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Florida Medical Entomology Laboratory

Florida Medical Entomology Laboratory

The 2012 West Nile Epidemic in Dallas County, Texas: Deja vu, All Over Again

In 2012, Dallas County, Texas experienced one of the largest West Nile outbreaks yet seen in the U. S. The epidemic is reviewed in Chung et al.1 Six hundred and fifteen cases met laboratory case criteria. There were 398 cases confirmed by clinical review with 19 deaths for a case incidence of 7.3/100,000 residents.

The timeline of the outbreak showed that Dallas County had excellent West Nile virus (WNV) surveillance information throughout that summer that included information about vector abundance and mosquito infection rates.1 The culprit was Culex quinquefasciatus.

Florida mosquito control and public health professionals have been discussing West Nile surveillance and appropriate mosquito-borne disease control since 1999 when WNV first entered the U. S.2-11 WNV surveillance programs are conducted in many Florida counties using sentinel chickens and/or vector surveillance. The Florida arbovirus action plan, revised in 2003, includes a mosquito control action plan for WNV using surveillance data.12 Thankfully, Florida has not experienced a West Nile epidemic on the scale of the 2012 Dallas County epidemic. However, there are examples where aggressive mosquito and mosquito-borne disease control actions have been conducted in Florida against WNV when surveillance information provided evidence of a high risk for WNV transmission. For example, Pinellas County Mosquito Control3 and Indian River Mosquito Control13 conducted aggressive spraying and/or issued medical alerts in response to sentinel chicken surveillance information showing high WNV transmission. Several other Florida mosquito control programs have taken similar actions based on surveillance data over the past few years.

There have been many articles, workshops and meetings, where the importance of WNV surveillance data has been demonstrated. Unfortunately, there are still situations when these data are not available, overlooked or even ignored. Yogi Berra, the legendary catcher for the New York Yankees, once quipped, "it's like déjà vu, all over again."

Surveillance information was available for WNV in Dallas County in 2012. It is apparent that by mid-June, the surveillance data showed a high risk for WNV transmission. The first WNV positive mosquito pool was detected in late May with weekly mosquito infection rates peaking at 53/100 in July. Trapping showed Cx. quinquefasciatus was widespread in the county. Ground spraying was initiated using trucks on June 21 targeting areas known to have abundant Cx. quinquefasciatus, but subsequent surveillance did not show substantial impact. Chung et al.1 show that aerial adulticiding began on August 16 through August 23 and also that cases declined during this period. We have seen a similar decline coincident with aerial adulticiding, although whether or not there is a cause and effect is not for certain. Recall similar examples reported in BuzzWords. The onset of West Nile symptoms or reporting of a case occurs several days after the actual transmission to that person. Therefore, the actual infection events, which are what we should be focused on, occurred several days previous to the adulticiding applications - well before the beginning of the aerial spray campaign. Aerial spraying occurred when transmission had already been declining, just as we witnessed in the 2003 WNV outbreak in Ft. Collins, Colorado, when aerial adulticiding occurred as the epidemic was already subsiding.5

Chung et al.1 conclude:

"Our findings support incorporating mosquito infection indices into response plans and closely monitoring the vector index in real time. The goal is to recognize significant increases above historically predictive thresholds of epidemic transmission when augmented mosquito control measures can prevent the most human illness. This requires continuing investments in robust mosquito surveillance programs...and establishment of local baseline patterns. Significant numbers of human cases may be reported too late to be a sensitive trigger for expanded intervention during the course of an epidemic."

Sound familiar? Chung et al.1 confirm previously established principles. For example, see reviews based on experiences in California14 or in Florida.15 Although one must take into account the delay in collecting surveillance information, i.e., to test mosquitoes and to report West Nile cases,  mosquito collections for abundance and reports of WNV-positive mosquitoes from Dallas County showedwtYogi increasing potential for WNV-transmission during June-July 2012. Chung et al.1 comment on the delays in surveillance reporting, and also the failure of the various Dallas County agencies to collate the available information they did have in hand.

Dallas County has instituted earlier surveillance for WNV in 2013, started its ground spraying program May 1 and hopefully will take other appropriate mosquito control measures if the surveillance information warrants, without basing decisions largely on the appearance of human West Nile cases.

