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An outlook for Arborival Transmission in Florida During the Second Half of 2007

The efficient amplification and transmission of mosquito-borne Flaviviruses (West Nile virus (WNV) and St. Louis encephalitis virus (SLEV)) in Peninsular Florida depends on a fine balance between wet and dry conditions during the spring and early summer months. Modeled Water Table Depth (WTD) provides a measure of the intensity of drought and wetting at a particular recording site throughout the year. The drier it is (the more intense the drought), the deeper below the surface is the water column. Figure 1, shown below, is a composite of WTD data collected in Indian River County (IRC), Florida during two SLE epidemics (1977 - top line and 1990 - bottom line), and the average of the WTD data reported during both epidemic years (middle line).

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Figure 1. The ideal sequence of dry and wet conditions that result in the efficient amplification and epidemic transmission of SLEV in Peninsular Florida.

The epicenter for both the 1977 and 1990 SLE epidemics occurred in IRC, indicating that the ecological and environmental factors driving the epidemics had their beginnings in IRC. Both of the epidemic WTD lines in Figure 1 are remarkably consistent. They begin with an initial dry-down (IDD) from week 1 through week 22. The IDD is followed by a period of initial wetting (IWET) from week 22 through week 27. The IWET period is followed by a secondary dry-down (SDD) from week 27 through week 32. Finally, the SDD is followed by a period of secondary wetting (SWET) from week 32 through the end of the year.

This dry-wet-dry-wet cycle in Peninsular Florida favors the amplification of SLEV and WNV by forcing vector mosquitoes and avian amplification hosts into contact in freshwater habitats during the IDD period. The forced contact of vectors and amplification hosts facilitates arboviral amplification and the IWET period allows the dispersal of infected vector mosquitoes and avian amplification hosts. The SDD period again forces mosquitoes and avian amplification hosts into focal habitats that contain freshwater and allow a second round of amplification. The SWET period allows dispersal of infected mosquitoes into habitats where they encounter and blood feed on humans and, in the case of WNV, horses, initiating epidemic and epizootic arboviral transmission.

The meteorologically-driven amplification and transmission of eastern equine encephalitis virus (EEEV) in North Florida and the Florida Panhandle is somewhat different than that for the Flaviviruses in Peninsular Florida described above. The EEEV is amplified between Culiseta melanura and resident/migratory birds in a hardwood swamp cycle during the autumn and winter months. Heavy spring droughts prevent the dispersal of EEEV from these swamp habitats while wet spring conditions allow infected mosquitoes and birds to disperse from the swamps carrying the EEEV with them. Once outside of the swamps, EEEV undergoes secondary amplification and transmission to humans and horses in habitats that are adjacent to the original amplification sites.

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Figure 2. The ideal sequence of drought and wetting that results in the efficient amplification and transmission of SLEV in Peninsular Florida (bottom line) and EEEV in North and Panhandle Florida (top line).

A comparison of the ideal modeled WTD curves for EEEV and Flavivirus amplification and transmission is shown in Figure 2. The basic meteorologically-driven amplification and transmission dynamics differ for EEEV and Flaviviruses. In general, wet springs (top curve in Figure 2) favor EEEV dispersal and transmission while dry springs (bottom curve in Figure 2) favor the amplification, dispersal, and transmission of Flaviriruses in Florida.

This brings us to the risk of arboviral amplification and transmission in Florida during the second half of 2007. As you undoubtedly know, Florida and the southeastern United States are suffering one of the worst droughts in recorded history. Figure 3 shows the Keetch-Byram Drought Index (KBDI) map for Florida from May 23, 2007. The KBDI map indicates the amount of moisture at the ground surface. The KBDI scale goes from 0 to 800. A reading of 0 indicates saturated soil while a reading of 750 to 800 is indicative of what one would expect to find in a desert.

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Figure 3. The May 23, 2007 Keetch-Byram Drought Index (KBDI) map for Florida. Drought Index values greater than 550 (dark grays if the map is being viewed in black and white) indicate severe drought. The entire state of Florida, with the exception of southern Dade and northwestern Monroe Counties, is currently dangerously dry.

As noted above, severe droughts do not favor mosquito production or arboviral amplification and transmission. The last time Florida experienced a severe, multi-year drought was in 1987, 1988, and 1989. It is evident from surveillance data (mosquito and arboviral) during those years that mosquito populations and arboviral transmission was greatly reduced in Florida (Day and Curtis, 1993). Figure 4 below shows the number of emerging Cx. nigripalpus females during a six year period from 1986 through 1991. It is evident that there was little mosquito reproduction, and consequently little arboviral transmission, during the period from 1987 through 1989. This period coincided with a severe drought in the Florida peninsular. These years also helped to set the stage for a major SLE epidemic in the same region during the late summer and autumn of 1990.

The severe drought under which Florida now suffers makes the likelihood of a major arboviral epidemic during the next six months unlikely. As we saw in 1987-1989, dry conditions curtail mosquito blood feeding and egg laying and severely impact the reproductive abilities of important arboviral vectors. This reduces host contacts and mosquito reproduction in general and, in so doing, reduces the amplification and transmission of mosquito-borne viruses.

Severe droughts, whether they last for three months or for three years, do end. The lesson from the 1987-1989 droughts and the subsequent SLE epidemic in 1990 is that these droughts may set the stage for major arboviral transmission events. Droughts reduce arboviral transmission to wild birds and serve to increase the number of susceptible wild birds when the drought ends. In addition, droughts prevent the flooding to temporary pools that are the favorite oviposition sites of Cx. nigripalpus, the major arboviral vector in the Florida peninsula. While remaining dry for months, or even years, these sites accumulate vegetation that, once flooded, produces a highly attractive and nutritious development site for larval mosquitoes. These oviposition sites become unusually productive for multiple generations of mosquitoes (see the highlighted Cx. nigripalpus emergence pattern for 1990 in Figure 4). While we may escape a major epidemic during this drier than normal year of 2007, we need to keep our mosquito and arboviral surveillance efforts current and we need to remain vigilant of all of the signs that will indicate conditions that are favorable to a severe mosquito-borne virus epidemic in the future.

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Figure 4. Culex nigripalpus emergence patterns in Indian River County, Florida from 1986 through 1991 (top frame). The mosquito emergence patterns are compared with heavy rainfall events (> 50mm during a 72 hr time period) during the same years (bottom frame).

References

  • Day and Curtis. 1993. Annual emergence patterns of Culex nigripalpus females before, during and after a widespread St. Louis encephalitis epidemic in south Florida. J. Am. Mosq. Control Association 9 (3):249-255

Jonathan F. Day, Professor
Florida Medical Entomology Laboratory
Department of Entomology and Nematology
University of Florida/IFAS
Vero Beach, FL 32962