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Press Briefing Transcript

CDC Telebriefing on West Nile Virus Update

Wednesday, September 5 at Noon ET

OPERATOR: Welcome and thank you for standing by all participants will be on listen only.  You may ask a question by pressing star 1 on your touch tone phone. Today’s conference is being recorded.  If you have any objection, you may disconnect at this time.  I'd like to turn meeting over to Lola Russell. Thank you, you may begin.

LOLA RUSSELL: Good afternoon.  Welcome to today's telebriefing on West Nile virus where CDC and the Texas Department of State Health Services will be discussing the most current West Nile numbers, virus numbers and will provide you with an update on the situation in Texas.  As you know, we are reporting record numbers of West Nile virus disease to date since it was first detected in the U.S. in 1999.  Our first speaker will be Dr. Lyle Petersen, who is director of CDC’s Division of Vector-Borne Infectious Diseases and Dr. David Lakey, who is commissioner for the Texas Department of State Health Services.  We will begin with a statement by Dr. Petersen, followed one by Dr. Lakey. After that you will have an opportunity to ask one question and a follow-up. Beginning with Dr. Petersen. 

LYLE PETERSEN: Good afternoon.  Thank you for joining us for this update about the 2012 West Nile virus disease outbreak in the United States.  The information I’m sharing with you was posted this morning on CDC’s West Nile virus website (  These national data are compiled from weekly reports provided through our ArboNET surveillance system by our partners at state public health departments nationwide.  I want to acknowledge their important contributions for helping us track what’s happening with the epidemic.  As we expected, the number of human cases of West Nile virus disease continues to arise in the United States.  We expect this increase to continue for the next several weeks, probably until October.  As someone who was sickened with West Nile virus infection several years ago, I can personally relate to the people’s concerns about getting this infection.  Through the combined efforts of federal, state, and local public health departments, we’re working to prevent the spread of infection.  My report today touches on three areas:  the latest national case numbers, our consultations with state officials in Louisiana and elsewhere in the aftermath of Hurricane Isaac, and some work we’ve done in Texas, which continues to report the highest number of cases.  Dr. Lakey will then share more details about the outbreak-control efforts in Texas.

As of September 4, 2012, 48 states have reported West Nile virus infections in people, birds, or mosquitoes.  The only states not reporting West Nile activity are Alaska and Hawaii.  Forty-four states have reported at least one human case.  A total of 1,993 cases of West Nile virus disease in people, including 87 deaths, have been reported to CDC.  Of these, 1,069 (54 percent) were classified as neuroinvasive disease (such as meningitis or encephalitis) and 924 (46 percent) were classified as non-neuroinvasive disease.  For comparison, the numbers reported last week were 1,590 total cases, 889 neuroinvasive disease cases, and 66 deaths. So, this week numbers represent about a 25 percent increase over last week. The 1,993 cases reported thus far in 2012 is the highest number of West Nile virus disease cases reported to CDC through the first week in September since West Nile virus was first detected in the United States in 1999.  As was true last week, over 70 percent of the cases have been reported from the following six states: Texas, South Dakota, Mississippi, Oklahoma, Michigan, and Louisiana.  And nearly 45 percent of all cases have been reported from Texas.

Based on past epidemics of West Nile disease, we expect the numbers will eventually show that this year’s epidemic peaked in mid- to late-August.  Although we may be past the historical peak, we expect that a great many cases of West Nile virus disease have not yet been reported, largely because of the lag between when a person gets sick and when the illness is reported.  Even if West Nile virus transmission were to stop today, we would continue to see reports of cases for several weeks.   

With regard to Hurricane Isaac, we have been consulting with officials in Louisiana, Mississippi, and other states since the storm made landfall.  Both our CDC program and the public health departments of southern and eastern coastal states have considerable experience in assessing the public health impact of hurricanes and floods. In general, experiences shown that such disasters do not increase the transmission of West Nile virus and other arboviruses.  However, small increases in the numbers of West Nile virus cases can occur, as was reported in several areas of Louisiana after Hurricane Katrina in 2005.  These cases were thought to be due to the unavoidable increased outdoor exposure people experienced when their houses were damaged and during recovery efforts.  Rain and flooding events can produce conditions that lead to explosive increases in mosquito populations, which in turn can pose a terrible nuisance to residents and those helping with recovery.  Aerial spraying of insecticide is sometimes used after hurricanes to control nuisance mosquitos.  CDC will continue to offer technical assistance to Louisiana, Mississippi, and other states affected by Hurricane Isaac.  

