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Florida Medical Entomology Laboratory

Florida Medical Entomology Laboratory

West Nile Surveillance and Florida Mosquito Control: Acting on Surveillance Information

Since West Nile virus (WNV) arrived in the U.S. in 1999, the Florida mosquito control and public health community has stressed the importance of West Nile surveillance using, among other things, sentinel chickens. Through Buzzwords we have advised mosquito control and public health organizations to believe in their surveillance data and to take appropriate actions based on available surveillance information. There are some general guidelines for using surveillance information in the Florida Mosquito Control Arbovirus Response Plan--West Nile (FMCARP-WN) that can be viewd by going to

Is Florida better at WNV surveillance today compared with where we were in 2001? Has our WNV surveillance effort been useful? Are we prepared for a significant WNV epidemic--the "big event?" Will mosquito control have enough advance warning of the big event to allow reduction of the impact of a major epidemic, when West Nile cases rise from 10-20 scattered around the state to 100s and perhaps 1000s?

Let's look at Florida County X in 2005. Here is a quick sketch of events:

  1. July 11, 2005--7 sentinel chicken WN presumptive positives distributed at 3 different sites. An overall county-wide seroconversion rate of 15% and a seroconversion rate of approximately 30% at the 3 WN positive sites, well above any background previously recorded.
  2. July 13, 2005 - County X believes in their surveillance data and issues a Medical Advisory, and aggressively launches mosquito control operations in the geographical areas identified by the 3 WN-positive sites.
  3. July 29, 2005--12 more WN-positive sentinel chickens are reported at the same 3 sites, with the majority located at 2 of these sites. This is a county-wide seroconversion rate of approximately 30% and approximately 70% of the positive chickens were located at 2 sites. County X continues aggressive control efforts in the geographical areas identified by the WN-positive sentinel chickens.
  4. August 1, 2005 the first human WNV case is reported in County X and the county is placed on Medical Alert. Mosquito control continues aggressive control operations in the areas defined by the 3 sentinel sites with positive chickens.

For those who have not already guessed--County X is Pinellas County!

Pinellas' handling of WNV in 2005 was absolutely right on target and is an illustration of the benefits provided by a well-run sentinel chicken surveillance program. Pinellas ramped up operations appropriately, and their surveillance activities and mosquito control responses were not based on the appearance of the first human case. Pinellas acted immediately and decisively on the surveillance data they had at hand. The surveillance information preceded not only the first human case by a week (frankly this is of little importance), it preceded by several weeks the appearance of a larger number of human cases that identified a clearly defined focus of WNV transmission that had already been identified by sentinel chicken surveillance.

Pinellas County Mosquito Control hit the ground running with the appearance of 7 seroconversions (a 15% overall seroconversion rate and a 30% seroconversion rate within the positive flocks) during a single week. This was well above the historical background transmission rate for Pinellas County. This control response was mounted in the absence of a single human case. The Pinellas County surveillance effort alone confirmed the appropriateness of the response. On July 29, 2005 they reported 12 more seroconversions in their sentinels (50-70% in the 3 sites if one subtracts the 7 original positives). There was no doubt that a clearly defined geographical region of Pinellas was at risk for WNV transmission. One could expect 2-60 actual human WNV cases per week based on a reported 15-70% county-wide sentinel chicken seroconversion rate (see the FMCARP-WN).

Pinellas County recognized that the sentinel seroconversions were not county-wide and were confined to a relatively small geographic area, bracketed by the 3 sentinel flocks, and clustered around the two most active sites. This is where control efforts were targeted. Pinellas County had identified a focal transmission zone of active WNV transmission in 2005, similar to what occurred in Coconut Grove, FL in 2004, where there were 9 cases among ca. 60,000 people for an incidence of 1 per 6700.

Pinellas County hit the ground running based on a well-designed and organized sentinel chicken surveillance program. The total number of WN human cases in Pinellas County during July and August was 18 among ca. 1,000,000 people in the county, with ca. 100,000+ people at high risk in the focal transmission area. Gauged by the Pinellas County arboviral surveillance program, WNV transmission never spread beyond the focal transmission zone, and actually declined in the transmission focus where 4 or fewer sentinel seroconversions per week were reported throughout August and September. This was commensurate with a reduction in transmission risk for humans where the number of cases also declined in August and September. The total number of human WN cases did not approach the potential 2-20 per week indicated by the initial sentinel chicken seroconversion rates.

Kudos to the Pinellas County Mosquito Control program (Sue Bartlett, Nancy Page, Jeffrey Ball, and their entire staff) and to the Pinellas County Health Department for their excellent surveillance program and their quick and decisive response to the early surveillance data. Mosquito control and public health worked together providing videos, public announcements, and public meetings. Pinellas County Mosquito Control was aggressive and along with the County Health Department they got the message out about the risk of WNV transmission and mosquito avoidance.

The final score card: surveillance data showed early on that there was a risk of WNV transmission in Pinellas County that could have resulted in 100-200 human cases. Continued arboviral surveillance in Pinellas County confirmed the risk. Aggressive vector control and public health mediated the outbreak and 18 human WN cases were reported during July and August. Victory can be declared.

We hope that the rest of Florida does as well when it is our turn.

Walter J. Tabachnick, - Retired
Jonathan Day, Professor,
Florida Medical Entomology Laboratory, University of Florida/IFAS