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Date: Mon, 28 Oct 2002
Posted by: Ninivaggi, Dominick (Dominick.Ninivaggi@co.suffolk.ny.us)

I would be particularly interested in avian serosurveys. I would like to know if our bird populations are building some level of immunity. If so, we may start to see some tapering off or cycling up and down of WNV. If not, the possibility exists we could see larger outbreaks when conditions are right (whatever that is) because there are plenty of susceptible birds to fuel viral cycling. I am also beginning to suspect that the geographical distribution of WNV activity may have less to do with the distribution of mosquitoes than it does with the distribution of birds. How else to explain the fact that we tend to see a lot of WNV activity (and 5 of our 7 human cases in 2002) in a part of the County that is relatively the least infested? Or have we just been fortunate not to have much spillover into bridge vectors in the most infested areas yet? What I'm really interested in knowing is are we likely to continue to see a few, sporadic human cases, or are we in real danger of having a situation like Cook County, IL, with hundreds of cases? Until we know otherwise, we have to guard against that possibility, with all that that implies in the way of control strategies.

-----Original Message-----
From: Michael Gochfeld [mailto:gochfeld@eohsi.rutgers.edu]
Sent: Monday, October 28, 2002 2:06 PM
To: westnilevirus-L@cornell.edu
Cc: bpb01@health.state.ny.us
Subject: serosurveys

I am elated that some places are considering serosurveys according to Bryon Backenson's email. He is right, they are indeed challenging to perform and to interpret. The easiest interpretations are from small villages where one can often see the history of a disease by the antibody distribution in people of different age.

In Sprawlsville, it's not clear how much of a geographic area to cover, and even presuming that there is an appropriately random sample, the statement of rate can only refer to the area represented by the sample. An assumption is that everyone in the population has an equal risk of exposure and infection. Other assumptions are that immigration and emigration don't occur or are irrelevant.

It's one thing if you're looking at an airborne exposure where a plume may cover a large areas (then all you have to do is deal with atmospheric physics, discrete modeling, wind roses, etc). But for a mosquito borne illness, where the mosquitoes are presumably not homogeneously distributed, it's even more challenging. Individuals in the sample don't have an equal likelihood of being contacted by a mosquito or an infected mosquito. Using GIS and mapping the distribution of positive sera and negative sera against known mosquito habitats would be more useful and there is a rapidly emerging field of disease mapping, but I haven't heard that any of the serosurveys (still precious few by this time) have been examined that way.

When I did a student rotation at a virus lab in Trinidad, we did a lot of serosurveys (of people, birds, rodents, etc) for ARBO viruses, and were able to draw conclusions regarding past prevalence of infection and the degree of current protection.

I think when I mentioned the Romania data I added the caveat from one CDC worker that the serosurvey there had not been extremely accurate.

Mike Gochfeld

"Bryon P. Backenson" wrote:

I may be wrong, but I believe that the "1 in 150" figure came not from the serosurvey done in Queens in 1999, but from the serosurvey done in 2000. This serosurvey was done in 3 locations (Staten Island NY, Suffolk County NY, Fairfield County CT), each of which covered a greater geographic area than the 1999 serosurvey did. The study consisted of taking blood and completion of a lengthy questionnaire. Approximately 750-850 individuals had blood drawn in each location, for a total of about 2400-2500 individuals. Using the number of positives found, the populations of the area, and the number of hospitalized cases in each area, CDC crunched the numbers. The result were the figures we've seen replicated in numerous news reports:

"Less than 1% of people in a given area will be exposed to West Nile virus."
"Of those people that are exposed, about 1 in 150 will develop symptoms that are serious enough to require hospitalization."

Sometimes the two statements above get misrepresented in the press.

I'm not sure if the data from 2002 will alter these figures any. I know that some locations are considering doing serosurveys, but to be done right, these are extremely time- and labor-intensive, particularly when you take into account the calculations that tell you the number of people required for statistical significance/adequate representation.

-- 
P. Bryon Backenson
Research Scientist and Assistant Director
Arthropod-Borne Disease Program
New York State Department of Health
ESP, Corning Tower Room 632
Albany, NY  12237
phone:  518-474-4568
fax:  518-473-1708
e-mail:  bpb01@health.state.ny.us

-- 
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