In Texas, we are working with public health officials to contain the outbreak in various communities, including Dallas County.  One way that we’ve done this is through CDC’s “Epi-aid” process.  Here’s how it works.  CDC’s Epi-aid teams, which include investigators from our Epidemic Intelligence Service, are deployed at the request of states to help tackle urgent public health problems.  CDC does this in collaboration with state and local health departments – so it’s not a matter of CDC coming in and taking over the response.  This interaction among federal, state, and local authorities has been crucial to the rapid response in Texas. 

Two CDC Epi-aid teams have recently returned from deployments to Texas.  One team, which was based in Austin from August 20 to 31, helped review and describe West Nile cases in Texas, evaluated electronic laboratory reports to estimate the numbers and trends in human cases, and assessed differences in West Nile virus seasons during the past decade.  The second team, which was based in Arlington from August 20 to September 4, worked closely with Texas officials to collect data for human and mosquito surveillance, and for evaluating the impact of insecticide spraying.  In just a moment, Dr. Lakey will share some of the preliminary results from the Arlington team’s work. 

Before I close my remarks, I would like to remind everyone about ways people can protect themselves from mosquito bites.  We expect West Nile virus transmission to continue until October in most regions of the country.  We encourage everyone to:

  • Wear insect repellents when you go outdoors. 
  • Wear long sleeves and pants during dawn and dusk.
  • Install or repair screens on windows and doors. 
  • Use air conditioning, if possible.
  • Empty standing water from items outside your home such as gutters, flowerpots, buckets, kiddie pools, and birdbaths.
  • Support your local community mosquito control programs.


And now Dr. Lakey will share some more details about the situation in Texas.  Dr. Lakey. 

DAVID LAKEY:  Thank you, Dr. Petersen.  And again I want to thank the media for being involved in communicating these public health messages. The ones that Dr. Petersen just outlined and those messages are really key to our overall response.  We continue to be very involved, very concerned about this West Nile outbreak.  It's an outbreak we're seeing in Texas and an outbreak that were are seeing in other parts of the United States.  Historically, August is the peak month for our human cases.  However, this year has been unpredictable.  The virus is somewhat unpredictable.  We have had, obviously, a large number of cases.  We have had intense efforts to decrease the number of mosquitoes.  We're still looking at where we are on the overall epidemiology curve. 

Having said that, as of this week, 2012 is now officially our worst year in the state of Texas for West Nile disease.  Our previous worst year was 2003 when we had had 439 cases of neuroinvasive disease and 40 deaths.  As of yesterday, if you look at the overall number of individuals that have been diagnosed with West Nile, we have 1,013 confirmed cases in our state.  We now have 40 deaths and 495 cases of West Nile neuroinvasive disease.  Again, the previous worst year was 2003 at which time we had 40 deaths and 439 cases of neuroinvasive disease.  As of yesterday, we have had 40 deaths and 495 cases of West Nile neuroinvasive disease.  Last week we reported we had 31 deaths, so as of this week, we added nine additional deaths.  Last week we reported 416 cases of neuroinvasive disease and again we are up to 495 cases as of today.  So when we look at the data, about a fourth of all the cases that have been reported in Texas, we reported last week.  Unfortunately, those numbers are going to continue to go up.  Those numbers will be updated today.  We know there will be additional deaths, unfortunately, and again, sadly, more folks that will be diagnosed with West Nile neuroinvasive disease. 

