A question about West Nile virus

[Lord Jim]

The following is from Tarrant has 1st West Nile case
(http://www.dfw.com/mld/dfw/news/local/4049944.htm) in today's Ft Worth Star Telegram. "The estimate is less than 1 percent of the mosquitoes in a given area are infected, and less than 1 percent of those are going to cause serious illness," he said.

The outbreak of the West Nile virus is now at its peak, Hathaway said. When area temperatures begin to cool, the mosquito population will begin to die out, she said.

Next year, the number of cases should not be as high because the population will begin to develop an immunity to the disease, Hathaway said.
My question is about that last statement. I understand that most people don't show symptoms so that segment can be ignored. Is he saying that by next year a significant number in the high risk segment will contact the virus enough to build an immunity without getting sick? And doesn't that mean that a significant portion of the population will have come in contact with the virus?

That would seem to indicate that the virus is wide spread but very few people get sick. But that's not my understanding. They spray a small area for mosquitoes around where they find an infection. That would indicate that only those in the general area are coming in contact with the virus and therefore are the only ones building an immunity to it.

So is it a wide spread virus with almost no one getting sick from it or a local virus that will pop up here and there for a long time?

[jk1245]

Do they mean the mosquito population will build an immunity to it perhaps? In the quote, the word "population" is used to mean the mosquito population.


As to your last question, from what I've read it sounds like it is a wide spread virus that few people get sick from.

[Floater]

[slight highjack]
Where does the name West Nile come from?
[/slight highjack]

[Colibri]

Originally posted by Floater
[slight highjack]
Where does the name West Nile come from?
[/slight highjack]

That's where the disease was first identified from. It has only shown up in the U.S. recently. It took awhile to figure out what it was, because of its origin. (And I'll continue the hijack by noting that it was my second cousin, Tracy McNamara, chief vetinarian at the Bronz Zoo, that first figured out that the disease that was killing all the local crows was West Nile .)

[Zazie]

And I am highjacking further... sorry...
What are the symptoms??? I heard a lot about the virus and never heard the news talk about the symptoms!

[light strand]

Diseases often have a somewhat arbitrary naming system. Usually the name reflects the point of recognition, not the point of origin. I can't be sure this is true about West Nile, but that would be my guess.

When a virus is first introduced into a population, that population is, of course, at it's must susceptible. Since this year it seems to have spread quite rapidly into unfamiliar populations, there are plenty of populations that need to gain immunity. Amongst these will be the avian, equine, and human populations. Although West Nile isn't very often fatal, it can indeed be relatively easily spread. In fact, if you had a summer flu, you may have very well contracted West Nile. However, you won't be getting it next year (I believe it's not terribly mutatable).

There is no reason for mosquitoes to acquire immunity as they are simply vectors and don't have any problems with carrying the disease. So it's the human, avian, and equine population that will be gaining immunity.

Now to answer the actual question. One percent of mosquitoes will carry the virus. This is actually a significant number of mosquitoes when you consider the numbers. That means that for every 100 active mosquitoes, one will be a carrier. Considering that there are literally hundreds of millions of active mosquitoes, there are a whole bunch of carriers. However, that really isn't the important number. The important figure is that one percent of people bitten by the infected mosquitoes will get encephalitis, which can lead to death.

Since less susceptible people, birds, and horses for the mosquitoes to use as blood meal, will be available next year, it translates into less cases.

[light strand]

The symptoms are very flu-like (as with most viruses) fever, chills, etc. It's when the sore neck and muscle aches show up that you're going to want to go see yor Doctor.

[Lord Jim]

Light strand, you are touching on the source of my confusion. In the quote he say 1% of the 1% are going to cause serious illness in a given area. My question is how big is the given area? If it is the 1 square mile or so around a known infection that they spray for mosquitoes, then the number seem in line. In Tarrant County, there is 1600 people per square mile which would mean only a couple of dozen high risk people. 1% of 1% of the mosquitoes in a one square mile area is a lot of mosquitoes. So one case would seem likely to me. That doesn't consider how many birds would be involved.

But if you expand it out to the whole county, there should be a lot more than just one case. Tarrant county has 1000 square miles and 1.5 million people, meaning tens of thousands of high risk people plus thousands of horses and millions of birds to be infected by the millions of mosquitoes.

So my confusion is that if the given area is only about 1 square mile then not much of the population is going to develope an immunity. If it is as big as the county, then I would guess that most people in Tarrant County have had contact with it, I've been bitten a few hundred times, but why aren't there more cases of illness and dead birds.