Many Florida counties have invested in surveillance for mosquito-borne diseases like West Nile and St. Louis encephalitis. Florida's public health and mosquito control leaders must use this information and the Florida arbovirus mosquito response plan to take appropriate actions. Why expend resources and the effort to collect surveillance information, and then pay little attention to the findings?

 What about dengue in Key West and elsewhere in Florida? Martin County is experiencing an outbreakwtAlbert of dengue virus transmission in the Rio-Jensen Beach region. Clearly there has been transmission of DENV-1 by Aedes aegypti in Martin County in 2013. Martin County Mosquito Control has conducted a heroic effort against Ae. aegypti in this region, conducting house to house sweeps against Ae. aegypti in roughly 600 houses in Rio and similarly in Jensen Beach. What were other actions by both mosquito control and public health professionals to protect the public? Have these efforts had an impact on transmission? Were other efforts considered to reduce transmission? Are mosquito and mosquito-borne disease efforts based on all the available information about transmission and will these plans effectively reduce transmission? This will be reviewed in the coming months when we have more information.

Florida can and must do better.

References and Selected Readings:
Relevant BuzzWords articles on using surveillance information for West Nile can be obtained in our Buzz Words Archive.

1. Chung, WM, et al. 2013. The 2012 West Nile encephalitis epidemic in Dallas, Texas. JAMA 310: (3): 297-307. doi: 10.1001/jama.2013.8267.
2. Tabachnick, WJ and JF Day. 2001. Sentinel chicken surveillance and West Nile virus in Florida. BuzzWords, Newsletter of the FMCA, 1(6): 8-9.
3. Tabachnick, WJ and JF Day. 2003. WN in Florida: Detection, surveillance, human cases and Florida's response policy. BuzzWords, Newsletter of the FMCA 3(5): 10-11.
4. Tabachnick, WJ. 2004. Florida's sentinel chicken surveillance program: Smart chickens. BuzzWords, Newsletter of the FMCA 4(1): 7-8.
5. Tabachnick, WJ. 2004. Sentinel surveillance and human risk for West Nile. BuzzWords, Newsletter of the FMCA 4(2): 10-12.
6. Tabachnick, WJ. 2005. West Nile surveillance and Florida mosquito control: Acting on surveillance information. BuzzWords, Newsletter of the FMCA 5(5): 6-8.
7. Tabachnick, WJ, JF Day and CR Rutledge. 2005. Florida West Nile surveillance: Estimating mosquito transmission frequencies. BuzzWords, Newsletter of the FMCA 5(2): 6.
8. Tabachnick, WJ. 2006. Mosquito surveillance and West Nile prediction: Lessons learned. BuzzWords, Newsletter of the FMCA 6(4): 9-10.
9. Tabachnick, WJ and JF Day. 2006. Sentinel chicken surveillance: Some pitfalls in analyzing the data. BuzzWords, Newsletter of the FMCA 6(5): 7-9.
10. Tabachnick, WJ and JF Day. 2006. Sentinel chicken surveillance for West Nile virus. BuzzWords, Newsletter of the Newsletter of the FMCA 6(6): 9-10.
11. Tabachnick, WJ. 2012. Predicting the risk for West Nile using sentinel chickens or monitoring mosquito infection rates. BuzzWords, Newsletter of the FMCA 12(5): 4-6.
12. Tabachnick, WJ. 2003. Florida Mosquito Control Arbovirus Response Plan – West Nile: Guidelines for Mosquito Control Responses. A report to the Florida Coordinating Council on Mosquito Control, The Florida Dept. of Agriculture and Consumer Services and the Florida Dept. of Health, 17 pp.
13. Day, JF and WJ Tabachnick. 2003. Great move by the Indian River County Health Department and the Florida Department of Health, September 2003. BuzzWords, Newsletter of the FMCA 3(6): 12-14.
14. Reeves, WC, MM Milby and WK Reisen. 1990. Development of a statewide arbovirus surveillance program and models of vector populations and virus transmission. In: WC Reeves (ed.), Epidemiology and Control of Mosquito-Borne Arboviruses in California, 1943-1987. California Mosquito and Vector Control Association, Sacramento, CA, pp. 431-459.
15. Day, JF. 2001. Predicting St. Louis encephalitis virus epidemics: Lessons from recent, and not so recent, outbreaks. Ann. Rev. Entomol. 46: 111-148. Doi: 10.1146/annurev.ento.46.1.111.

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