Now having said that, there does look like there's some promising indicators that in some parts of the state of Texas that we may have peaked.  Especially in the north Texas area.  Again, it's too early to say for sure, but some of the data that we're looking at looks like we may have peaked in the north Texas area.  We're closely evaluating the disease incident rates in all our counties and especially those that have been hardest hit.  We're trying to put that data available to you online.  So we advise the media to go to our website  Where we have the county by county data, but  also incidence rates by age group and by the diagnosis.  We think that will be helpful to you.  We are looking at other ways to follow this outbreak using some real time intelligence, using monitoring our laboratory diagnostic data through the national electronic data surveillance system.  Again, that's a new approach for us.  We're still looking to see how helpful that will be for us to figure out the path of this outbreak.

Now one component of our response has been aerial spraying.  Again, that's just one component of our response besides the public health messages that we have already talked about.  But we have provided aerial spraying in Dallas and Denton.  Those missions were accomplished last week.  And we have had the CDC Epi-aid team come and help us look at the effectiveness of the spraying in the north Texas area and specifically Dallas county.  The preliminary data, again, seems to be encouraging.  In those areas that used aerial applications of insecticides where there was a substantial reduction in mosquito populations that carried West Nile virus.  In that area, the areas that did not receive spraying, the number of mosquitoes actually went up.  In the areas that had aerial spraying, the number of the mosquito that carries West Nile virus is decreased by 93 percent, where the weather allowed us to do two consecutive treatments.  Again, in the areas that we weren't able to do consecutive treatments, there was a reduction, but not at the 93 percent level.  We also had encouraging data related to the mosquito pools.  When we gather mosquitoes and we test those mosquitoes on whether or not they have the West Nile virus, the percentage of those pools that have West Nile virus in them has gone down substantially.  For example, if you look at the north Texas area back in July, early August, about half of the pools we tested were positive for West Nile.  That rate now is down to 5 percent, 6 percent. 

So again, some encouraging data that the number of mosquitoes that are present are less likely to be infected with West Nile, at least in the north Texas area.  When we look at other parts of our state, however, we continue to see a large number of those pools that are still infected with West Nile.  For example, in Travis county the Austin area, about 28 percent of those mosquito pools are positive for West Nile.  So again, unfortunately, officially as of this week, this is the worst year ever for West Nile in the state of Texas.  But we are starting to see some encouraging signs related to the mosquito populations, the number of them that are infected, at least in certain parts of our state that we may have peaked.  Again, I want to thank the CDC for their ongoing support and the local jurisdictions that have been helping us in this outbreak.  And again I want to thank the media for their critical support in getting those public health messages out, not only in Texas, but across the United States throughout this outbreak. Thank you very much. 

LOLA RUSSELL: We’ll take the first question, Melinda. 

OPERATOR: Our first question comes from Ken Kalthoff, NBC Dallas.  Your line is open. 

KEN KALTHOFF: Good morning to you.  In your briefing that was posted on August 29th and Dr. Lakey just referred also to the area that was not sprayed two nights in a row, that was zone one of Dallas county and also a northeastern portion of zone two in Dallas county.  On the August 29th report it said that further analysis is pending and Dr. Lakey you said the results were not quite as good in those areas that were not sprayed two consecutive nights, but what more do we know than that at this time and has any of this further analysis been completed? 

DAVID LAKEY:  Lyle, you want to take that question? 

LYLE PETERSEN:  I thought you were taking the question.  Okay.  I'm sorry.  Yeah, the area that did not get two consecutive nights of spraying, we're still analyzing the data.  It's going to take a while to sort out because many of these areas were sprayed at different times, sometimes using truck managed spraying, sometimes not.  And in combination with varying times between the first and second spray episodes.  So we expect that the optimal treatment or the optimal reduction is going happen in areas where the weather permitted two consecutive nights of spraying.  For the other areas, it's going to take more time to sort out.  We don't expect to have any conclusive results in those areas for a number of weeks until we look at the spray maps in detail and observe and look at the mosquito collection sites in detail and that's just going to take a bit of time. 

KEN KALTHOFF: If I might follow up then, Dr. Petersen or Dr. Lakey, is something more needed in those areas that did not get those consecutive treatments if those areas which were among the hardest hit in this area did not receive the optimal results, is some great additional amount of ground spraying or perhaps another couple passes of aerial spraying justified in order for those area to catch up with the lack of improvement that they have seen? 

