered to meet the case definition. But we are of course tracking all of these individuals, and as we get more information, we'll be probably figuring out a way to report them separately so that people can keep a track of the overall expansion of the problem.

Over here.

QUESTION: Last year folks heard about transmission by blood donations, organ transplants, and even baby's milk, nursing. And what do we know now? First of all, it sounds like not everyone has these tests at the blood centers, so it's possible that blood is being donated and something can get through. Where are we on organ donations? If people hear about this and are wondering--putting aside the risk of course if I'm in a really bad car crash, I need something, you know, I need the blood more than I need to worry about West Nile virus. But what are the risks now? What do we know about detection, detecting early?

DR. GERBERDING: Jay, can you take that question, please?

DR. EPSTEIN: Well, I'll try. You know, we are not directly regulating organ transplantation at FDA, and of course we're not going to be able to control naturally-acquired infections by breast milk or transplacentally. What I can tell you is that the tests that have been made available experimentally to screen the blood supply are being used in some places also to screen organ donors, and this of course is permitted by the FDA.

As far as the magnitude of risk, well, that would be related to the extent of the epidemic which has yet fully to evolve in 2003, but there's no reason to think that these tests wouldn't be equally effective on other kinds of donor.

DR. GERBERDING: I think where we are at this moment is that we can never say there's no risk, but a remarkably speedy advance in testing and test availability for this particular indication, looking for the actual virus, per se, has put us in much better shape this year than last year. So this is clearly an improvement and a big step forward.

Question here.

QUESTION: Of all the states earmarked there, are there any that are at any more risk than others as far as seeing any human outbreaks, or can we just not tell at this point?

DR. GERBERDING: You know, if we could really precisely put all of the predictive variables together, we might be able to give you a projection on that, but the truth is there are so many factors. The weather plays an important role, the rainfall, the density of the mosquito population in the birds, the amount of time the virus itself has to replicate and get up to higher and higher titers, the state of immunity of the horses in a particular jurisdiction, many places are using the equine vaccine. So it is really impossible to predict. What we are doing of course is looking at the mosquito populations, looking at the birds, the dead birds, because that indicates a fair amount of virus activity if it is having a lethal impact on birds, and then doing everything we can to detect the initial human cases.

But public health jurisdictions know if mosquitoes with West Nile have been identified in their locale, and that's a stimulus to step up the mosquito abatement programs, and also an extra warning for the people in that community to be safe and take the precautions they need.

QUESTION: Following up to that, we've seen an excessive amount of rain, not only in our area here, but in Texas and all. Has that raised some concerns with the CDC, seeing so much rain in the Southeastern United States and even up in the Midwest here recently?

DR. GERBERDING: Well, there certainly has been a lot of rain in the South. We can vouch for that. And whether or not that's a help to the mosquito population or whether it's helping to keep the population down because they're being rinsed away, I can't say, but I think that because we can't accurately predict, we just have to err on the side of caution.

I'll take a phone question, please.

OPERATOR: And that's from Steven Smith of the Boston Globe. Please go ahead.

QUESTION: Hi, Dr. Gerberding. Thanks for doing this.

You were saying at the start of the briefing there are 32 states reporting activity at this juncture compared with how many from last year? About 20 or do you have a precise figure?

DR. GERBERDING: Yes, about 20 states, at this time last year, were reporting West Nile Virus activity in either birds, mosquitoes, animals or humans.

QUESTION: And can you more broadly characterize the scope of the activity this year. Is it a matter that there are just isolated reports coming from these 32 states? What's your broad characterization of the scope of the activity?

DR. GERBERDING: Well, one way of looking at this would be to think of it as an additive phenomenon. So once a state has West Nile in the bird population or in the Southern states where mosquitoes winter over, they can survive the winter. I mean, once you have it established in a jurisdiction, it's likely to persist there until either something changes to interrupt the transmission cycle or the weather doesn't allow ongoing propagation. And so the fact that we had "X" number of states affected by West Nile last year, and this is early in this season, what we're really seeing is the perpetuation of where it had gotten to last year.

Now, what remains to be seen is whether or not it will emerge on the West Coast because those were not states that were affected by West Nile last year. It was an isolated situation in Los Angeles. And so those are the areas that would really represent further migration of the virus to new areas, and it's just too soon to see whether or not that's going to be the case or not this year. Again, it will depend on many things, including bird migration patterns, weather and mosquito abatement procedures, and most importantly will depend on whether or not people take the steps necessary to fight the bite.

I'll take a telephone question, please.

OPERATOR: And that's from John Pope from the Times. Please go ahead.

QUESTION: Hi, Dr. Gerberding. I have two questions related to surveillance.

Can you give an estimate of when you might know whether that fifth case you mentioned will be either confirmed as a West Nile case or not?

