March 29, 2023 | Volume 3 | Issue 2 | As of Week 12
Welcome to the Florida Disease Activity Update from the desk of Dr. Jonathan Day.
It continues to be Clarke’s privilege to share Dr. Day's weekly analysis of arbovirus disease activity in Florida with mosquito control professionals across the state. Our shared goal with Dr. Day is to provide timely and actionable information that mosquito control programs can use to make operational decisions and protect public health from vector-borne diseases.

An archive of all past newsletter issues remains available on the Clarke website.
A QUESTION FROM OUR READERS

Q. Jim from Ft. Lauderdale, FL, asks: “How do you use environmental monitoring to forecast arboviral transmission in Florida?”

A. Part 2, Flavivirus (SLEV and WNV)
The cycling of rainfall and drought and their effects on mosquito and wild bird populations are critical components of EEEV, SLEV, and WNV transmission and monitoring. This week, the effect of rainfall and drought on Flavivirus (SLEV and WNV) transmission in Florida will be discussed. 
 
The Keetch-Byram drought index (KBDI) is produced and published daily by the FDACS, Florida Forest Service and is a continuous reference scale for estimating soil dryness, making it an excellent predictor of standing surface water that can be used as a proxy to estimate mosquito production. The drier the surface soil layer, the less likely it is that adult mosquitoes will have access to flooded oviposition sites and an ability to disperse across large tracks of dry land.
 
Daily KBDI readings are compared with a Modeled Water Table Depth (MWTD) analysis developed in 2005 by Shaman and Day. The MWTD measures the depth of the water column below the ground's surface. Wet conditions bring the water column closer to the surface, resulting in flooding, standing surface water, and low KBDI readings (100 to 300) that favor mosquito oviposition and dispersal. 
 
MWTD levels were monitored during two significant arboviral transmission seasons (SLEV in 1990 and EEEV in 1991). The focus of this week will be MWTD conditions reported during the SLE epidemic of 1990 (Figure 1). 
Figure 1. Daily MWTD levels during 1990 (red line and red arrows), an SLE epidemic year in Florida.
In 1990, a widespread rural SLE epidemic remarkably similar to an SLE epidemic in 1977 was reported in South Florida. The index case for the 1990 SLE epidemic was a 22-year-old male who, like the index case for the 1977 SLE epidemic, lived in Fellsmere, Florida (Indian River County). The onset date for the 1990 index case was July 28, with an approximate infection date of July 19. The 1990 SLE epidemic continued for 25 weeks in 28 Florida counties, where 226 laboratory-confirmed human cases and 11 deaths (case-fatality ratio of 4.9%) were reported. The final human case was a 72-year-old male from Polk County with an onset date of 1/8/91 and an approximate infection date of 12/30/90.
 
The MWTD profile for 1990 (red line in Figure 1) provides a Flavivirus epidemic year signature that is characterized by a continuously drying winter and spring, a rapid wetting period in early July, a secondary dry down in mid-July and early August, and extreme wetting from August through October (red arrows in Figure 1).
 
A recent KBDI image (3/26/23) is shown in Figure 2. The Florida Panhandle has been wet for the entire winter and probably will not support high levels of Flavivirus transmission unless a deep drought occurs this spring. North Florida has been wet and is currently reporting a drying trend. If this dry-down continues in North Central Florida, WNV transmission may appear there later this summer. South Florida has been extremely dry for most of 2023. If this continues and is followed by wetting in June, WNV transmission in South Florida is likely this summer.
 
This week's take-home message: watch South Florida for accelerated WNV transmission this summer.
Figure 2. The KBDI output for 3/26/2023 shows wet surface conditions (blue) in the Florida Panhandle, drying (green and yellow) in North and Central Florida, and drought (orange and red) in most of South Florida.
THE CURRENT OUTLOOK FOR ARBOVIRAL TRANSMISSION IN FLORIDA

It is early in the 2023 arboviral transmission season in Florida (Table 1). Spring migration is in full swing, with waves of warblers and other species moving north through the state. The breeding season for Florida resident birds will begin soon. Northern Cardinals are particularly abundant this year in South Florida.

Rainfall patterns so far this year favor EEEV transmission in North Central Florida (see the answer to A Question From Our Readers in Volume 3, Issue 1 of the Newsletter) and WNV transmission in South Florida (see the answer to A Question Form Our Readers above). Spring and summer amplification of EEEV and WNV in migrant and resident breeding birds will determine the temporal and spatial intensity of arboviral transmission in Florida later this year.
 