DAVID LAKEY:   I think at this time, this is David Lakey, I think at this time we need to continue to monitor it closely.  Monitor not only the number of mosquitoes, but that the number of mosquitoes that are positive for the virus.  Again, we're getting some encouraging data now related to the decrease that has incurred in the number of mosquito pools that are positive.  So I think we continue to look at that data and the mosquito data before we decide we need to add any additional spraying.  The local health department is doing some monitoring, if they see in a specific area there's an increase, we can -- number of mosquitoes, number of them are positive, we can add additional spraying at that time whether it's land-based or aerial.  But we're going to continue to closely monitor that information. 

LOLA RUSSELL: Next question, please. 

OPERATOR:  Mike Stobbe, the Associated Press.  Your line is open. 

MIKE STOBBE: Hi.  Thanks for taking my question.  I will have a follow-up in a minute. My first question is anymore discovery or thinking about why this is such a bad year?  Especially in Texas? 

LYLE PETERSEN: Yeah, this is Lyle Petersen.  Mosquito-borne outbreaks, historically no matter what the disease is have been very difficult to predict where they are going to happen and how big these outbreaks are going to be.  The reason is because there's a very complicated ecology to the transmission of these viruses in nature.  And it's difficult to sort out all of the factors which may influence each of -- each of which influenced this arboviral transmission or transmission of these mosquito-borne viruses.  One of the things we are closely looking at is the effect of weather on this year's outbreak.  We know that West Nile virus outbreaks tend to occur when the temperature is above normal, and of course, this year's heat wave was record setting.  So we think the temperature may have -- may be influencing this year's outbreak.  But of course, many areas that have had record high temperatures this summer have not had outbreaks. So we think temperature may be a permissive factor to promoting West Nile virus outbreak, but certainly isn't the only factor involved.  So the end result is we really don't know at this point in time what factors may have contributed to this outbreak, we think it's weather related.  Why it occurred in Dallas more than other areas is a matter of speculation at this point, and it's one of the things that we're going to be looking at very carefully when all is said and done at the end of the season. 

MIKE STOBBE:  Okay.  My follow up to the extent you can address it.  We have so much in the news now not only on West Nile but Hantavirus, heartland virus, plague. It seems like -- I mean, are there any connections that can be drawn or is this kind of coincidence?  Is heat ---these are vector-borne diseases, is heat the contributor in each case or do they seem to contradict each other in some instances? 

LYLE PETERSEN:  Temperature is certainly a factor in mosquito-borne outbreaks, but the effects of weather are very complicated on all of these vector-borne diseases.  But I think what you have observed is that there's a lot of vector-borne diseases in the news here in the United States, but there's also new outbreaks occurring in Europe and elsewhere which I think just points to the fact that these vector-borne diseases are among the fastest emerging diseases in the world at the current time.  The reasons for this are quite complex.  It has to do with the changes in ecology, population growth, movement of man and animals and vectors, and, so these are, I think, just illustrative of the growing concerns about vector-borne diseases in the world today. 

LOLA RUSSELL: Next question, please.

OPERATOR:  Shannon Pettypiece from Bloomberg news, your line is open. 

SHANNON PETTYPIECE: Before my question if you can just identify who that, who was speaking. I think I missed it.  But my question is I know the number of cases is up from last week.  Are you seeing this spreading to more states or -- yeah, can you say anything about other states that are being impacted a lot more than they were a week or two ago at this time? 

LYLE PETERSEN: This is Lyle Petersen.  I just answered the last question and I’m going to answer your question now.  It's an excellent question.  What we are seeing is that we're seeing more cases increase -- excuse me.  More cases are occurring in the northern parts of the United States.  This is not unexpected because the cases in the northern part of the United States tend to occur a couple of weeks later on average.  Of course, there are always exceptions, but on average, they occur a little bit later than cases in the southern United States.  This is most likely due to the fact that the spraying in summer starts a little earlier in the south than it does in the north. 

