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The Florida West Nile Forecast for 2006

Florida mosquito control professionals are dusting off their crystal balls about now. The 2006 mosquito and mosquito-borne disease transmission cycle in Florida began with the usual fanfare on January 1. We are already in the April--June arbovirus amplification phase of the Florida arbovirus cycle. This is the period when West Nile virus (WNV) and St. Louis encephalitis virus (SLEV) circulate in Florida's birds and mosquitoes. The amplification phase sets up the subsequent transmission phases. Depending on the bird numbers and immune status, mosquito populations, and rainfall patterns, the stage can be set for increased transmission in the later Early (July--September) and Late (October - December) transmission phase. If Florida is to have a widespread WNV epidemic in 2006 then WNV amplification needs to occur starting now until mid-summer. So it is important for Florida mosquito control and health professionals to begin paying attention to conditions in their respective locations.

It is still early in the season, and any predictions about arboviruses are expected to be fraught with uncertainty. However, we do have a variety of indicators that must be brought to bear so that we can make predictions to determine risk; much the same way we use hurricane forecasting information to assess the risk from hurricanes. Figure 1 is the August 30, 2004 hurricane track for Hurricane Frances.

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Fig. 1. Hurricane Frances, August 30, 2004.

Note how the uncertainty of the western end of the track is figured into the projection, such that the further the hurricane is from an area, the greater the uncertainty of the track. Naturally, the closer the hurricane is to an area (southeast portion of the track in Fig. 1), the more certain the prediction. For the Florida arbovirus system we are still a long way away from an informative and accurate prediction.

Wouldn't it be nice if we could provide WNV risk maps that convey risk information similar to that provided by Fig. 1? We suggest that a WNV risk prediction map can indeed be made for specific regions and locations in Florida. Such a risk map is available at the UF IFAS Florida Medical Entomology Laboratory "Encephalitis Information System" web site (http://eis.ifas.ufl.edu/eis1.htm).

The Encephalitis Information System provides a broad based assessment for WNV, SLEV and Eastern equine encephalitis virus (EEV) risk throughout Florida with a discussion of the surveillance and environmental indicators used as the basis for assigning risk. Of course, since the surveillance indicators are based on a regional perspective, the smaller focal changes in many indicators will be missed, giving rise to the possibility that a focal arbovirus outbreak could occur like the WNV outbreaks in Pinellas County in 2005, and Coconut Grove in 2004.

Let's focus on the current Florida situation and assess the current arbovirus amplification phase. What should local mosquito control and public health professionals focus on during this phase to assess the potential for elevated arbovirus transmission in the subsequent Early and Late Transmission Phases? If you would like to learn more about arbovirus transmission phases and the biological and environmental triggers that increase the risk of arbovirus transmission in Florida see the following review article: J. F. Day. 2001. Predicting St. Louis encephalitis virus epidemics: Lessons from recent, and not so recent, outbreaks. Annual Review of Entomology 46:111-38. Anything that improves the seasonal avian reproductive cycle, and improves the potential for bird to mosquito to bird contact serves to increase arbovirus amplification. Two environmental conditions are particularly important for increased viral amplification: 1) Hard winter freezes in peninsular Florida serve to kill cold-sensitive ground cover and open up the under story in hammocks which improves the reproductive success of ground feeding birds such as Mourning Doves and Common Grackles. 2) Winter droughts serve to increase mosquito bird contact by bringing birds and mosquitoes together around remaining water sources. If either of these conditions occurs, they increase the risk of arbovirus transmission later in the year. As most of you likely know, most of Florida is currently experiencing severe drought as evidenced by the most recent Keetch-Byram Drought Index for April 9, 2006 which is shown in Figure 2.

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Fig. 2. Keetch-Byram Drought Index for April 9, 2006

Florida is still several months away from the equivalent of land fall by Hurricane WNV or SLEV. However keep abreast of the amplification conditions in your area. The risk we seek to predict is the risk of a substantial outbreak, unlike anything Florida has experienced in recent years. This would be the "big event" when arboviral transmission becomes epidemic with 100s of cases in a localized region, rather than the focal 20 cases, or the sporadic 20-100 cases scattered around the state. Florida's widespread drought (Fig. 2) may be the first step in an elevated arboviral transmission season.

Florida has experienced this before without a subsequent increase in arbovirus transmission or an epidemic. However, for each region of the state we will also need to assess the wild bird populations, their immune status, and their reproductive success during the next three months. Then we will need to assess the water parameters, and rainfall patterns, and real-time arboviral transmission as measured by sentinel chicken flocks. Areas where all of the triggers fall into place are the likely future arboviral transmission hotspots. Time will tell whether or not this will be a big year for arboviral transmission in Florida. Keep a careful watch.

Walter J. Tabachnick and Jonathan F. Day
Florida Medical Entomology Laboratory University of Florida/IFAS