Imported cases of dengue continue to be reported in Florida (see below) and continue to pose a threat of outbreaks of locally-acquired dengue throughout all of South Florida. Table 1 summarizes the current status of arboviral transmission in Florida.
Table 1. Summary of mosquito-borne disease transmission and imported cases in Florida as of March 25, 2023
Dengue Viruses

Imported cases of dengue continue to be reported in Florida (Figure 3).
Figure 3. The spatial distribution of imported human dengue cases in Florida during 2023. Counties shaded red indicate sites of at least one imported case during the past two weeks.
Recently imported dengue cases increase the risk of locally-acquired dengue transmission, and the six Florida counties shaded red in Figure 3 should be aware of possible locally-acquired dengue transmission in the coming weeks. Historically (2009-2023) locally-acquired dengue has been reported in nine Florida counties (Figure 4). Most of the transmission (87%) has been reported in Monroe and Miami-Dade Counties. Locally-acquired dengue transmission has been reported as far north as Volusia County. A single case of locally-acquired dengue has been reported in Miami-Dade County, with onset in January 2023.
Figure 4. The spatial distribution of locally-acquired human dengue cases in Florida from 2009-2023.
Malaria
 
Seven imported human malaria cases have been reported in Florida so far in 2023. These include single introductions into Broward, Duval, Leon, Miami-Dade, and Osceola Counties and two introductions into Pinellas County. Locally-acquired human cases of malaria usually appear in Florida after mid-July.
 
Eastern Equine Encephalitis Virus
 
North Central Florida is currently the region where transmission of EEEV will most likely occur in the coming months. Seven EEEV antibody-positive sentinel chickens (Figure 5) and two EEEV-positive equines (Figure 6) have been reported in Florida so far in 2023.
Figure 5. The spatial distribution of EEEV antibody-positive sentinel chickens in Florida during the first three months of 2023.
Figure 6. The spatial distribution of EEEV-positive equines in Florida during the first three months of 2023.
St. Louis Encephalitis Virus
 
There has been no evidence of SLEV transmission in Florida so far in 2023.
West Nile Virus

There has been little evidence of WNV transmission in Florida so far in 2023. One WNV antibody-positive sentinel chicken was reported in Orange County in February, and one WNV-positive equine was reported in Hernando County in early January.
OPERATIONAL STRATEGIES TO CONSIDER
It is too early in the 2023 Florida arboviral transmission season to make meaningful predictions about disease transmission later in the year.

Imported dengue cases continue to be reported in Florida, so there is a possibility of emerging foci of locally-acquired dengue. Vector control agencies in Broward, Collier Hillsborough, Miami-Dade, and Volusia Counties should be aware of the recent imported dengue cases and the possibility that these may result in locally-acquired cases of dengue.

Environmental conditions in North Florida currently favor EEEV transmission, but there are no indications of an impending major outbreak. West Nile virus and SLEV transmission is currently low, although the environmental conditions in South Florida favor enhanced mid-summer Flavivirus transmission. 
ABOUT DR. JONATHAN DAY
Jonathan Day, Professor Emeritus of Medical Entomology from the University of Florida, is a national expert on mosquitoes and other blood-feeding arthropods that transmit diseases to humans, domestic animals, and wildlife. In collaboration with other researchers, Dr. Day has developed an effective system for monitoring and predicting epidemics of mosquito-borne diseases.
Acknowledgments: This analysis would not be possible without the tireless efforts of multiple agencies across Florida. At least 27 Florida agencies collect serum samples from sentinel chickens each week and mail them to the Florida Department of Health Tampa Branch Laboratory for analysis, compilation and reporting. Data are summarized by researchers at the Florida Department of Health in Tallahassee and reported weekly as the Florida Arbovirus Surveillance Report.

Contributors to this summary and full report include: Andrea Morrison, PhD, MSPH, Rebecca Zimler, PhD, MPH, and Danielle Stanek, DVM, Florida Department of Health, Bureau of Epidemiology; Lea Heberlein-Larson, DrPH; Alexis LaCrue, PhD, MS; Maribel Castaneda, and Valerie Mock, BS, Florida Department of Health Bureau of Public Health Laboratories, and Carina Blackmore, DVM, PhD, FDOH Division of Disease Control and Health Protection. And, Dr. Rachel Lacey, Florida Department of Agriculture and Consumer Services, Animal Disease Diagnostic Laboratory in Kissimmee, FL.

Daily updates of the Keetch-Byram Drought Index (KBDI) are produced by the Florida Department of Agriculture and Consumer Services, Forest Service.

All of the graphics used in issues of this Newsletter are designed and developed by Gregory Ross.
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