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Published Date: 2012-08-25 11:36:04
Subject: PRO/AH/EDR> West Nile virus - USA (09) human, comment
Archive Number: 20120825.1263077

WEST NILE VIRUS - USA (09) HUMAN, COMMENT
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Date: Fri 24 Aug 2012
From: Blane Hollinger [edited] <blaineh@bcm.edu>

As a person who was extensively involved in the St. Louis encephalitis (SLE) outbreak in Dallas and Corpus Christi in 1966 as a member of the Arbovirus Infections Section in the Bureau of Laboratories at the CDC, it might be of interest to discuss the rationale for aerial spraying since this virus is in the same Flaviviridae family as WNV and shares similar vectors.

While reduction of the vector mosquitoes is a goal to spraying, many of these vectors seek asylum in storm sewers or culverts during this activity and for several days thereafter. _Culex_ mosquitoes usually live only a few weeks during the warm summer months and are weak fliers that do not move far from their home sites although exceptions certainly exist. What is important is that the duration of viremia in infected birds that might be sufficient for infecting the vector is of short duration (3-5 days) so that when the mosquitoes finally reemerge, they are unable to reestablish the transmission cycle.

During the course of the SLE epidemic in 1966, most of Dallas county was sprayed aerially over a period of 7 days with an ultra-low volume, high-concentration malathion mist. Prior to spraying, an [SLE virus] infection rate of 1 in every 167 mosquitoes tested was observed among 10 888 mosquitoes collected. During the post-spray period in which over 57 000 mosquitoes were collected, only 2 isolations of SLE virus were made and these were from mosquitoes collected about 3 weeks after spraying. So while the virus activity was greatly reduced, it was not entirely eliminated. Nevertheless, the infection rate was so drastically low that large-scale transmission was highly unlikely and the human epidemic declined during the 2-3 weeks after the spraying, beginning approximately 6 days after that event.

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Communicated by:
F. Blaine Hollinger, M.D.
Professor of Medicine, Molecular Virology & Epidemiology
Director, Eugene B. Casey Hepatitis Research Center
Baylor College of Medicine
Houston, TX 77030
<blaineh@bcm.edu>

[One hopes that the spraying that has been going on in the Dallas county, Texas area is equally effective in reducing the field infection rates and of West Nile virus in _Culex_ mosquitoes there with fewer human cases. If pre- and post-spraying infection rates are going to be determined during this outbreak, ProMED would appreciate the receiving the results when they become available. ProMED thanks Dr. Hollinger for his comments.

A HealthMap/ProMED-mail map can be accessed at: http://healthmap.org/r/1hiS. - Mod. TY
]

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