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Date: Mon, 28 Oct 2002
Posted by: Richard Lampman (rlampman@inhs.uiuc.edu)

First, I would like to thank everyone that provided advice on the interpretation of "1 in 150". The Queens, NY survey in 1999 (Lancet 2001; 358: 261-64) used a 1 to 140 ratio. I misinterpreted the statement in the CDC presentation as 1) coming from the Queens survey and 2) meaning the ratio of serious symptoms to all those having symptoms. But,thanks to the comments of people on this listserv, I now have a much better understanding of the "1:150" (it would seem to provide a conservative ballpark estimate of 100,000 WNV-infections in Illinois and about 500,000 WNV-infections in the US according to October's total cases).

If possible, I'd like comments on the list below that I'm developing to summarize WNV transmission characteristics (with a focus on the east-central states).

The hallmarks of West Nile virus (WNV) transmission in the US have been --

1) rapid range expansion from an epicenter in New York City in 1999 to 45 states and 5 provinces of Canada in just four years;

2) high mortality in corvids (crows and bluejays) and some raptor species (hawks and owls);

3) high seroprevalence in common urban birds with considerable year-to-year, intra- and inter-specific variation in seroprevalence;

4) evidence of bridge vectors to mammals (e.g., Cx. salinarius in coastal New York) that explains a relatively high rate of transmission to humans (about 3500 cases in 2002) and horses (over 11,000 reported cases in 2002) (but, the role of bridge vectors has not been widely demonstrated);

5) ability to overwinter in vectors and re-emerge early in the season at multiple sites;

6) a transmission cycle in east-central US that is similar to St. Louis encephalitis virus (SLEV) with Culex species amplifying the virus in avian hosts (but dissimilar from SLE in that it reappears annually in the same areas, as in 5 above, whereas in the east-central states SLEV appears to be sporadically re-introduced);

7) focal transmission patterns in some areas based on human serosurveys and the seroconversion rates in stationary sentinels;

8) extensive spill-over of WNV into atypical hosts, mosquito species, insects, and vertebrates (for example WNV or antibodies to WNV have been detected in dogs, a wolf, skunks, bats, bears, and squirrels). Each year since 2000 over 20 mosquito species have been found with WNV in a wide range of genera including Culex, Anopheles, Aedes, Ochlerotatus, Psorophora, and Uranotaenia mosquito species (most detections of WNV have been from Culex species, but this may partially reflect the use of gravid traps as the primary vector collection method; it should be kept in mind that detection of WNV RNA in a species is not evidence of the species' ability to become infected and transmit WNV to a host);

9) geographical differences in environmental and ecological factors the modify the specifics of transmission (transmission of SLEV is often divided into 3 geographical patterns based on different vectors; and

10) non-vector transmission routes (in humans, this includes blood transfusion from infected donors; also bird-to-bird and prey-to-raptor transmissions have been demonstrated in small-scale laboratory studies, but have not been verified in the field).

Rich Lampman

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