[light strand]

Ah the old "a given area" stat.

What that statistic actually means is that on average in an infected, trapped population, one percent of mosquitoes will be found to be carriers.

In other words: When there is found to be dead crows in a county, the heath department is sent out to trap mosquitoes (generally with Co2 traps). When these animals are tested for viral presence, and those numbers are compiled with the numbers of all other trapped mosquitoes in other infected communities you will get a result of roughly one percent carrier mosquitoes.

It's a meaningless statistic thrown about to ease the fears of the public. The percentage of carrier mosquitoes will vary with the number of infected birds and vice versa. But the public seems to accept the one percent of one percent, and so the powers that be will continue to quote it.

[Lord Jim]

Well, we are getting somewhere. So the "given area" is probably about one square mile. After thinking about the critters and birds around here, except for a few hawks and such, not many would travel outside of a one mile square.

So if I look at Tarrant, Dallas and Harris counties, we have 50 cases of the 67 in Texas plus a few pending. These three counties have a population of 7 million, in about 3500 square miles, an average density of about 2000 per square mile. With 1% of the mosquitoes as carriers, it would seem to me that only about half in the "given area" are going to be bitten by a carrier. With 50 cases, plus a few mis-diagnosted cases and a few pending, we are probably talking about 100 "given areas". That's about 100,000 people likely to have been bitten by carrier mosquitoes. In a population area with 7 million that's only about 1.5% that are likely to be immune next year. That doesn't seem to be a very significant figure. I'm only guessing but I'd think that the birds and mammals affected would have about the same distribution rate, although I have no idea what the actual numbers might be.

So, is our immunity actually going to change things significantly over the next few years? That's my basic question.

[YEP]

Have you taken into account the portion of 1% of infected mosquitoes may not even feed on humans?

Culiseta melanura strictly preys on birds for their blood meals

[brachyrhynchos]

Originally posted by light strand
...It's a meaningless statistic thrown about to ease the fears of the public. The percentage of carrier mosquitoes will vary with the number of infected birds and vice versa. But the public seems to accept the one percent of one percent, and so the powers that be will continue to quote it.
I'm glad to see you say this, light strand (you changed your name, bio-brat - I had to turn on the sigs to see who was this familiarly-sounding poster. Looks good on ya! Is that a DNA reference?). I am wondering how this figure was derived.

When mosquitoes are tested, they are tested as "pools," with any pool containing up to either 50 or 100 mosquitoes. They're trapped in an area using CDC traps (aided with CO2 as light strand mentioned, gravid traps (with the attractant given the technical name of "stink water"), or NJ light traps. The mosquitoes are separated according to species and sometimes to feeding stage. Some pools may contain only one mosquito, particularly if it is of a species of concern, such as Aedes albopictus or Ochlerotatus japonicus, but generally pools are of the full compliment (i.e., 50 or 100 individuals). This pool is then ground up and is tested. A positive may represent one infected mosquito in that pool or 33 or 76 or all of them. So, for this first point, there is a large amount of error since the value of a pool being positive does not indicate how many within the pool were actually positive in themselves.

Secondly, the number of positive pools do change through the season. (As background, I work with the Mosquito Research and Control Unit in the Department of Entomology at Rutgers, and part of our job is to do arboviral surveillance, including West Nile and eastern equine encephalitis. I do the bird work.) In the beginning of the season (here in NJ, March/April), we need to trap mosquitoes at the site of where a dead crow was found in order to find positive mosquitoes. If we trap next door, we don't find them. Later on in the season, we can put a trap much farther out and find positive mosquitoes.

So, why the seasonal change? We think the rapid spread at the local landscape level (not nationally) represents the spread of the main group of organisms that amplify the virus: juvenile birds. The timing corresponds fairly well with the emancipation of juvenile birds from territories (not just leaving the nest, but moving around off-territory). At least this is our currently working hypothesis. :)

YEP brings up an important point. Most mosquito species have a preferred host, some being attracted to birds, some mammals, some herptiles. The ones that are of concern to health folks are the bridge vectors - those that feed on both birds and mammals. Culex pipiens, the species of mosquitoes most associated with West Nile here in the States will feed on both. They are a competent (though not the most competent) bridge vector and the one we tend to focus on. Here's a list of the mosquito species in New Jersey (http://www-rci.rutgers.edu/~insects/njspp.htm), many of which have species accounts that indicate who they like to suck on.

