Published Date: 2002-09-09 23:50:00
Subject: PRO/AH/EDR> West Nile virus, human - USA (CA) (02)
Archive Number: 20020909.5265
WEST NILE VIRUS, HUMAN - USA (CALIFORNIA) (02)
A ProMED-mail post
ProMED-mail is a program of the
International Society for Infectious Diseases
Date: 8 Sep 2002
From: Jack Woodall <firstname.lastname@example.org>
This is a comment -- which could well be overtaken by events by the time it
is posted -- on last week's report of a probable case of WNV infection in a
resident of Los Angeles County (see ProMED-mail reference at end). A human
case in the absence of any bird or horse positives is epidemiologically
very unlikely. Let's look at the evidence.
From the press release of 6 Sep 2002 from the California Dept. of Health
1) A Los Angeles County woman is being evaluated as a probable case of West
Nile virus. If confirmed, the woman would be the first reported case of
West Nile virus in California. We have no other evidence that the virus has
arrived. The woman became ill on 10 Aug 2002 and was hospitalized on 12 Aug
2002 ... she reports no mosquito bites, blood transfusions, organ
transplants, or travel to areas where West Nile virus has been confirmed.
2) State and local public health officials have been conducting an expanded
surveillance program for West Nile virus since 2000, searching for evidence
of infection in people, mosquitoes, birds, and other animals. However,
tests of mosquitoes, chickens, and dead birds have shown no evidence of the
virus in California.
And this from California's Vector-Borne Diseases Surveillance Bulletin #19
for Arbovirus Surveillance - Week 35, Fri 30 Aug 2002 (the latest posted
Sun 9 Sep 2002):
"Tested for Western equine encephalitis (WEE) & St Louis encephalitis (SLE)
Suspect equine viral encephalitis: Year to date (YTD): 27 This week 0. All
Sentinel chicken sera tested for antibody: (YTD) 15 475 This week 911
Positive this week: WEE 1 (YTD 33) SLE 8 (YTD 19)."
Now WNV belongs to the same (flavivirus) group as SLE virus (but not WEE
virus) and cross-reacts with SLE virus in some tests. So note California's
posted disclaimer (asterisks * added by me):
The DHS Viral and Rickettsial Disease Laboratory (VRDL) forwards serum
specimens from sentinel chickens in which antibodies are detected in new
geographic areas to the Davis Arbovirus Research Unit (DARU) for additional
testing for *WNV*, SLE, and WEE antibodies by a different ELISA and plaque
reduction neutralization test (PRNT)."
The table does not say that the 8 SLE reactors this week are final results;
if not, some or all could possibly turn out to be WNV converters.
Mosquito pools tested for virus (final results): (YTD) 2300 This week 193
Positive this week: WEE 4 (YTD 22), SLE 5 (YTD 5) Other 1 (YTD 1) "This is
the first report of mosquitoes positive for SLE in the state this year.
[The 4] _Culex quinquefasciatus_ pools were collected 9 and 24 Jul 2002,
and the _Culex tarsalis_ pool was collected July 24 [all] in Imperial
County." -- but a table shows they were only received on 13-14 Aug 2002.
The disclaimer says:
All mosquito pools in which SLE is detected are subsequently tested for *WNV*.
Since the mosquito tests for this week (ending 30 Aug 2002) are final
results, the mosquitoes were infected with SLE virus, not WNV. But these
were mosquitoes collected 5 weeks or more earlier. So it is possible that
there are some WNV-infected mosquitoes among those in the freezer waiting
to be tested; 258 mosquito pools are listed as having been received the
week ending 30 Aug 2002 (dates of collection not given).
A total of 123 dead birds was received by the lab the week ending 24 Aug
2002; 26 have been tested so far, all (presumably) negative. Of those, 13
of 9 species were from LA County, and 2 of them have been tested (1 crow, 1
blackbird -- 4 crows & a blue jay were pending).
The woman lives in LA County, with no history of travel. Sentinel chickens
posted in LA County were bled between 29 Jul and 21 Aug 2002, but the
specimens were only received by the lab on 26-27 Aug 2002, so it may be
some time before results are available. I was not able to determine from
the tables whether mosquito collections are being made in LA County. Dead
birds are being received from LA County, and results are pending. So we may
find retrospectively that WNV had in fact been circulating in LA County at
the time the woman was presumably infected -- around the beginning of
August 2002. My recommendation (as the former head of the New York State
Arbovirus Laboratory) is that the California lab leave that for later
research, ask the field people to identify the most recent mosquito
collections to species right away, and concentrate on testing the most
recent specimens from all sources received, to find out where in California
the virus is active now.
CDC website map:
ProMED-mail Associate Editor
Director, Nucleus for Investigating Emerging Diseases
Federal University of Rio de Janeiro, Brazil