SHANNON PETTYPIECE:  So -- okay.  So northern states you're seeing an increase but mostly, sort of weather --, because sort of the climate patterns of those areas. 

LYLE PETERSEN: Most likely, that is true, yes. 


LOLA RUSSELL:  Do you have a follow up? 

SHANNON PETTYPIECE:   Nope.  I think that should do it.  Thank you. 

LOLA RUSSELL:  Okay.  Next question please, Melinda. 

OPERATOR:   Neale Todd from Medpage Today. 

TODD NEALE:  Thanks for taking my call.  I had a question about some of the numbers.  The CDC numbers 888 cases in Texas but the state numbers have 1,013. I was wondering if you can explain the discrepancy. 

LYLE PETERSEN:  Yes.  This is Lyle Petersen again.  The CDC numbers are updated weekly and so we take all cases that are reported as of 3:00 a.m. on Tuesday mornings.  So cases that are reported by states after that are not reflected in the CDC numbers.  It is impossible for us to keep up with all 50 states every time they decide to update their numbers.  So we update our numbers once a week.  This week the numbers may have been particularly impacted because Monday was a holiday.  And so many cases that states may have heard about subsequently to 3:00 a.m. on Tuesday may not be reflected in CDC's numbers.  So we actually may see a bigger jump next week from cases that didn't make it into the weekly numbers this week. 

TODD NEALE:  Thank you. 

LOLA RUSSELL:  Next question, please. 

OPERATOR:   Sherry Jacobson, Dallas Morning news. 

SHERRY JACOBSON: Hello.  Dr. Lakey, I was wondering if you could be a little more specific about the amount of West Nile that was found in mosquito pools in the areas of Dallas county that did not get the consecutive aerial spraying and I’m really interested because this was the hardest-hit area in Dallas county with a disproportion number of cases and deaths.  But it also there's a great amount of anxiety on the part of the people who live there.  What exactly did you find? 

DAVID LAKEY:   So the CDC is doing the full analysis right now.  It's complicated because there are areas that got land-based spray versus no spray and the collection of the mosquitoes differently.  So the CDC Epi-aid team is doing that full analysis.  Now if you look at overall positivity rates in Dallas county, last week out of 36 pools that were tested here in our agency only two of them were PCR positive.  So the percent of infected mosquitoes has gone down to about 6 percent for Dallas county as a whole.  But the more granular information that the CDC is working on that information right now through that Epi-aid team. 

SHERRY JACOBSON: Okay.  So you really just can't say at all if aerial spraying was effective in the hardest-hit area? 

DAVID LAKEY:   We know in the area that got two consecutive aerial sprays that the number of the mosquitoes that carry West Nile went down 93 percent. 


LOLA RUSSELL:  Next question, please. 

OPERATOR:   Robert Lowes, Medscape Medical News. 

ROBERT LOWES: Thank you for taking my question.  Dr. Lakey, I wanted to ask about areas where the number of mosquito, infected mosquitoes, or the percentage that were infected went up. These were areas that were not sprayed at all either through land-based spraying and aerial spraying is that correct? 

DAVID LAKEY:   There's a couple -- I want to make sure I understand your question.  If you look at Texas as a whole, the percentage of infected mosquitoes has gone down in the north Texas area but is staying up in the central Texas area.  We're still seeing about 28 percent of the mosquitoes, as of earlier this week in Travis county, about 28 percent are still positive for the virus. 

ROBERT LOWES: But didn't you say there was an increase in the unsprayed areas? 

DAVID LAKEY: The CDC analysis in Dallas county when they sprayed, when they looked at dividing the county up into different areas, the areas that got two consecutive sprays had a 93 percent reduction in the mosquitoes themselves.  The areas that received no aerial spraying as a whole had an increase in the number of mosquitoes.  In the areas that got two consecutive sprays but it was separated because of weather, they couldn't do it back to back, had a reduction.  The CDC is doing that analysis to see how solid of a percentage they can put out there related to the specific reduction in that area. 

ROBERT LOWES: So then right now you don't have specific numbers to put on those increases and decreases. 