So, will human population immunity levels change significantly? Most probably, but the extent will depend on both natural and anthropogenic factors. The virus will not be found uniformly. There will be endemic foci, or areas where the virus will reside in avian populations at some "resting" level. At times, the viral load will increase, and outbreaks will occur. Depending on how frequent these outbreaks are, the human population exposed to them will either be more immune (if outbreaks are frequent enough) or less immune (if they are very infrequent, and new, non-immune humans are added to the population between outbreaks). Overlaying this all is the effect of local mosquito control. While it might make sense to have a population that is fully immune, we do this at the risk of loosing some to the disease as well as exposing ourselves to other, more dangerous arboviral diseases. So, we rely on mosquito control to reduce the threat of exposure, and thus reduce the effect of increasing more immune individuals to the population.

[Lord Jim]

My question still has to do with the immunity factor not the amount or rate of infections.

If an infection occurs 10 miles away and I get bitten several times a day, is it likely that I'll be immune? If so, how long does it take? If I gave blood today, should I consider that I likely have the virus? Or should I figure I'm already immune?

See it's the immunity thing I'm having trouble with. It just seems like a very small part of the population is going to be immune next year.

[brachyrhynchos]

A serosurvey of residents in Queens after the initial outbreak suggested a 2.6% seroconversion, while surveys a year later in Staten Island and Connecticut showed lower rates (here's an Morbidity and Morlaity Weekly Report summary about the surveys (http://www.cdc.gov/mmwr/preview/mmwrhtml/mm5003a1.htm)).

Okay, let's assume that this is the rate 10 miles away from you. What does this mean for you? Well, it still depends on how far into the season you are (localized verses amplified) and what mosquitoes are in your area (bridge vectors or not) as well as how prevalent they are. Your risk is more likely to increase as the season progresses, but is still relatively small (that is, it's not like seroprevalence is 50%). If the bridge vector has a pronounced preference for humans, the risk gets smaller (even biting infected humans in unlikely to result in an infected mosquito since we are "dead-end" hosts - we don't produce a high enough viremia to pass it on to mosquitoes). In addition to all this, not all mosquitoes are equally good at spreading the virus (the competency I referred to earlier). If the mosquito that lives with you is not very competent, then your risk is even smaller.

I wish I was less wishy-washy about this, but we're on the steep part of the learning curve with this virus. We're finding things we haven't expected (a grad student found exposure in deer and black bear :eek: ) and the expansion of the range was swifter and more intense than a number of us expected. Many of us thought it would reach California this year (as it has), but I don't think we forsaw such activity in the Midwest.

[Mirage]

The West Nile Virus mainly targets the bird population. This is why CDC officials focus on towns where large amounts of dead birds are discovered. Birds are also blamed for the spread of the virus, as noted earlier mosquitoes are mearly a vector, they just transfer blood. You need an infected organism in the area to host the virus before the bug can give it to you.

Birds are the ones who are hit the hardest by this, and they are the ones who will develop immunity the quickest. With less sick birds there will be fewer mosquitoes carying the virus and less old people dying.

[cantara]

So, a couple more questions then...but first a little background...

Last summer we found a dead blue jay on our front lawn, but by the time the city had come to collect it, it had decomposed too much for any testing to be done...I suspect it was due to West Nile since others in the area were soon reported as the first confirmed cases. In the last month a neighbour has come down with an illness that looked like meningitis, but was ruled out. They suspect that it could be West Nile, but I haven't heard the results yet. This week I saw a dead blue jay and an almost dead crow on our block. We have a storm culvert just below us that has remained relatively dry this summer due to the low amounts of rain, so there are pools of standing water.

My questions...
1) Given the high likelyhood of West Nile virus in the local bird population and the availablility of 'vectors', are we significantly more at risk?
2) Is the virus more dangerous to our 16-month old kids?
3) Can they produce a vaccine to protect against it?

[brachyrhynchos]

Originally posted by Mirage
Birds are the ones who are hit the hardest by this, and they are the ones who will develop immunity the quickest. With less sick birds there will be fewer mosquitoes carying the virus and less old people dying. You're right in the respect that we believe our native bird population should develop immunity to WN. But mosquitoes will still get infected because of an interesting phenomena we think is the main factor of re-introducing the virus to the mosquito population each year. Many mosquitoes in temperate zones will overwinter as either eggs or larvae. These eggs or larvae don't carry virus. (There are some indications that "transovarial transmission" or from infected female to her offspring may occur in some arboviruses, but this appears to be a relatively rare phenomenon.)

