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Florida Medical Entomology Laboratory

Florida Medical Entomology Laboratory

West Nile Virus in Florida: The Calm Before the Storm?

According to the Florida Department of Health's West Nile web page, the 2002 Florida tally for West Nile(WN) virus as of Jan. 13, 2002 was: 28 human cases (2 fatalities), 450 dead infected birds, 1033 sentinel chickens, 499 horses, 56 positive mosquito pools (73% Culex nigripalpus) and six other animals. Fifty-six Florida counties had WN virus activity. There is no doubt that in 2002 there was active WN virus transmission throughout much of the state, yet Florida was spared substantial numbers of human cases compared to other states including Illinois, Indiana, Louisiana, Michigan, Mississippi, Missouri, Nebraska,Ohio and Texas, all of which reported at least 100 human cases.

Why did Florida dodge the WN virus bullet in 2002 and what can we expect in the future?

We were extremely fortunate that conditions in Florida during the 2002 arboviral transmission season were clearly not optimal for flavivirus transmission. St. Louis encephalitis (SLE) virus transmission in 2002 was negligible with only a single confirmed human case and two positive mosquito pools. There was substantial eastern equine encephalitis (EEE) virus activity during 2002, but EEE has a different amplification and transmission cycle from WN and SLE viruses. Readers should review previous Buzz Words columns and other information that can be found at the FMEL web page ( for more information about important factors necessary for maintaining and amplifying WN virus in nature. For other source information, readers are encouraged to look at a comprehensive review of SLE issues (Day, J. F.2001. Predicting St. Louis encephalitis virus epidemics: Lessons from recent and not so recent outbreaks. Ann. Rev. of Entomol. 46: 111-138). Here Jon Day discusses at length the necessary primers and triggers for a full-blown SLE virus epidemic.

A major factor in the flavivirus amplification cycle that was missing in the Florida SLE transmission zone during 2002 was a wet spring and wet early summer. This is a necessary precursor for large mosquito populations that coincide with nesting birds, allowing explosive arboviral amplification. However, despite these sub-optimal amplification conditions, there was substantial sporadic WN virus transmission during 2002. One can only imagine the WN transmission activity we will see in Florida when conditions are more conducive for the amplification of this virus. Researchers at the FMEL reported mosquito transmission of WN virus in a focal area in the Florida Panhandle during 2001. If the environmental conditions that lead to the North Florida WN virus transmission are found in more populous areas of South Florida, 1000-2000 human WN cases could occur (Rutledge, C. R. et al. 2003. Culex nigripalpus Theobald (Diptera: Culicidae)transmission of West Nile virus in Florida: Infection rates in Florida Culex mosquitoes do not accurately reflect transmission rates in nature. Journal of Medical Entomology, accepted for publication).

The FMEL strongly urges Florida mosquito control and public health officials to prepare now for this potential catastrophe. Previous editorial columns, presentations at FMCA and workshop meetings, papers, and the proceedings of the FMEL sponsored West Nile Workshop (see the FMEL web page) have summarized the surveillance protocols necessary to monitor mosquito arboviral transmission by utilizing sentinel chicken flocks and the FMEL Arbovirus Rapid Deployment System (ARDS). ARDS can provide real time information to determine specific high-risk regions for WN virus transmission so that public health and mosquito control strategies can be targeted to areas for maximum impact.

It is imperative that Florida mosquito control organizations and public health departments begin to implement their plan to employ strategies that will reduce WN virus transmission early in the year. Plans need to be developed in each jurisdiction that are tailored to specific regional factors. Prudent professionals recognize that success will hinge on an investment now in developing an appropriate real time surveillance system, using sentinel chickens, coupled with plans to employ an ARDS design strategy to provide more accurate and timely transmission information. This will require the assistance of external experts on arboviral disease epidemiology and surveillance. This assistance is available through the FMEL, state level support, and local support.

Members of the FMCA were gratified to hear Commissioner Bronson, the Florida Commissioner of Agriculture and Consumer Services, at the Fall FMCA Meeting. His commitment to provide support for mosquito control and WN virus intervention will hopefully result in essential resources that the FMEL and mosquito control districts can utilize for developing effective surveillance programs in individual Florida Counties. Florida desperately needs a substantial increase in financial support from the state level directed at improved WN surveillance and control. Effective surveillance will not be easy to develop and implement. The current Florida sentinel flock system has wide gaps in coverage across the state and flock placement, husbandry, maintenance, and bleeding protocols are applied inconsistently from county to county. The WN dead bird reporting system is inaccurate and problematic at the least. Florida has the capability of instituting an accurate, real-time, statewide, arbovirus surveillance reporting system. The design of this system demands a research base to identify flexible goals that are likely to be changed depending on local circumstances,and a long term commitment to collect, analyze, and quickly report data. Consider the costs to the State of Florida in direct health costs, adverse publicity, and lost tourism from 1000 human cases compared with a relatively small investment in research and development of surveillance protocols that have the ability to minimize a human arboviral epidemic.

I challenge public health and mosquito control professionals to ponder three important questions:

  1. Are you prepared to identify high-risk WN virus transmission zones in the state and in individual counties priorto the onset of a significant number of human cases?
  2. Are you prepared to target mosquito control and WN prevention strategies to specific identified high-risk regions?
  3. Have you invested any resources tobegin to develop an appropriate WN virus surveillance and response plan for your jurisdiction?

When the WN storm hits Florida, and high-risk regions are experiencing 50 new human WN cases a day, it will be too late to implement effective surveillance efforts designed to identify and target high-risk areas within individual districts. Prudent professionals are investing now in preparation of the WN storm. To do nothing prior to the major WN epidemic might be considered negligence. Without a surveillance investment, there can be no information to target control efforts in high-risk areas that will result in efficient, effective and environmentally proper mosquito control. In the absence of high quality WN virus surveillance data, mosquito control will be forced into a wide area control strategy that will be costly, inefficient, likely ineffective, and, unfortunately, may be environmentally improper.

The time to invest, plan and implement is now--the calm before the storm.

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