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.

The cycling of rainfall and drought and their impacts on mosquito and wild bird populations are critical components of EEEV, SLEV, and WNV transmission. This week, the impact of rainfall and drought on the transmission of EEEV 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, 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 the ability to disperse from breeding and resting sites.

Daily KBDI readings are compared to 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 that favor mosquito oviposition and dispersal. 

MWTD levels were monitored during two significant arboviral transmission seasons (SLEV in 1990 and EEEV in 1991) (Figure 1). The focus this week will be the EEEV transmission season of 1991.  

The transmission of EEEV was unusually active in Florida during 1991. Five human EEE cases were reported. Two in Duval County (onsets of 6/8 and 6/15), two in Bradford County (onsets of 6/15 and 7/7), and one in Washington County (onset of 7/7). Sentinel chicken seroconversions to EEEV were high in North and Central Florida from April-September. Finally, of the 160 EEEV-positive equines reported that year, 145 (91%) were reported in May (16 positives), June (48), July (68), and August (13) during a period of drying (Figure 1).

The MWTD profile for 1991 (blue line in Figure 1) provided an EEE epidemic year signature and was characterized by a dry winter, wet spring, dry summer, and wet autumn.

A recent KBDI image (3/12/23) is shown in Figure 2. The Florida Panhandle has been wet for the entire winter and probably will not support high levels of EEE transmission unless a drought occurs later this year. North Florida has been wet and is currently reporting a drying trend, and South Florida has been extremely dry.

With the exception of imported and locally acquired dengue viruses, arboviral transmission in Florida during the 2022 transmission season was below normal.

Table 1 summarizes the expected and observed transmission levels for dengue, malaria, eastern equine encephalitis, St. Louis encephalitis, and West Nile viruses. 

The expected values for DENV, SLEV, and malaria were determined from reviews of The Florida Department of Health Annual Florida Arbovirus Surveillance Summaries for 2010-2021 (12 years). The expected values for EEEV transmission (sentinel chickens, EEEV-positive equines, and human EEE cases) were determined from reviews of Florida Arbovirus Surveillance Summaries for 2004-2021 (18 years). The expected values for WNV transmission (sentinel chickens, positive equines, and human WN cases) were determined from reviews of Florida Arbovirus Surveillance Summaries for 2001-2021 (21 years).

Observed values for 2022 were gathered from weekly FDOH Florida Arbovirus Surveillance reports.

Dengue viruses (DENV) were by far the most active arboviruses in Florida in 2022.

During the 12 years from 2010 to 2021, an average of 105 imported dengue cases were reported in Florida. In 2022, there were 916 imported cases, a difference of 811 cases (Table 1, Figure 3).

Most of the imported cases (543) were the DENV 3 serotype, and most of the travelers (836) had a travel connection to Cuba. Imported dengue cases were reported throughout Florida, but the main focus was South Florida, where 756 (83%) cases were reported. 

During the 12-year period from 2010 to 2021, an average of 16 locally-acquired dengue cases was reported in Florida. In 2022, there were 68 locally-acquired cases of DENV reported in four Florida counties, a difference of 52 cases (Table 1, Figure 4).

One of the most important observations from the 2022 dengue transmission season is that there were a number of locally-acquired dengue cases associated with imported cases. The vectors of DENV, Aedes aegypti and Ae. albopictus have become exceedingly abundant in the Florida Peninsula, and vector control programs must assume that imported DENV can, and will, translate into foci of locally-acquired dengue.

Imported malaria cases in 2022 approached the 12-year average (expected = 68, observed = 63). It is extremely rare for imported malaria cases to produce a focus of locally-acquired malaria. The reasons for this were discussed in the answer to a Question From Our Readers in Volume 2, Issue 2 (4/27/22) of the Newsletter. Even though locally-acquired malaria transmission is rare, it does occur as evidenced by the eight cases of locally-acquired human malaria reported in Palm Beach County in 2003, with onset dates from 7/12 to 9/14/2003.

Transmission of WNV to sentinel chickens in Florida was above normal (expected = 413, observed = 492, difference = +79) in 2022 (Table 1). However, the number of WNV equines was below normal (expected = 60, observed = 5), as was the number of reported human WN cases (expected = 23, observed = 6).

One of the most important lessons from the 2022 Florida arboviral transmission season is the potential threat of imported dengue cases to result in locally-acquired dengue (Figures 3 and 4).

A number of the 68 locally-acquired dengue cases reported in four Florida counties during 2022 were associated with imported cases. Aedes aegypti and Ae. albopictus, the known vectors of DENV, are abundant around households and businesses throughout South and Central Florida. I have a significant population of both species in my yard (see Confessions of a Mosquito Producer, Newsletter, Volume 2, Issue 23, 11/16/22).

The large populations of these mosquito species in Florida increase the probability that an imported dengue case may seed a workplace or neighborhood with DENV that infects local vector mosquitoes. This may result in a focus of locally-acquired human dengue. Both of these vector species are notoriously difficult to control. With the increasing number of imported dengue cases in Florida, the successful local control of these species, along with an intense public education campaign regarding the hows, whens, and whys of dengue transmission, is more necessary than ever.

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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