When mosquito eggs develop and the larvae emerge, they are non-infected. How do mosquitoes pick up the virus from a population of birds that are immune? We think that the virus is actually "hiding" in the immune birds. The infected birds show no sign of illness, but at some point, when they get stressed, the virus comes out of hiding or "recrudesces" very briefly. The virus reproduces quickly while the birds immune system begins to react. The bird's antibodies do their job, and the virus retreats. But in that short (sometimes hours) period of time, the virus is circulating in the bird's blood, available for transfer.

Birds get stressed during migration, when they look for a territory, defending the territory, getting a mate, raising the kids: in short, during the breeding season. It's at this time when many mosquitoes emerge, and females are looking for a bloodmeal to lay eggs.These non-infected females pick up the virus during that recrudescent flash. If they live long enough, they may lay several batches of eggs and go back for another bloodmeal. Hey, there's a nice juicy, defenseless baby bird! A non-immune baby bird will amplify the virus more readily before its immune system kicks in. The kid will likely survive due to genetic shifts avian populations go through under pressure from the virus and will be there next year as an adult when the virus recrudesces again.

This cycle goes unnoticed by most of us when it remains with the birds and the bird-feeding mosquitoes. But when conditions are right (climate, habitat) and bridge vector populations explode, then the virus has more of a chance to spread to the bridge vectors, and we have an outbreak. (This is the "stable system" scenario, where viruses exist in populations that have adjusted to their presence. West Nile, on the other hand, is an emerging disease in the Western Hemisphere, exploiting immunologically naive populations. It is possible that the patterns of infection we see today will change in the future.)

[brachyrhynchos]

Originally posted by cantara
We have a storm culvert just below us that has remained relatively dry this summer due to the low amounts of rain, so there are pools of standing water.
Call you local mosquito control agency. They will come out and trap at the culvert (to find which mosquitoes are there) and treat it. They may larvicide, pupacide, or possibly adulticide. They may find ways of eliminating the standing water.

Originally posted by cantara
My questions...
1) Given the high likelyhood of West Nile virus in the local bird population and the availablility of 'vectors', are we significantly more at risk?
2) Is the virus more dangerous to our 16-month old kids?
3) Can they produce a vaccine to protect against it?
1.) You are more at risk in an area where the virus is than in areas where it isn't (obviously), but how significantly more at risk is impossible for me to say. Are you over 60 (unlikely with a 16-month old :))? Do you have diabetes or any other condition that compromises your immune system (AIDS, tissue transplant)? If yes to any of these, your risk goes up. Fortunately, this is a relatively easy disease to avoid by reducing the chance for exposure. Taking care of the culvert problem is one step.
You can take steps around your house and when you go out. (http://www-rci.rutgers.edu/~insects/ipm.htm) Wearing clothing with long sleeves and long pants (ugh in hot weather) and using a DEET-based product should be considered.
2.) I am unaware of any child younger than 7 years (I think) that has contracted WN resulting in hospitalization. In this respect, WN is different from EEE, which can hit young kids. I am reluctant to recommend applying a DEET-based product to toddlers (I don't have a bottle in front of me to read applications), but perhaps there is a product available for youngsters? Anyone know?
3.) Right now there is no available human vaccine, but one may be on the horizon (http://news.bbc.co.uk/1/hi/health/1295940.stm).

[samclem]

If this question has been asked, I'm sorry. I haven't read every thread currently on the board.

My local talk show(I know, I know) propounds the theory that the WNV was introduced to the US by terrorist.

Is there a scientific answer to these cretins?

[light strand]

Thanks brachyrhynchos, yes it is a DNA reference. While I have you here, I have a question myself about West Nile. Is there a variance in virulence? I've noticed that the occurrence of encephalitis and deaths seem to be clustered (like those in New Orleans). Is this just because of higher incidence mosquitoes, and hence carriers, or is it virulence?

samclem I've heard this claim myself, and in fact, I believe that a Senator has asked that it be investigated. Personally I think it very unlikely (read: not a snowball's chance in hell) that this was introduced as a terrorist act.

We've had this guy floating around as an identified viral species since ~1998. Which means it's probable been around since well before than. It usually takes the CDC a while before it actually gives names to new diseases, so it was probably attributed to something else (like viral meningitis) for years.

Also it's very similar to another indigenous species. The virus that causes St. Louis encephalitis has been around for a very long time. It seem silly to introduce a species to which the natives may already carry immunity .

