Florida's 2004 West Nile Transmission Season

We will most remember 2004 as the year of the Hurricanes. On the brighter side, thankfully Florida once again escaped a major West Nile epidemic. Here are Florida's West Nile numbers for the year as of Nov. 20, 2004. Human cases: 41 (two deaths), Sentinel chickens: 310, Horses: 6, Wild Birds: 35. Seven of Florida's counties had human cases and West Nile virus activity was detected in 34 of Florida's 67 counties. Despite the large amounts of rainfall, and the resulting explosion of mosquito populations in large portions of the state, West Nile transmission remained close to what has been observed each year since its introduction in 2001.

The reasons for the absence of a Florida West Nile epidemic in 2004 include, 1) the failure of substantial West Nile amplification during the avian breeding season (April-June) and 2) the late timing of major rainfall events associated with the hurricanes which occurred during the early transmission period (July-September) of the Florida arboviral cycle. This validates that without viral amplification, which typically occurs during the spring and summer, there is little chance of epidemic transmission during the late summer and fall (August-November). The FMEL Encephalitis Information System at continually collated epidemiologically relevant factors during the year and assessed them in terms of the risk for West Nile transmission. Readers are encouraged to visit this site for explanations. It is reassuring to note that the predictions made throughout the year on EIS were accurate and right on target. The factors indicating substantial risk of West Nile virus transmission to humans were never in place during 2004.

We must continue to improve our surveillance capabilities during upcoming years to ensure we will be prepared for the anticipated West Nile epidemic in Florida. So what can we learn from 2004? Although Florida did, yet again, escape a substantial West Nile epidemic, the West Nile virus transmission reported in Miami-Dade County from June-September 2004 was very troubling and could be an omen of a future West Nile epidemic. Twenty of Florida's human West Nile cases were in Miami-Dade. Perhaps this is not all that troubling considering the county population is ca. 2.3 million. However, when one considers the 9 human cases in Coconut Grove/Coral Gables, population 60,431, the disease incidence for this area is 1 in 6700 people during the course of the summer. This incidence is among the highest observed in the U. S. during 2004. Certainly Coconut Grove and Coral Gables presented a very focal outbreak that did not spread substantially to other parts of the county. Consider that this same incidence throughout Miami-Dade would produce a staggering 350 West Nile cases. The same incidence throughout Florida would produce a catastrophic 2,400 West Nile cases. We will be working with Marlon Nelms and the Miami-Dade Mosquito Control District over the next few months to evaluate what happened. Why did this one small area have such a high incidence? Why did transmission not spread to other regions? What can be done to ensure that the situation will not repeat in the same area during 2005, or more troubling--worsen and spread?

We will continue working with all Florida Mosquito Control programs to develop the appropriate actions to reduce the risk of a catastrophic West Nile epidemic. However, consider Coconut Grove and Coral Gables. What are appropriate steps to take in 2005 if there are indications that transmission is again at the 2004 levels? What should be the response if, for example, ARDS data shows mosquito transmission frequencies of 1/1000 or greater frequency? What should local mosquito control provide? What coordinated steps are needed by mosquito control and public health agencies? What constitutes a medical threat level that requires strong advice to the public that outdoor nighttime activities dramatically increase exposure to mosquitoes and risk of disease? What level (50? 100? 1,000 human cases?) might constitute a medical disaster situation requiring external assistance from the state and perhaps FEMA? What disease incidence levels should be used? What surveillance levels (mosquito transmission levels? sentinels? mosquito pools?) require that appropriate actions be taken in advance of large numbers of human cases?

What may seem like academic questions in December 2004 will become all too real in July-August 2005. We were very close in Miami-Dade during the summer of 2004 to dealing with these issues and questions during a potentially dangerous epidemic situation.

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