Secondly, because so many West Nile cases may be overlooked because a lot of folks can simply fight off the vaccine with their own well-functioning immune systems, are you doing anything to increase the sort of self-reporting or whatever to make sure that a more exact case or case count can be reached?

DR. GERBERDING: Thank you.

The second part of your question that pertains to the underestimation or the potential for underestimating the true magnitude of human infection because we tend to test the most seriously ill patients is one that we anticipate.

And I mentioned earlier about the availability of the commercial tests now means that people will likely be tested when they have less-serious forms of the illness and will have to rely on the confirmatory testing to make sure that they represent true cases. But we expect this year that our sensitivity of detection of human infection will be increased, both because the blood donors are being screened, and we may pick up asymptomatic people that way, but also because the commercial test is more accessible, and we may be seeing milder cases of infection.

If we want to know where the virus is and where it's going, we need to use a combination of measures, and the mosquito-trapping programs, the bird surveillance and of course the illness in horses is still a very reliable predictor of where we will next see human disease. So that was why we put so much emphasis on these other kinds of surveillance because they tell us that soon you may be likely to see serious human cases in that particular area.

And from the standpoint of the patient or the individual, what's most relevant is the illness, and so knowing where the activity of the virus is highest gives people a head-start on taking the steps to protect themselves.

I'll take a question here.

QUESTION: I knew that last week the CDC reported a study on the results of the spraying insecticides in communities and the relatively low incidence of people getting sick, which is good news. Are the states taking the measures that they need to earlier and better than they did last year or is that end of it--because it's kind of hard to tell folks in the dead of summer to wear long sleeves, even though it's the best way to protect your skin, and they can do their part, but are the states doing what they need to do, too?

DR. GERBERDING: We have a number of ways of monitoring the overall mosquito control programs in states, and all of the states, funded by CDC, have been participating for several years now in a program to combat West Nile through a number of modalities, including what we would call integrated pest management, which is the strategy for preventing mosquitoes from breathing in the first place by emptying their breeding sites, by treating the larva of mosquitoes with larvacide, and then, when necessary, spraying adult mosquitoes.

And so far the information we have coming into CDC suggests that the larvacide programs are getting off to an excellent start this year and that many states are acting earlier than human cases to try to prevent the outbreak from speeding up.

So I think we have good news, in terms of the measures the state and local health departments are taking.

DR. GERBERDING: We'll take a telephone question, please.

OPERATOR: And that's from John Lauerman with the Bloomberg News. Please go ahead.

QUESTION: Thanks for taking my question. I just wanted to understand a little bit better what's going on with the testing, the blood testing. The case that's just a single case so far has been turned up by the test, and can you tell me which case it was that turned up this one case.

DR. GERBERDING: Jay, can you please address the question?

DR. EPSTEIN: Yes, certainly.

At the moment, there's one confirmed case, and it was picked up by the Roche Diagnostics test. As I said earlier, that was in the Gulf Coast Regional Blood Center. However, there are a small number of additional suspect positives widely geographically spread that are being further investigated. Of course, since there is now a human epidemic, we do expect to see more pickups among donors. After all, most infections, about 80 percent, are asymptomatic, so they will be seen in donors.

DR. GERBERDING: Thank you. I have time for one more question. I'll take it from the phone, please.

OPERATOR: And that's from the line of Denise Grady, of the New York Times. Please go ahead.

QUESTION: Thank you. In the early days of the epidemic in New York, I remembered that there were, people who were in affected areas were tested to find out whether they had ever been exposed, and I wonder if any of that testing is being done now and if you have any sense of what kind of levels of immunity you might expect over time as the infection spreads.

I wonder if it, by the time the vaccine becomes available, there may be a lot of people who are already immune naturally.

DR. GERBERDING: Thank you. Now, I think you're referring to a sort of a door-to-door survey that was done in Queens and other areas of New York at the very onset of the West Nile emergence in the United States, and we've, also, at CDC been working with partners at state and local health departments to do a similar study in other jurisdictions. I would love to be able to say that, you know, we're approaching 100-percent immunity as we go forward, but the true sero-prevalence is still very low, and the vast majority of the population remains fully susceptible to West Nile Virus infection.

So unless we have repeated cycles year after year and vast numbers of people get infected each year, I don't think we're going to be able to achieve human population immunity through natural exposure any time soon. We still look forward to that good work going on at NIH in vaccine development to take us where we may need to be to get full protection.

But West Nile Virus is a very fascinating and complicated problem for all of us, and there are many advances this year compared to where we were just one year ago. So I think we have reason to be optimistic. But in terms of the shape and magnitude of the problem this year, it's too soon to tell for sure, but the news indicates that people need to be prepared and to protect themselves.

So that's the bottom-line message today--fight the bite.

Thank you.

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