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Date: Tue, 29 Oct 2002
Posted by: Richard Lampman (rlampman@inhs.uiuc.edu)

I think one of the most informative discussions we could generate would be on the geographic differences in the temporal relative abundance of Culex pipiens, Cx. restuans, and Culex salinarius. It always seemed strange to me that several monographs on SLE strongly suggested Cx. pipiens was the primary epidemic vector, but suddenly with WNV its status was seemingly downgraded to bird transmission amplifier.

In central Illinois, we have several decades of data showing Cx. restuans appears early in the year, coincidental with the proposed onset of the enzootic cycle of SLEV or WNV in birds, and Culex pipiens begins to increase in relative abundance between late June and July. The two species seem to be in about equal abundance (cross-over) sometime between mid-July and late August, although the date of cross-over varies from year to year. After cross-over, Cx. pipiens becomes the more abundant species, although occasionally there is evidence of a late season rebound in Cx. restuans. In 2002, the rise in Cx. pipiens seemed to correlate to the rise in positive Culex spp. mosquito pools and was temporally consistent with the increase in mammal cases (humans and horses). Based on oviposition traps, we catch few Cx. salinarius, although this may reflect trap bias. Do other researchers have data to show Cx. salinarius oviposits readily in the standard Culex infusion trap? Also, the feeding preferences of these Culex species can generate considerable debate, perhaps reflecting regional or seasonal differences in vector behavior or relative ability of the researcher to ID the species. A recent paper suggests that as many as half of the pools thought to be pure Culex salinarius were actually mixed with other Culex species (VIRAL IMMUNOLOGY, Volume 14, Number 4, 2001, Pp. 319-338, Bernard and Kramer).

Many aspects of the behavior of Cx. pipiens and Cx. restuans are poorly defined in my opinion. In some of the early field studies, it is difficult to determine whether separation of Cx. restuans from Cx. pipiens was based on morphological differences or time of the year. Do a search on Cx. restuans and you find very little on its field ecology. Generally, it is said to be similar to Cx. pipiens, but I think this is an overstatement. First, its mating behavior must be different because you can't rear it easily in a cage (and we've tried to select for cage-mating). Second, PCR ID of Culex from stormwater tunnels indicated that Cx. pipiens was the overwhelming species in that type of habitat. If the two species were similar in their choice of overwintering quarters why was Cx. restuans so under-represented? Does Cx. restuans go through some amazing bottleneck in numbers or are there differences in overwintering preferences? Cx. restuans is a native Neartic species probably less adapted to humans than Cx. pipiens (or is it?). Why have the early season isolates from Cx. restuans been so low?

Rich Lampman

I think that one of the big uncertainties involves the relative role of Culex pipiens/restuans vs "bridge vectors" in human transmission. In our County alone, we have had the following situations:

1 - Babylon, 2000: Many isolations in Culex "pre" with little transmission to people or nearby horses (granted, we did adulticide the area). In 2002, one human case detected at about the same time as WNV in Culex pre, with biting reported by residents in the area.

2 -Nesconset 2001: Isolations in Culex pre plus Cq. perturbans and a human case (effective adulticiding not allowed due to environmental concerns). Detection of WNV in Culex pre led to effective aerial adulticiding in 2002, with no human cases in the area.

3 - Huntington, 2002: A small number of isolations in Culex pre, low numbers of all species, but 5 human cases in a single township. Detection of WNV in Culex occured at about the same time as the report of the first human case. There were numerous isolations of WNV in Culex and birds in Huntington 1999-2001, with no known human cases.

The first two situations made us think we are on the right track by considering adulticiding primarily in areas where spillover in bridge vectors seemed likely, since our experience in 2000 seemed to suggest you can have a lot of WNV in Culex with little transmission to people. Our experience in the third situation leads us to think perhaps we should have adulticided Huntington more and earlier, even though the situation did not seem that bad. Perhaps our adulticiding in 2000 kept WNV spillover to humans to a minimum in Babylon, and if we had adulticided there sooner in 2002, we could have prevented that case.

The point I'm trying to make is that we have tended to treat Culex pre as a bird biter, and not that great a threat to humans. We may need to rethink that, and blood meal studies are underway that may help. Needless to say these observations almost certainly do not apply to other regions of the US, especially the South.

As far as effectiveness of interventions goes, I note that we had only 2 of our 34 positive mosquito pools in areas that received aerial adulticide. All had WNV isolations pre-spray. One of the 7 human cases might have been exposed a few days after aerial adulticide in a treated area, the others were pre-spray or in untreated areas.

-----Original Message-----
From: Richard Lampman [mailto:rlampman@inhs.uiuc.edu]
Sent: Monday, October 28, 2002 5:34 PM
To: WESTNILEVIRUS-L@cornell.edu
Subject: Characteristics of WNV transmission

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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