DAVID LAKEY: For the areas in Dallas county that did not receive aerial spraying, and Lyle Petersen, you can jump in if you want, they had a 10 to 20 percent increase in the number of mosquitoes, the mosquitoes that carry West Nile virus. 

ROBERT LOWES: And there was no land spraying either in those areas? 

DAVID LAKEY: Yes, there was in some of those areas.  That's the complicated analysis that the CDC is currently doing is pulling out the areas that got land-based spray, that may have gotten three consecutive land-based spray.  There's differences in the way -- the number of traps for mosquitoes, that's part of the analysis that the CDC Epi-aid team is doing right now. 

LOLA RUSSELL: Can we have the next question please? 

OPERATOR: Thank you. Umair Irfan, Climate Wire. 

UMAIR IRFAN: Hi.  Thank you for taking my call.  For Dr. Petersen, you mentioned a couple weeks ago that the percentage of people that are typically infected but don't show symptoms is roughly 80 percent and only 20 percent actually end up developing West Nile fever.  Then you extrapolate from the cases that are reported to the number of infected individuals.  Have you verified if this is still the case?  Has this infection pattern held out for the current outbreak? 

LYLE PETERSEN: We don't have data about the current outbreak right now.  We certainly know the number of neuroinvasive disease cases, but to try and figure out the number of total number of West Nile fever cases is not an easy task.  It would take some time to do. 

UMAIR IRFAN: When do you expect to get those numbers? 

LYLE PETERSEN: Well, we may or may not decide to do that kind of analysis.  It's a major study to try and figure out how many West Nile fever cases there were and how many infections there were.  So we may or may not decide to do that study.  We do not believe that the proportion -- that these proportions changed this year.  But we are looking at the virus that's circulating this year to see if there have been any changes which could cause a change in pathogenicity of the virus.  However, we don't believe that's the case.  We just believe there are environmental conditions that occurred for whatever reason this year that just promoted a large West Nile outbreak.  Right now, we have no evidence to suggest that the virus has changed somehow or that there's a higher proportion of seriously-ill people.  But we are looking into that, at least from the viral standpoint. 

UMAIR IRFAN: Thank you. 

LOLA RUSSELL: Next question, please. 

OPERATOR: Next question is from Mayra Moreno, CBS San Antonio.  Your line is open.

MAYRA MORENO: Thank you for taking my question.  I heard that about 89 of the thousand or so cases in Texas were actually discovered by screening donated blood.  Do you guys have any numbers on that or can you specify any more information? 

DAVID LAKEY: This is David Lakey.  The 89 number is the same number I was told today about the number of asymptomatic individuals that have gone to donate blood that had been tested positive for West Nile virus in the state of Texas. 

MAYRA MORENO: That's just people that have been tested positive, none of the deaths or anything related to that?

DAVID LAKEY: Those are asymptomatic individuals. They went to donate blood. As part of a screening of their blood before it was given to somebody they tested for a whole host of infectious diseases, one of which is West Nile.  So there have been 89 of those that have turned out to be positive.  Those individuals are asymptomatic, so having no symptoms, fever, of West Nile disease. 

LOLA RUSSELL: Can I have the next question please? 

OPERATOR: Eben Brown, Fox News radio. 

EBEN BROWN: Good afternoon.  Thank you for taking the question.  I apologize.  I jumped into the conference a little late.  Could someone go over the numbers you may have given out at the top of this, in terms of the increase in number of cases and the number of deaths? 

LOLA RUSSELL: We'll share those.  They are available on our website.  Do you have a question?  We can make those available to you afterwards. 

EBEN BROWN: No, but I was just hoping to get the audio for a radio broadcast. 

LOLA RUSSELL: We will have the audio file that will be available as soon as this is over. 

EBEN BROWN: Thank you. 

LOLA RUSSELL: You're welcome.  Next question please. 

OPERATOR: Lena Sun, Washington Post. 

LENA SUN: Yes, hi.  Dr. Petersen, I was wondering where you could maybe talk a little bit more about the different kinds of environmental factors that would have to align to create this perfect storm of outbreaks?  In past years there have been these outbreaks in different parts of the country.  I was wondering if folks have been able to tease out what it is.  Is it certain kinds of light infections of blue jays and hot summers and certain kinds of rainfall that come together to produce this -- these perfect alignments for an outbreak to occur? 