Another flaw in the theory is that the virus isn't very virulent. It's the lazy terrorist that wants to kill crows, horses, and old people. If this virus behaves like its sister (St Louis) then folks should have immunity within a couple of years, and it'll be just a faint memory like its sister. Especially if, as has been mentioned, there is a vaccine on the horizon.

[rsa]

Sen. Patrick Leahy suggests possibility of terrorist link with West Nile virus should be explored (http://www.boston.com/news/daily/12/leahy_west_nile.htm)

I think the Senator is getting a little paranoid personally.

For anyone who is worried about exposure to West Nile, remember that mosquitos generally feed only at dawn and dusk so avoiding being outdoors at those times should significantly reduce your risk.

[brachyrhynchos]

Gee, of course my most brilliant post to date became hamster food (and apparently unedible at that :D ). But dagnabbit, I made a cute little graph, so I'm posting again (the truncated version):

Ditto with West Nile no go as bioweapon.

The virus here in the western hemisphere is nearly identical with a strain that has been going around in Israel the past few years. This strain is a bit more virulent than some other outbreaks (worldwide rates vay between 3 and 15 percent). The first year here had mortality rates around 10 percent. This year, rates (as of a few days ago), vary between 0 and 33 percent (my graph). (http://www.rci.rutgers.edu/~insects/wn.jpg) (So, yeah, there seems to be variability, not surprisingly related to some degree to N.) Note that above 30-some cases, the rates fall below 10 percent.

[light strand]

Great graph brachyrhynchos, thanks!

Now for another question: Have you heard of "polio-like" symptoms? Do you have any info?

[Lord Jim]

I want to thank everyone for the information. I know there are lots of questions about this out there.

An article earlier this week said that up to 200,000 people had been exposed to WNV since 1999 when the first outbreak in Queens occured. That's a big number but a very low percentage of the population. It's not even 2% of the population of the New York area where it started not counting the rest of the country that it has spread to and this is after 3 years.

I hope I'm right in saying that this is a bird disease that us mammals are caught up in the fringe of. I wish the news coverage would pay more attention to what's happening to the birds.

I know it's big news when a person is sick and supposedly no one cares if a bird gets sick, but really, some of us do care. Plus, if I knew that a bunch of birds were dying in my neighborhood, I'd be paying more attention than just reading about some old man getting sick 10 miles from here.

So, protecting yourself with long clothing and not going out in the mornings and evenings and using repelant is good. But the things that will protect the birds and consequently slow the virus are getting rid of standing water, changing birdbaths often. Our city give tablets for where there is standing water. Kill mosquitoes, not just repel them. Destroy their breeding places. These actions will save the birds and save the people and other mammals in the process.

If I'm wrong, please correct me, but the focus on human cases by the media seems to me to miss the main problem with this virus.

Ok, down off my soapbox.

Jim

[brachyrhynchos]

ProMed Ahead had a report about the polio-like symptoms (http://www.promedmail.org/pls/askus/f?p=2400:1001:376298::NO::F2400_P1001_BACK_PAGE,F2400_P1001_PUB_MAIL_ID:1003,19359). I had heard about weakness and paralysis being longterm aftereffects before, but never in terms of etiology. And the longterm effects are not pretty. In another thread, I had posted: ....if you're one of those that end up hospitalized, the outlook is not quite a bed of roses. This is an encephalitic disease, and the long-term morbidity (http://www.annals.org/issues/v137n3/full/200208060-00009.html) is rather unpleasant "One-year follow-up of the 1999 New York patients by the New York City Department of Health found frequent persistent symptoms (fatigue, 67%; memory loss, 50%; difficulty walking, 49%; muscle weakness, 44%; and depression, 38%).".... Fortunately, it is fairly easy to avoid getting by following Lord Jim's last post.


Originally posted by Lord Jim
I know it's big news when a person is sick and supposedly no one cares if a bird gets sick, but really, some of us do care. Amen. It's concerned a lot of us, especially some of the more sensitive species, such as the Florida Scrub Jay and California Condor.
Originally posted by Lord Jim
So, protecting yourself with long clothing and not going out in the mornings and evenings and using repelant is good. But the things that will protect the birds and consequently slow the virus are getting rid of standing water, changing birdbaths often. Our city give tablets for where there is standing water. Kill mosquitoes, not just repel them. Destroy their breeding places. These actions will save the birds and save the people and other mammals in the process.
And know which species of mosquitoes to direct this toward. We have 64 species (linked earlier), many of which pose minor or no health threat. Some, like Toxorhynchites, may even be beneficial (http://pherec.org/entguides/EntGuide3.html) under certain circumstances. :)