LYLE PETERSEN: That's an excellent question and somebody could probably write a Ph.D. thesis on this.  But I'll try and give a very simple explanation.  The transmission of West Nile virus is depending upon a number of factors.  One is the population of mosquitoes.  Simply put, the more mosquitoes you have or the more vector mosquitoes, the particular kind of mosquitoes that spread West Nile, the more you have the greater likelihood you are of having an outbreak.  The second thing is how many susceptible birds you have.  So as you pointed out, there's a number of species of birds ranging in the hundreds now that can become infected with West Nile virus.  So the distribution of particular species of birds, which may vary in different parts of the country, is critically important.  And then thirdly, it's kind of the combination of how these birds and mosquitoes interact with each other.  If you've got let's say infected mosquitoes near a birds roost, where there’s hundreds of susceptible birds, that would promote an outbreak or promote transmission of the virus in nature.  So it has to do with the spatial distributions of the birds and mosquitoes. 

The other factor that's involved is how infectious these mosquitoes actually are.  The infectiousness of the mosquitoes that spread West Nile is influenced by temperature.  So temperature can increase the infectivity of mosquitoes simply because it raises the levels of virus in the mosquitoes, so the hotter it is, the mosquitoes tend to become more infectious.  But temperature also infects how long a mosquito may live, which is a critical factor.  Temperature will also affect breeding sites for mosquitoes.  Rainfall is obviously going to affect breeding sites for mosquitoes like if you have a flood it washes out all the breeding sites.  If you have a drought, it may eliminate them.  So you have to have just the right amount of rainfall to produce a large number of mosquito breeding sites. 

And further complicating the matter is cities are complicated environments.  For example, even in droughts, you can find a lot of water in cities simply because people water their lawn and have birdbaths and the likes and create plenty of breeding sites.  For that reason, we find outbreaks of West Nile in places like phoenix, which are in the middle of the desert and it's 115 degrees.  So it's all very complicated, but you can see that a variety of environmental factors, temperature, rainfall, and early onsets of spring or late onsets of spring or late onsets of fall, etcetera, can affect all of these parameters that affect transmission of West Nile virus in nature.  So it's complicated. 

LENA SUN: Okay. 

LOLA RUSSELL: Melinda, can I have the next question please? 

OPERATOR: Bob Roehr, British Medical Journal. 

BOB ROEHR: Thank you very much.  One of the earlier questions asked about the emerging infectious diseases.  How much of that is simply our ability to be able to detect and segment out this is one infectious disease as opposed to the other where the symptoms are often similar? 

LYLE PETERSEN: Yeah, you cut out a little bit on your question, so if I don't answer it properly, please repeat it.  There's a couple of factors involved.  Certainly our ability to identify new pathogens is increased.  And for example, the new heartland virus which was reported in the New England Journal of Medicine, last week was actually discovered because we were doing a study on another vector-borne pathogen.  We accidentally discovered this new virus, which has probably been there all the time.  We just hadn't looked for it and didn't have the tools to actually discover it.  So part of this is clearly related to our better ability to identify new pathogens.  But probably the bigger factor is the fact that the world is a smaller place right now.  Birds, mosquitoes, other animals, people are all moving around the planet at a record rate.  The spread of goods and commodities is tremendously increased and along with all of this movement, pathogens move around the planet.  And we have very substantial evidence that this is a major driving factor of what's happening.  And West Nile is the perfect example.  It arrived in New York City in 1999.  Genetic evidence suggests that the virus that was imported originally came from Africa.  Well there's no way a bird can fly from Africa to New York City or mosquitoes certainly can't fly that far.  So it was brought in somehow.  So it really is, these pathogens are moving around and vector-borne disease pathogens are very susceptible to movement.  Once they get established in a new area, they can cause a huge problem, as we have observed this year with West Nile. 

BOB ROEHR: Great.  Just a follow up, there was a suggestion, if I understand your data correctly, that the mosquitoes carrying West Nile virus are more vulnerable to spraying than other mosquitoes.  Is that an accurate description or not? 

LYLE PETERSEN: That's not an accurate description.  We know that the vector mosquitoes are certainly susceptible and highly susceptible to the kind of intervention that we can give using very low amounts of pesticide.  But other mosquitoes are susceptible as well.  For example, when I talked earlier about mosquito control efforts in the hurricane areas, the flooding there causes tremendous number of what's called nuisance mosquitoes.  These aren't mosquitoes that spread disease, but they are present in tremendous numbers after a flooding event.  And those mosquitoes can act very effectively be controlled by judicious amounts of pesticides. 

LOLA RUSSELL: Thank you.  Melinda, next question please. 

OPERATOR: Ninoska Marcano, Kids Health. 

NINOSKA MARCANO: Hi, thank you for taking my call.  I wanted to know if you could go over the number of pediatric neuroinvasive figures and also discuss a little bit if you are taking any particular action with school officials or recreational centers in terms of preventive measures towards family and kids?  Thank you. 

LYLE PETERSEN: Yeah, I do not have the data of neuroinvasive disease cases in front of me.  But if you look at the 1,993 cases of West Nile virus reported this year, which we have age data on, 85 of them are reported for the age less than 20 years of age.  So a small proportion of the reported cases are in children.  However, as I noted earlier, more than half of the reported cases are neuroinvasive disease, which we know are rather uncommon in children.  Can occur, but are uncommon.  So most of the children who do get infected, will either have no symptoms at all or West Nile fever and the cases of West Nile fever don't tend to end out in our statistics.  But we can get the data on neuroinvasive disease cases after the call if you're interested. 

DAVID LAKEY: Dr. Petersen, this is David Lakey from Texas.  Our Texas data is on our website divided up with neuroinvasive and West Nile fever.  So if you go to, you can pull out the table that has it by age and whether its neuroinvasive or West Nile fever. 

LOLA RUSSELL: Next question.  This will be our last question. 

OPERATOR: Amy Maxmen, Nature. 

AMY MAXMEN: Hi, thanks a lot, I have two questions.  Somebody asked earlier about if the CDC would be looking into how many asymptomatic cases there were and I think that there were given information about whether the virus might of become more pathogenic this year than usual. And I’m wondering if the CDC isn't sure if they are looking into right now, is there somebody else who might be and kind of a related question to that is anybody at the CDC addressing the long-term effects of West Nile?  Will they stick around for a little bit longer even though they have already left Texas. 

LYLE PETERSEN: We are conducting studies on the long-term effects of West Nile.  We have several cohorts that we have been following now for ten or more years, so we are looking into the long-term effects of West Nile.  We think particularly for the cases of neuroinvasive disease they are quite significant.  We've published interim analysis already on some of these cohorts, but we are definitely looking into that.  As far as looking at asymptomatic cases, we  last did-- it's not an easy thing to do because basically you have to do a serosurvey in a large population of people to see who got infected, which right now, is more difficult because a certain percentage of the population has already been infected.  So these kinds of serosurveys to figure out how many people got infected in any given year is more difficult than it used to be just because of the background rate of infections from previous years.  We last did this kind of a study in North Dakota in 2010, which the results are published and I can get you that reference later, if you wish.  But it basically demonstrated that what we knew about the proportion of asymptomatic infections in neuroinvasive disease cases hadn't really changed since the virus was first discovered in 1999.  Did that answer your question? 

LOLA RUSSELL: All right.  Thank you so much, everyone, for joining us for today's telebriefing on West Nile virus.  As a reminder, if you would like to speak with Dr. Lakey and have additional questions, you can reach out to Carrie Williams at 512-776-7119.  Carrie is the director of communications with the Texas Department of State Health Services or if you have additional questions for Dr.  Petersen or CDC staff, reach out to us at 404-639-3286.  An audio file will be available very shortly and we can provide that for you and a transcript will be available later on today.  Thank you again for joining us. 

OPERATOR: Thank you.  This does conclude the conference.  You may disconnect at this time.  


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